GameBoy FAQ

Pan Docs
--------

Overview
--> About the Pan Docs
--> Game Boy Technical Data
--> Memory Map

I/O Ports
--> Video Display
--> Sound Controller
--> Joypad Input
--> Serial Data Transfer (Link Cable)
--> Timer and Divider Registers
--> Interrupts
--> CGB Registers
--> SGB Functions

CPU Specifications
--> CPU Registers and Flags
--> CPU Instruction Set
--> CPU Comparision with Z80

Cartridges
--> The Cartridge Header
--> Memory Bank Controllers
--> Gamegenie/Shark Cheats

Other
--> Power Up Sequence
--> Reducing Power Consumption
--> Sprite RAM Bug
--> External Connectors

About the Pan Docs
------------------

 =================================================================
       Everything You Always Wanted To Know About GAMEBOY *
 =================================================================

                     * but were afraid to ask

        Pan of -ATX- Document Updated by contributions from:
     Marat Fayzullin, Pascal Felber, Paul Robson, Martin Korth
             CPU, SGB, CGB, AUX specs by Martin Korth

                  Last updated 10/2001 by nocash
               Previously updated 4-Mar-98 by kOOPa

Forward
The following was typed up for informational purposes regarding the inner
workings on the hand-held game machine known as GameBoy, manufactured and
designed by Nintendo Co., LTD. This info is presented to inform a user on how
their Game Boy works and what makes it "tick". GameBoy is copyrighted by
Nintendo Co., LTD. Any reference to copyrighted material is not presented for
monetary gain, but for educational purposes and higher learning.

Available Document Formats
The present version of this document is available in Text and Html format:
  http://www.work.de/nocash/pandocs.txt
  http://www.work.de/nocash/pandocs.htm
Also, a copy of this document is included in the manual of newer versions of
the no$gmb debugger, because of recent piracy attacks (many thanks and best
wishes go to hell) I have currently no intention to publish any such or
further no$gmb updates though.

Game Boy Technical Data
-----------------------

  CPU          - 8-bit (Similar to the Z80 processor)
  Clock Speed  - 4.194304MHz (4.295454MHz for SGB, max. 8.4MHz for CGB)
  Work RAM     - 8K Byte (32K Byte for CGB)
  Video RAM    - 8K Byte (16K Byte for CGB)
  Screen Size  - 2.6"
  Resolution   - 160x144 (20x18 tiles)
  Max sprites  - Max 40 per screen, 10 per line
  Sprite sizes - 8x8 or 8x16
  Palettes     - 1x4 BG, 2x3 OBJ (for CGB: 8x4 BG, 8x3 OBJ)
  Colors       - 4 grayshades (32768 colors for CGB)
  Horiz Sync   - 9198 KHz (9420 KHz for SGB)
  Vert Sync    - 59.73 Hz (61.17 Hz for SGB)
  Sound        - 4 channels with stereo sound
  Power        - DC6V 0.7W (DC3V 0.7W for GB Pocket, DC3V 0.6W for CGB)

Memory Map
----------

The gameboy is having a 16bit address bus, that is used to address ROM, RAM,
and I/O registers.

General Memory Map
  0000-3FFF   16KB ROM Bank 00     (in cartridge, fixed at bank 00)
  4000-7FFF   16KB ROM Bank 01..NN (in cartridge, switchable bank number)
  8000-9FFF   8KB Video RAM (VRAM) (switchable bank 0-1 in CGB Mode)
  A000-BFFF   8KB External RAM     (in cartridge, switchable bank, if any)
  C000-CFFF   4KB Work RAM Bank 0 (WRAM)
  D000-DFFF   4KB Work RAM Bank 1 (WRAM)  (switchable bank 1-7 in CGB Mode)
  E000-FDFF   Same as C000-DDFF (ECHO)    (typically not used)
  FE00-FE9F   Sprite Attribute Table (OAM)
  FEA0-FEFF   Not Usable
  FF00-FF7F   I/O Ports
  FF80-FFFE   High RAM (HRAM)
  FFFF        Interrupt Enable Register

Jump Vectors in First ROM Bank
The following addresses are supposed to be used as jump vectors:
  0000,0008,0010,0018,0020,0028,0030,0038   for RST commands
  0040,0048,0050,0058,0060                  for Interrupts
However, the memory may be used for any other purpose in case that your
program doesn't use any (or only some) RST commands or Interrupts. RST
commands are 1-byte opcodes that work similiar to CALL opcodes, except that
the destination address is fixed.

Cartridge Header in First ROM Bank
The memory at 0100-014F contains the cartridge header. This area contains
information about the program, its entry point, checksums, information about
the used MBC chip, the ROM and RAM sizes, etc. Most of the bytes in this area
are required to be specified correctly. For more information read the chapter
about The Cartridge Header.

External Memory and Hardware
The areas from 0000-7FFF and A000-BFFF may be used to connect external
hardware. The first area is typically used to address ROM (read only, of
course), cartridges with Memory Bank Controllers (MBCs) are additionally using
this area to output data (write only) to the MBC chip. The second area is
often used to address external RAM, or to address other external hardware
(Real Time Clock, etc). External memory is often battery buffered, and may
hold saved game positions and high scrore tables (etc.) even when the gameboy
is turned of, or when the cartridge is removed. For specific information read
the chapter about Memory Bank Controllers.

Video Display
-------------

Video I/O Registers
--> LCD Control Register
--> LCD Status Register
--> LCD Interrupts
--> LCD Position and Scrolling
--> LCD Monochrome Palettes
--> LCD Color Palettes (CGB only)
--> LCD VRAM Bank (CGB only)
--> LCD OAM DMA Transfers
--> LCD VRAM DMA Transfers (CGB only)

Video Memory
--> VRAM Tile Data
--> VRAM Background Maps
--> VRAM Sprite Attribute Table (OAM)
--> Accessing VRAM and OAM

LCD Control Register
--------------------

FF40 - LCDC - LCD Control (R/W)
  Bit 7 - LCD Display Enable             (0=Off, 1=On)
  Bit 6 - Window Tile Map Display Select (0=9800-9BFF, 1=9C00-9FFF)
  Bit 5 - Window Display Enable          (0=Off, 1=On)
  Bit 4 - BG & Window Tile Data Select   (0=8800-97FF, 1=8000-8FFF)
  Bit 3 - BG Tile Map Display Select     (0=9800-9BFF, 1=9C00-9FFF)
  Bit 2 - OBJ (Sprite) Size              (0=8x8, 1=8x16)
  Bit 1 - OBJ (Sprite) Display Enable    (0=Off, 1=On)
  Bit 0 - BG Display (for CGB see below) (0=Off, 1=On)

LCDC.7 - LCD Display Enable
CAUTION: Stopping LCD operation (Bit 7 from 1 to 0) may be performed during
V-Blank ONLY, disabeling the display outside of the V-Blank period may damage
the hardware. This appears to be a serious issue, Nintendo is reported to
reject any games that do not follow this rule.
V-blank can be confirmed when the value of LY is greater than or equal to 144.
When the display is disabled the screen is blank (white), and VRAM and OAM can
be accessed freely.

--- LCDC.0 has different Meanings depending on Gameboy Type ---

LCDC.0 - 1) Monochrome Gameboy and SGB: BG Display
When Bit 0 is cleared, the background becomes blank (white). Window and
Sprites may still be displayed (if enabled in Bit 1 and/or Bit 5).

LCDC.0 - 2) CGB in CGB Mode: BG and Window Master Priority
When Bit 0 is cleared, the background and window lose their priority - the
sprites will be always displayed on top of background and window,
independently of the priority flags in OAM and BG Map attributes.

LCDC.0 - 3) CGB in Non CGB Mode: BG and Window Display
When Bit 0 is cleared, both background and window become blank (white), ie.
the Window Display Bit (Bit 5) is ignored in that case. Only Sprites may still
be displayed (if enabled in Bit 1).
This is a possible compatibility problem - any monochrome games (if any) that
disable the background, but still want to display the window wouldn't work
properly on CGBs.

LCD Status Register
-------------------

FF41 - STAT - LCDC Status   (R/W)
  Bit 6 - LYC=LY Coincidence Interrupt (1=Enable) (Read/Write)
  Bit 5 - Mode 2 OAM Interrupt         (1=Enable) (Read/Write)
  Bit 4 - Mode 1 V-Blank Interrupt     (1=Enable) (Read/Write)
  Bit 3 - Mode 0 H-Blank Interrupt     (1=Enable) (Read/Write)
  Bit 2 - Coincidence Flag  (0:LYC<>LY, 1:LYC=LY) (Read Only)
  Bit 1-0 - Mode Flag       (Mode 0-3, see below) (Read Only)
            0: During H-Blank
            1: During V-Blank
            2: During Searching OAM-RAM
            3: During Transfering Data to LCD Driver

The two lower STAT bits show the current status of the LCD controller.
  Mode 0: The LCD controller is in the H-Blank period and
          the CPU can access both the display RAM (8000h-9FFFh)
          and OAM (FE00h-FE9Fh)

  Mode 1: The LCD contoller is in the V-Blank period (or the
          display is disabled) and the CPU can access both the
          display RAM (8000h-9FFFh) and OAM (FE00h-FE9Fh)

  Mode 2: The LCD controller is reading from OAM memory.
          The CPU  access OAM memory (FE00h-FE9Fh)
          during this period.

  Mode 3: The LCD controller is reading from both OAM and VRAM,
          The CPU  access OAM and VRAM during this period.
          CGB Mode: Cannot access Palette Data (FF69,FF6B) either.

The following are typical when the display is enabled:
  Mode 2  2_____2_____2_____2_____2_____2___________________2____
  Mode 3  _33____33____33____33____33____33__________________3___
  Mode 0  ___000___000___000___000___000___000________________000
  Mode 1  ____________________________________11111111111111_____

The Mode Flag goes through the values 0, 2, and 3 at a cycle of about 109uS. 0
is present about 48.6uS, 2 about 19uS, and 3 about 41uS. This is interrupted
every 16.6ms by the VBlank (1). The mode flag stays set at 1 for about 1.08
ms.

Mode 0 is present between 201-207 clks, 2 about 77-83 clks, and 3 about
169-175 clks. A complete cycle through these states takes 456 clks. VBlank
lasts 4560 clks. A complete screen refresh occurs every 70224 clks.)

LCD Interrupts
--------------

INT 40 - V-Blank Interrupt
The V-Blank interrupt occurs ca. 59.7 times a second on a regular GB and ca.
61.1 times a second on a Super GB (SGB). This interrupt occurs at the
beginning of the V-Blank period (LY=144).
During this period video hardware is not using video ram so it may be freely
accessed. This period lasts approximately 1.1 milliseconds.

INT 48 - LCDC Status Interrupt
There are various reasons for this interrupt to occur as described by the STAT
register ($FF40). One very popular reason is to indicate to the user when the
video hardware is about to redraw a given LCD line. This can be useful for
dynamically controlling the SCX/SCY registers ($FF43/$FF42) to perform special
video effects.

LCD Position and Scrolling
--------------------------

FF42 - SCY - Scroll Y   (R/W)
FF43 - SCX - Scroll X   (R/W)
Specifies the position in the 256x256 pixels BG map (32x32 tiles) which is to
be displayed at the upper/left LCD display position.
Values in range from 0-255 may be used for X/Y each, the video controller
automatically wraps back to the upper (left) position in BG map when drawing
exceeds the lower (right) border of the BG map area.

FF44 - LY - LCDC Y-Coordinate (R)
The LY indicates the vertical line to which the present data is transferred to
the LCD Driver. The LY can take on any value between 0 through 153. The values
between 144 and 153 indicate the V-Blank period. Writing will reset the
counter.

FF45 - LYC - LY Compare  (R/W)
The gameboy permanently compares the value of the LYC and LY registers. When
both values are identical, the coincident bit in the STAT register becomes
set, and (if enabled) a STAT interrupt is requested.

FF4A - WY - Window Y Position (R/W)
FF4B - WX - Window X Position minus 7 (R/W)
Specifies the upper/left positions of the Window area. (The window is an
alternate background area which can be displayed above of the normal
background. OBJs (sprites) may be still displayed above or behinf the window,
just as for normal BG.)
The window becomes visible (if enabled) when positions are set in range
WX=0..166, WY=0..143. A postion of WX=7, WY=0 locates the window at upper
left, it is then completly covering normal background.

LCD Monochrome Palettes
-----------------------

FF47 - BGP - BG Palette Data  (R/W) - Non CGB Mode Only
This register assigns gray shades to the color numbers of the BG and Window
tiles.
  Bit 7-6 - Shade for Color Number 3
  Bit 5-4 - Shade for Color Number 2
  Bit 3-2 - Shade for Color Number 1
  Bit 1-0 - Shade for Color Number 0
The four possible gray shades are:
  0  White
  1  Light gray
  2  Dark gray
  3  Black
In CGB Mode the Color Palettes are taken from CGB Palette Memory instead.

FF48 - OBP0 - Object Palette 0 Data (R/W) - Non CGB Mode Only
This register assigns gray shades for sprite palette 0. It works exactly as
BGP (FF47), except that the lower two bits aren't used because sprite data 00
is transparent.

FF49 - OBP1 - Object Palette 1 Data (R/W) - Non CGB Mode Only
This register assigns gray shades for sprite palette 1. It works exactly as
BGP (FF47), except that the lower two bits aren't used because sprite data 00
is transparent.

LCD Color Palettes (CGB only)
-----------------------------

FF68 - BCPS/BGPI - CGB Mode Only - Background Palette Index
This register is used to address a byte in the CGBs Background Palette Memory.
Each two byte in that memory define a color value. The first 8 bytes define
Color 0-3 of Palette 0 (BGP0), and so on for BGP1-7.
  Bit 0-5   Index (00-3F)
  Bit 7     Auto Increment  (0=Disabled, 1=Increment after Writing)
Data can be read/written to/from the specified index address through Register
FF69. When the Auto Increment Bit is set then the index is automatically
incremented after each  to FF69. Auto Increment has no effect when
 from FF69, so the index must be manually incremented in that case.

FF69 - BCPD/BGPD - CGB Mode Only - Background Palette Data
This register allows to read/write data to the CGBs Background Palette Memory,
addressed through Register FF68.
Each color is defined by two bytes (Bit 0-7 in first byte).
  Bit 0-4   Red Intensity   (00-1F)
  Bit 5-9   Green Intensity (00-1F)
  Bit 10-14 Blue Intensity  (00-1F)
Much like VRAM, Data in Palette Memory cannot be read/written during the time
when the LCD Controller is reading from it. (That is when the STAT register
indicates Mode 3).
Note: Initially all background colors are initialized as white.

FF6A - OCPS/OBPI - CGB Mode Only - Sprite Palette Index
FF6B - OCPD/OBPD - CGB Mode Only - Sprite Palette Data
These registers are used to initialize the Sprite Palettes OBP0-7, identically
as described above for Background Palettes. Note that four colors may be
defined for each OBP Palettes - but only Color 1-3 of each Sprite Palette can
be displayed, Color 0 is always transparent, and can be initialized to a don't
care value.
Note: Initially all sprite colors are uninitialized.

RGB Translation by CGBs
When developing graphics on PCs, note that the RGB values will have different
appearance on CGB displays as on VGA monitors:
The highest intensity will produce Light Gray color rather than White. The
intensities are not linear; the values 10h-1Fh will all appear very bright,
while medium and darker colors are ranged at 00h-0Fh.
The CGB display will mix colors quite oddly, increasing intensity of only one
R,G,B color will also influence the other two R,G,B colors.
For example, a color setting of 03EFh (Blue=0, Green=1Fh, Red=0Fh) will appear
as Neon Green on VGA displays, but on the CGB it'll produce a decently washed
out Yellow.

RGB Translation by GBAs
Even though GBA is described to be compatible to CGB games, most CGB games are
completely unplayable on GBAs because most colors are invisible (black). Of
course, colors such like Black and White will appear the same on both CGB and
GBA, but medium intensities are arranged completely different.
Intensities in range 00h..0Fh are invisible/black (unless eventually under
best sunlight circumstances, and when gazing at the screen under obscure
viewing angles), unfortunately, these intensities are regulary used by most
existing CGB games for medium and darker colors.
Newer CGB games may avoid this effect by changing palette data when detecting
GBA hardware. A relative simple method would be using the formula
GBA=CGB/2+10h for each R,G,B intensity, probably the result won't be perfect,
and (once colors became visible) it may turn out that the color mixing is
different also, anyways, it'd be still ways better than no conversion.
Asides, this translation method should have been VERY easy to implement in GBA
hardware directly, even though Nintendo obviously failed to do so. How did
they say, This seal is your assurance for excellence in workmanship and so on?

LCD VRAM Bank (CGB only)
------------------------

FF4F - VBK - CGB Mode Only - VRAM Bank
This 1bit register selects the current Video Memory (VRAM) Bank.
  Bit 0 - VRAM Bank (0-1)
Bank 0 contains 192 Tiles, and two background maps, just as for monochrome
games. Bank 1 contains another 192 Tiles, and color attribute maps for the
background maps in bank 0.

LCD OAM DMA Transfers
---------------------

FF46 - DMA - DMA Transfer and Start Address (W)
Writing to this register launches a DMA transfer from ROM or RAM to OAM memory
(sprite attribute table). The written value specifies the transfer source
address divided by 100h, ie. source & destination are:
  Source:      XX00-XX9F   ;XX in range from 00-F1h
  Destination: FE00-FE9F
It takes 160 microseconds until the transfer has completed (80 microseconds in
CGB Double Speed Mode), during this time the CPU can access only HRAM (memory
at FF80-FFFE). For this reason, the programmer must copy a short procedure
into HRAM, and use this procedure to start the transfer from inside HRAM, and
wait until the transfer has finished:
   ld  (0FF46h),a ;start DMA transfer, a=start address/100h
   ld  a,28h      ;delay...
  wait:           ;total 5x40 cycles, approx 200ms
   dec a          ;1 cycle
   jr  nz,wait    ;4 cycles
Most programs are executing this procedure from inside of their VBlank
procedure, but it is possible to execute it during display redraw also,
allowing to display more than 40 sprites on the screen (ie. for example 40
sprites in upper half, and other 40 sprites in lower half of the screen).

LCD VRAM DMA Transfers (CGB only)
---------------------------------

FF51 - HDMA1 - CGB Mode Only - New DMA Source, High
FF52 - HDMA2 - CGB Mode Only - New DMA Source, Low
FF53 - HDMA3 - CGB Mode Only - New DMA Destination, High
FF54 - HDMA4 - CGB Mode Only - New DMA Destination, Low
FF55 - HDMA5 - CGB Mode Only - New DMA Length/Mode/Start
These registers are used to initiate a DMA transfer from ROM or RAM to VRAM.
The Source Start Address may be located at 0000-7FF0 or A000-DFF0, the lower
four bits of the address are ignored (treated as zero). The Destination Start
Address may be located at 8000-9FF0, the lower four bits of the address are
ignored (treated as zero), the upper 3 bits are ignored either (destination is
always in VRAM).

Writing to FF55 starts the transfer, the lower 7 bits of FF55 specify the
Transfer Length (divided by 10h, minus 1). Ie. lengths of 10h-800h bytes can
be defined by the values 00h-7Fh. And the upper bit of FF55 indicates the
Transfer Mode:

Bit7=0 - General Purpose DMA
When using this transfer method, all data is transferred at once. The
execution of the program is halted until the transfer has completed. Note that
the General Purpose DMA blindly attempts to copy the data, even if the LCD
controller is currently accessing VRAM. So General Purpose DMA should be used
only if the Display is disabled, or during V-Blank, or (for rather short
blocks) during H-Blank.
The execution of the program continues when the transfer has been completed,
and FF55 then contains a value if FFh.

Bit7=1 - H-Blank DMA
The H-Blank DMA transfers 10h bytes of data during each H-Blank, ie. at
LY=0-143, no data is transferred during V-Blank (LY=144-153), but the transfer
will then continue at LY=00. The execution of the program is halted during the
separate transfers, but the program execution continues during the 'spaces'
between each data block.
Note that the program may not change the Destination VRAM bank (FF4F), or the
Source ROM/RAM bank (in case data is transferred from bankable memory) until
the transfer has completed!
Reading from Register FF55 returns the remaining length (divided by 10h, minus
1), a value of 0FFh indicates that the transfer has completed. It is also
possible to terminate an active H-Blank transfer by writing zero to Bit 7 of
FF55. In that case reading from FF55 may return any value for the lower 7
bits, but Bit 7 will be read as "1".

Confirming if the DMA Transfer is Active
Reading Bit 7 of FF55 can be used to confirm if the DMA transfer is active
(1=Not Active, 0=Active). This works under any circumstances - after
completion of General Purpose, or H-Blank Transfer, and after manually
terminating a H-Blank Transfer.

Transfer Timings
In both Normal Speed and Double Speed Mode it takes about 8us to transfer a
block of 10h bytes. That are 8 cycles in Normal Speed Mode, and 16 'fast'
cycles in Double Speed Mode.
Older MBC controllers (like MBC1-4) and slower ROMs are not guaranteed to
support General Purpose or H-Blank DMA, that's because there are always 2
bytes transferred per microsecond (even if the itself program runs it Normal
Speed Mode).

VRAM Tile Data
--------------

Tile Data is stored in VRAM at addresses 8000h-97FFh, this area defines the
Bitmaps for 192 Tiles. In CGB Mode 384 Tiles can be defined, because memory at
0:8000h-97FFh and at 1:8000h-97FFh is used.

Each tile is sized 8x8 pixels and has a color depth of 4 colors/gray shades.
Tiles can be displayed as part of the Background/Window map, and/or as OAM
tiles (foreground sprites). Note that foreground sprites may have only 3
colors, because color 0 is transparent.

As it was said before, there are two Tile Pattern Tables at $8000-8FFF and at
$8800-97FF. The first one can be used for sprites and the background. Its
tiles are numbered from 0 to 255. The second table can be used for the
background and the window display and its tiles are numbered from -128 to 127.

Each Tile occupies 16 bytes, where each 2 bytes represent a line:
  Byte 0-1  First Line (Upper 8 pixels)
  Byte 2-3  Next Line
  etc.
For each line, the first byte defines the least significant bits of the color
numbers for each pixel, and the second byte defines the upper bits of the
color numbers. In either case, Bit 7 is the leftmost pixel, and Bit 0 the
rightmost.

So, each pixel is having a color number in range from 0-3. The color numbers
are translated into real colors (or gray shades) depending on the current
palettes. The palettes are defined through registers FF47-FF49 (Non CGB Mode),
and FF68-FF6B (CGB Mode).

VRAM Background Maps
--------------------

The gameboy contains two 32x32 tile background maps in VRAM at addresses
9800h-9BFFh and 9C00h-9FFFh. Each can be used either to display "normal"
background, or "window" background.

BG Map Tile Numbers
An area of VRAM known as Background Tile Map contains the numbers of tiles to
be displayed. It is organized as 32 rows of 32 bytes each. Each byte contains
a number of a tile to be displayed. Tile patterns are taken from the Tile Data
Table located either at $8000-8FFF or $8800-97FF. In the first case, patterns
are numbered with unsigned numbers from 0 to 255 (i.e. pattern #0 lies at
address $8000). In the second case, patterns have signed numbers from -128 to
127 (i.e. pattern #0 lies at address $9000). The Tile Data Table address for
the background can be selected via LCDC register.

BG Map Attributes (CGB Mode only)
In CGB Mode, an additional map of 32x32 bytes is stored in VRAM Bank 1 (each
byte defines attributes for the corresponding tile-number map entry in VRAM
Bank 0):
  Bit 0-2  Background Palette number  (BGP0-7)
  Bit 3    Tile VRAM Bank number      (0=Bank 0, 1=Bank 1)
  Bit 4    Not used
  Bit 5    Horizontal Flip            (0=Normal, 1=Mirror horizontally)
  Bit 6    Vertical Flip              (0=Normal, 1=Mirror vertically)
  Bit 7    BG-to-OAM Priority         (0=Use OAM priority bit, 1=BG Priority)
When Bit 7 is set, the corresponding BG tile will have priority above all OBJs
(regardless of the priority bits in OAM memory). There's also an Master
Priority flag in LCDC register Bit 0 which overrides all other priority bits
when cleared.

As one background tile has a size of 8x8 pixels, the BG maps may hold a
picture of 256x256 pixels, an area of 160x144 pixels of this picture can be
displayed on the LCD screen.

Normal Background (BG)
The SCY and SCX registers can be used to scroll the background, allowing to
select the origin of the visible 160x144 pixel area within the total 256x256
pixel background map. Background wraps around the screen (i.e. when part of it
goes off the screen, it appears on the opposite side.)

The Window
Besides background, there is also a "window" overlaying the background. The
window is not scrollable i.e. it is always displayed starting from its left
upper corner. The location of a window on the screen can be adjusted via WX
and WY registers. Screen coordinates of the top left corner of a window are
WX-7,WY. The tiles for the window are stored in the Tile Data Table. Both the
Background and the window share the same Tile Data Table.

Both background and window can be disabled or enabled separately via bits in
the LCDC register.

VRAM Sprite Attribute Table (OAM)
---------------------------------

GameBoy video controller can display up to 40 sprites either in 8x8 or in 8x16
pixels. Because of a limitation of hardware, only ten sprites can be displayed
per scan line. Sprite patterns have the same format as BG tiles, but they are
taken from the Sprite Pattern Table located at $8000-8FFF and have unsigned
numbering.

Sprite attributes reside in the Sprite Attribute Table (OAM - Object Attribute
Memory) at $FE00-FE9F. Each of the 40 entries consists of four bytes with the
following meanings:

Byte0 - Y Position
Specifies the sprites vertical position on the screen (minus 16).
An offscreen value (for example, Y=0 or Y>=160) hides the sprite.

Byte1 - X Position
Specifies the sprites horizontal position on the screen (minus 8).
An offscreen value (X=0 or X>=168) hides the sprite, but the sprite
still affects the priority ordering - a better way to hide a sprite is to set
its Y-coordinate offscreen.

Byte2 - Tile/Pattern Number
Specifies the sprites Tile Number (00-FF). This (unsigned) value selects a
tile from memory at 8000h-8FFFh. In CGB Mode this could be either in VRAM Bank
0 or 1, depending on Bit 3 of the following byte.
In 8x16 mode, the lower bit of the tile number is ignored. Ie. the upper 8x8
tile is "NN AND FEh", and the lower 8x8 tile is "NN OR 01h".

Byte3 - Attributes/Flags:
  Bit7   OBJ-to-BG Priority (0=OBJ Above BG, 1=OBJ Behind BG color 1-3)
         (Used for both BG and Window. BG color 0 is always behind OBJ)
  Bit6   Y flip          (0=Normal, 1=Vertically mirrored)
  Bit5   X flip          (0=Normal, 1=Horizontally mirrored)
  Bit4   Palette number  **Non CGB Mode Only** (0=OBP0, 1=OBP1)
  Bit3   Tile VRAM-Bank  **CGB Mode Only**     (0=Bank 0, 1=Bank 1)
  Bit2-0 Palette number  **CGB Mode Only**     (OBP0-7)

Sprite Priorities and Conflicts
When sprites with different x coordinate values overlap, the one with the
smaller x coordinate (closer to the left) will have priority and appear above
any others. This applies in Non CGB Mode only.
When sprites with the same x coordinate values overlap, they have priority
according to table ordering. (i.e. $FE00 - highest, $FE04 - next highest,
etc.) In CGB Mode priorities are always assigned like this.

Only 10 sprites can be displayed on any one line. When this limit is exceeded,
the lower priority sprites (priorities listed above) won't be displayed. To
keep unused sprites from affecting onscreen sprites set their Y coordinate to
Y=0 or Y=>144+16. Just setting the X coordinate to X=0 or X=>160+8 on a sprite
will hide it but it will still affect other sprites sharing the same lines.

Writing Data to OAM Memory
The recommened method is to write the data to normal RAM first, and to copy
that RAM to OAM by using the DMA transfer function, initiated through DMA
register (FF46).
Beside for that, it is also possible to write data directly to the OAM area by
using normal LD commands, this works only during the H-Blank and V-Blank
periods. The current state of the LCD controller can be read out from the STAT
register (FF41).

Accessing VRAM and OAM
----------------------

CAUTION
When the LCD Controller is drawing the screen it is directly reading from
Video Memory (VRAM) and from the Sprite Attribute Table (OAM). During these
periods the Gameboy CPU may not access the VRAM and OAM. That means, any
attempts to write to VRAM/OAM are ignored (the data remains unchanged). And
any attempts to read from VRAM/OAM will return undefined data (typically a
value of FFh).

For this reason the program should verify if VRAM/OAM is accessable before
actually reading or writing to it. This is usually done by reading the Mode
Bits from the STAT Register (FF41). When doing this (as described in the
examples below) you should take care that no interrupts occur between the wait
loops and the following memory access - the memory is guaranted to be
accessable only for a few cycles directly after the wait loops have completed.

VRAM (memory at 8000h-9FFFh) is accessable during Mode 0-2
  Mode 0 - H-Blank Period,
  Mode 1 - V-Blank Period, and
  Mode 2 - Searching OAM Period
A typical procedure that waits for accessibility of VRAM would be:
  ld   hl,0FF41h    ;-STAT Register
 @@wait:            ;\
  bit  1,(hl)       ; Wait until Mode is 0 or 1
  jr   nz,@@wait    ;/
Even if the procedure gets executed at the  of Mode 0 or 1, it is still
proof to assume that VRAM can be accessed for a few more cycles because in
either case the following period is Mode 2 which allows access to VRAM either.
In CGB Mode an alternate method to write data to VRAM is to use the HDMA
Function (FF51-FF55).

OAM (memory at FE00h-FE9Fh) is accessable during Mode 0-1
  Mode 0 - H-Blank Period, and
  Mode 1 - V-Blank Period
Beside for that, OAM can be accessed at any time by using the DMA Function
(FF46). When directly reading or writing to OAM, a typical procedure that
waits for accessibilty or OAM Memory would be:
  ld   hl,0FF41h    ;-STAT Register
 @@wait1:           ;\
  bit  1,(hl)       ; Wait until Mode is -NOT- 0 or 1
  jr   z,@@wait1    ;/
 @@wait2:           ;\
  bit  1,(hl)       ; Wait until Mode 0 or 1 -BEGINS-
  jr   nz,@@wait2   ;/
The two wait loops ensure that Mode 0 or 1 will last for a few clock cycles
after completion of the procedure. In V-Blank period it might be recommended
to skip the whole procedure - and in most cases using the above mentioned DMA
function would be more recommended anyways.

Note
When the display is disabled, both VRAM and OAM are accessable at any time.
The downside is that the screen is blank (white) during this period, so that
disabling the display would be recommended only during initialization.

Sound Controller
----------------

--> Sound Overview
--> Sound Channel 1 - Tone & Sweep
--> Sound Channel 2 - Tone
--> Sound Channel 3 - Wave Output
--> Sound Channel 4 - Noise
--> Sound Control Registers

Sound Overview
--------------

There are two sound channels connected to the output terminals SO1 and SO2.
There is also a input terminal Vin connected to the cartridge. It can be
routed to either of both output terminals. GameBoy circuitry allows producing
sound in four different ways:

   Quadrangular wave patterns with sweep and envelope functions.
   Quadrangular wave patterns with envelope functions.
   Voluntary wave patterns from wave RAM.
   White noise with an envelope function.

These four sounds can be controlled independantly and then mixed separately
for each of the output terminals.

Sound registers may be set at all times while producing sound.

(Sounds will have a 2.4% higher frequency on Super GB.)

Sound Channel 1 - Tone & Sweep
------------------------------

FF10 - NR10 - Channel 1 Sweep register (R/W)
  Bit 6-4 - Sweep Time
  Bit 3   - Sweep Increase/Decrease
             0: Addition    (frequency increases)
             1: Subtraction (frequency decreases)
  Bit 2-0 - Number of sweep shift (n: 0-7)
Sweep Time:
  000: sweep off - no freq change
  001: 7.8 ms  (1/128Hz)
  010: 15.6 ms (2/128Hz)
  011: 23.4 ms (3/128Hz)
  100: 31.3 ms (4/128Hz)
  101: 39.1 ms (5/128Hz)
  110: 46.9 ms (6/128Hz)
  111: 54.7 ms (7/128Hz)

The change of frequency (NR13,NR14) at each shift is calculated by the
following formula where X(0) is initial freq & X(t-1) is last freq:
  X(t) = X(t-1) +/- X(t-1)/2^n

FF11 - NR11 - Channel 1 Sound length/Wave pattern duty (R/W)
  Bit 7-6 - Wave Pattern Duty (Read/Write)
  Bit 5-0 - Sound length data (Write Only) (t1: 0-63)
Wave Duty:
  00: 12.5% ( _-------_-------_------- )
  01: 25%   ( __------__------__------ )
  10: 50%   ( ____----____----____---- ) (normal)
  11: 75%   ( ______--______--______-- )
Sound Length = (64-t1)*(1/256) seconds
The Length value is used only if Bit 6 in NR14 is set.

FF12 - NR12 - Channel 1 Volume Envelope (R/W)
  Bit 7-4 - Initial Volume of envelope (0-0Fh) (0=No Sound)
  Bit 3   - Envelope Direction (0=Decrease, 1=Increase)
  Bit 2-0 - Number of envelope sweep (n: 0-7)
            (If zero, stop envelope operation.)
Length of 1 step = n*(1/64) seconds

FF13 - NR13 - Channel 1 Frequency lo (Write Only)

Lower 8 bits of 11 bit frequency (x).
Next 3 bit are in NR14 ($FF14)

FF14 - NR14 - Channel 1 Frequency hi (R/W)
  Bit 7   - Initial (1=Restart Sound)     (Write Only)
  Bit 6   - Counter/consecutive selection (Read/Write)
            (1=Stop output when length in NR11 expires)
  Bit 2-0 - Frequency's higher 3 bits (x) (Write Only)
Frequency = 131072/(2048-x) Hz

Sound Channel 2 - Tone
----------------------

This sound channel works exactly as channel 1, except that it doesn't have a
Tone Envelope/Sweep Register.

FF16 - NR21 - Channel 2 Sound Length/Wave Pattern Duty (R/W)
  Bit 7-6 - Wave Pattern Duty (Read/Write)
  Bit 5-0 - Sound length data (Write Only) (t1: 0-63)
Wave Duty:
  00: 12.5% ( _-------_-------_------- )
  01: 25%   ( __------__------__------ )
  10: 50%   ( ____----____----____---- ) (normal)
  11: 75%   ( ______--______--______-- )
Sound Length = (64-t1)*(1/256) seconds
The Length value is used only if Bit 6 in NR24 is set.

FF17 - NR22 - Channel 2 Volume Envelope (R/W)
  Bit 7-4 - Initial Volume of envelope (0-0Fh) (0=No Sound)
  Bit 3   - Envelope Direction (0=Decrease, 1=Increase)
  Bit 2-0 - Number of envelope sweep (n: 0-7)
            (If zero, stop envelope operation.)
Length of 1 step = n*(1/64) seconds

FF18 - NR23 - Channel 2 Frequency lo data (W)
Frequency's lower 8 bits of 11 bit data (x).
Next 3 bits are in NR24 ($FF19).

FF19 - NR24 - Channel 2 Frequency hi data (R/W)
  Bit 7   - Initial (1=Restart Sound)     (Write Only)
  Bit 6   - Counter/consecutive selection (Read/Write)
            (1=Stop output when length in NR21 expires)
  Bit 2-0 - Frequency's higher 3 bits (x) (Write Only)
Frequency = 131072/(2048-x) Hz

Sound Channel 3 - Wave Output
-----------------------------

This channel can be used to output digital sound, the length of the sample
buffer (Wave RAM) is limited to 32 digits. This sound channel can be also used
to output normal tones when initializing the Wave RAM by a square wave. This
channel doesn't have a volume envelope register.

FF1A - NR30 - Channel 3 Sound on/off (R/W)
  Bit 7 - Sound Channel 3 Off  (0=Stop, 1=Playback)  (Read/Write)

FF1B - NR31 - Channel 3 Sound Length
  Bit 7-0 - Sound length (t1: 0 - 255)
Sound Length = (256-t1)*(1/256) seconds
This value is used only if Bit 6 in NR34 is set.

FF1C - NR32 - Channel 3 Select output level (R/W)
  Bit 6-5 - Select output level (Read/Write)
Possible Output levels are:
  0: Mute (No sound)
  1: 100% Volume (Produce Wave Pattern RAM Data as it is)
  2:  50% Volume (Produce Wave Pattern RAM data shifted once to the right)
  3:  25% Volume (Produce Wave Pattern RAM data shifted twice to the right)

FF1D - NR33 - Channel 3 Frequency's lower data (W)
Lower 8 bits of an 11 bit frequency (x).

FF1E - NR34 - Channel 3 Frequency's higher data (R/W)
  Bit 7   - Initial (1=Restart Sound)     (Write Only)
  Bit 6   - Counter/consecutive selection (Read/Write)
            (1=Stop output when length in NR31 expires)
  Bit 2-0 - Frequency's higher 3 bits (x) (Write Only)
Frequency  =  4194304/(64*(2048-x)) Hz  =  65536/(2048-x) Hz

FF30-FF3F - Wave Pattern RAM
Contents - Waveform storage for arbitrary sound data

This storage area holds 32 4-bit samples  that are played back upper 4 bits
first.

Sound Channel 4 - Noise
-----------------------

This channel is used to output white noise. This is done by randomly switching
the amplitude between high and low at a given frequency. Depending on the
frequency the noise will appear 'harder' or 'softer'.

It is also possible to influence the function of the random generator, so the
that the output becomes more regular, resulting in a limited ability to output
Tone instead of Noise.

FF20 - NR41 - Channel 4 Sound Length (R/W)
  Bit 5-0 - Sound length data (t1: 0-63)
Sound Length = (64-t1)*(1/256) seconds
The Length value is used only if Bit 6 in NR44 is set.

FF21 - NR42 - Channel 4 Volume Envelope (R/W)
  Bit 7-4 - Initial Volume of envelope (0-0Fh) (0=No Sound)
  Bit 3   - Envelope Direction (0=Decrease, 1=Increase)
  Bit 2-0 - Number of envelope sweep (n: 0-7)
            (If zero, stop envelope operation.)
Length of 1 step = n*(1/64) seconds

FF22 - NR43 - Channel 4 Polynomial Counter (R/W)
The amplitude is randomly switched between high and low at the given
frequency. A higher frequency will make the noise to appear 'softer'.
When Bit 3 is set, the output will become more regular, and some frequencies
will sound more like Tone than Noise.
  Bit 7-4 - Shift Clock Frequency (s)
  Bit 3   - Counter Step/Width (0=15 bits, 1=7 bits)
  Bit 2-0 - Dividing Ratio of Frequencies (r)
Frequency = 524288 Hz / r / 2^(s+1)     ;For r=0 assume r=0.5 instead

FF23 - NR44 - Channel 4 Counter/consecutive; Inital (R/W)
  Bit 7   - Initial (1=Restart Sound)     (Write Only)
  Bit 6   - Counter/consecutive selection (Read/Write)
            (1=Stop output when length in NR41 expires)

Sound Control Registers
-----------------------

FF24 - NR50 - Channel control / ON-OFF / Volume (R/W)
The volume bits specify the "Master Volume" for Left/Right sound output.
  Bit 7   - Output Vin to SO2 terminal (1=Enable)
  Bit 6-4 - SO2 output level (volume)  (0-7)
  Bit 3   - Output Vin to SO1 terminal (1=Enable)
  Bit 2-0 - SO1 output level (volume)  (0-7)
The Vin signal is received from the game cartridge bus, allowing external
hardware in the cartridge to supply a fifth sound channel, additionally to the
gameboys internal four channels. As far as I know this feature isn't used by
any existing games.

FF25 - NR51 - Selection of Sound output terminal (R/W)
  Bit 7 - Output sound 4 to SO2 terminal
  Bit 6 - Output sound 3 to SO2 terminal
  Bit 5 - Output sound 2 to SO2 terminal
  Bit 4 - Output sound 1 to SO2 terminal
  Bit 3 - Output sound 4 to SO1 terminal
  Bit 2 - Output sound 3 to SO1 terminal
  Bit 1 - Output sound 2 to SO1 terminal
  Bit 0 - Output sound 1 to SO1 terminal

FF26 - NR52 - Sound on/off
If your GB programs don't use sound then write 00h to this register to save
16% or more on GB power consumption. Disabeling the sound controller by
clearing Bit 7 destroys the contents of all sound registers. Also, it is not
possible to access any sound registers (execpt FF26) while the sound
controller is disabled.
  Bit 7 - All sound on/off  (0: stop all sound circuits) (Read/Write)
  Bit 3 - Sound 4 ON flag (Read Only)
  Bit 2 - Sound 3 ON flag (Read Only)
  Bit 1 - Sound 2 ON flag (Read Only)
  Bit 0 - Sound 1 ON flag (Read Only)
Bits 0-3 of this register are read only status bits, writing to these bits
does NOT enable/disable sound. The flags get set when sound output is
restarted by setting the Initial flag (Bit 7 in NR14-NR44), the flag remains
set until the sound length has expired (if enabled). A volume envelopes which
has decreased to zero volume will NOT cause the sound flag to go off.

Joypad Input
------------

FF00 - P1/JOYP - Joypad (R/W)
The eight gameboy buttons/direction keys are arranged in form of a 2x4 matrix.
Select either button or direction keys by writing to this register, then
read-out bit 0-3.
  Bit 7 - Not used
  Bit 6 - Not used
  Bit 5 - P15 Select Button Keys      (0=Select)
  Bit 4 - P14 Select Direction Keys   (0=Select)
  Bit 3 - P13 Input Down  or Start    (0=Pressed) (Read Only)
  Bit 2 - P12 Input Up    or Select   (0=Pressed) (Read Only)
  Bit 1 - P11 Input Left  or Button B (0=Pressed) (Read Only)
  Bit 0 - P10 Input Right or Button A (0=Pressed) (Read Only)
Note: Most programs are repeatedly reading from this port several times (the
first reads used as short delay, allowing the inputs to stabilize, and only
the value from the last read actually used).

Usage in SGB software
Beside for normal joypad input, SGB games mis-use the joypad register to
output SGB command packets to the SNES, also, SGB programs may read out
gamepad states from up to four different joypads which can be connected to the
SNES.
See SGB description for details.

INT 60 - Joypad Interrupt
Joypad interrupt is requested when any of the above Input lines changes from
High to Low. Generally this should happen when a key becomes pressed (provided
that the button/direction key is enabled by above Bit4/5), however, because of
switch bounce, one or more High to Low transitions are usually produced both
when pressing or releasing a key.

Using the Joypad Interrupt
It's more or less useless for programmers, even when selecting both buttons
and direction keys simultaneously it still cannot recognize all keystrokes,
because in that case a bit might be already held low by a button key, and
pressing the corresponding direction key would thus cause no difference. The
only meaningful purpose of the keystroke interrupt would be to terminate STOP
(low power) standby state.
Also, the joypad interrupt does not appear to work with CGB and GBA hardware
(the STOP function can be still terminated by joypad keystrokes though).

Serial Data Transfer (Link Cable)
---------------------------------

FF01 - SB - Serial transfer data (R/W)
8 Bits of data to be read/written

FF02 - SC - Serial Transfer Control  (R/W)
  Bit 7 - Transfer Start Flag (0=No Transfer, 1=Start)
  Bit 1 - Clock Speed (0=Normal, 1=Fast) ** CGB Mode Only **
  Bit 0 - Shift Clock (0=External Clock, 1=Internal Clock)
The clock signal specifies the rate at which the eight data bits in SB (FF01)
are transferred. When the gameboy is communicating with another gameboy (or
other computer) then either one must supply internal clock, and the other one
must use external clock.

Internal Clock
In Non-CGB Mode the gameboy supplies an internal clock of 8192Hz only
(allowing to transfer about 1 KByte per second). In CGB Mode four internal
clock rates are available, depending on Bit 1 of the SC register, and on
whether the CGB Double Speed Mode is used:
    8192Hz -  1KB/s - Bit 1 cleared, Normal
   16384Hz -  2KB/s - Bit 1 cleared, Double Speed Mode
  262144Hz - 32KB/s - Bit 1 set,     Normal
  524288Hz - 64KB/s - Bit 1 set,     Double Speed Mode

External Clock
The external clock is typically supplied by another gameboy, but might be
supplied by another computer (for example if connected to a PCs parallel
port), in that case the external clock may have any speed. Even the
old/monochrome gameboy is reported to recognizes external clocks of up to
500KHz. And there is no limitiation into the other direction - even when
suppling an external clock speed of "1 bit per month", then the gameboy will
still eagerly wait for the next bit(s) to be transferred. It isn't required
that the clock pulses are sent at an regular interval either.

Timeouts
When using external clock then the transfer will not complete until the last
bit is received. In case that the second gameboy isn't supplying a clock
signal, if it gets turned off, or if there is no second gameboy connected at
all) then transfer will never complete. For this reason the transfer procedure
should use a timeout counter, and abort the communication if no response has
been received during the timeout interval.

Delays and Synchronization
The gameboy that is using internal clock should always execute a small delay
between each transfer, in order to ensure that the opponent gameboy has enough
time to prepare itself for the next transfer, ie. the gameboy with external
clock must have set its transfer start bit before the gameboy with internal
clock starts the transfer. Alternately, the two gameboys could switch between
internal and external clock for each transferred byte to ensure
synchronization.

Transfer is initiated by setting the Transfer Start Flag. This bit is
automatically set to 0 at the end of Transfer. Reading this bit can be used to
determine if the transfer is still active.

INT 58 - Serial Interrupt
When the transfer has completed (ie. after sending/receiving 8 bits, if any)
then an interrupt is requested by setting Bit 3 of the IF Register (FF0F).
When that interrupt is enabled, then the Serial Interrupt vector at 0058 is
called.

XXXXXX...

Transmitting and receiving serial data is done simultaneously. The received
data is automatically stored in SB.

The serial I/O port on the Gameboy is a very simple setup and is crude
compared to standard RS-232 (IBM-PC) or RS-485 (Macintosh) serial ports. There
are no start or stop bits.

During a transfer, a byte is shifted in at the same time that a byte is
shifted out. The rate of the shift is determined by whether the clock source
is internal or external.
The most significant bit is shifted in and out first.

When the internal clock is selected, it drives the clock pin on the game link
port and it stays high when not used. During a transfer it will go low eight
times to clock in/out each bit.

The state of the last bit shifted out determines the state of the output line
until another transfer takes place.

If a serial transfer with internal clock is performed and no external GameBoy
is present, a value of $FF will be received in the transfer.

The following code causes $75 to be shifted out the serial port and a byte to
be shifted into $FF01:

    ld   a,$75
    ld  ($FF01),a
    ld   a,$81
    ld  ($FF02),a

Timer and Divider Registers
---------------------------

FF04 - DIV - Divider Register (R/W)
This register is incremented at rate of 16384Hz (~16779Hz on SGB). In CGB
Double Speed Mode it is incremented twice as fast, ie. at 32768Hz. Writing any
value to this register resets it to 00h.

FF05 - TIMA - Timer counter (R/W)
This timer is incremented by a clock frequency specified by the TAC register
($FF07). When the value overflows (gets bigger than FFh) then it will be reset
to the value specified in TMA (FF06), and an interrupt will be requested, as
described below.

FF06 - TMA - Timer Modulo (R/W)
When the TIMA overflows, this data will be loaded.

FF07 - TAC - Timer Control (R/W)
  Bit 2    - Timer Stop  (0=Stop, 1=Start)
  Bits 1-0 - Input Clock Select
             00:   4096 Hz    (~4194 Hz SGB)
             01: 262144 Hz  (~268400 Hz SGB)
             10:  65536 Hz   (~67110 Hz SGB)
             11:  16384 Hz   (~16780 Hz SGB)

INT 50 - Timer Interrupt
Each time when the timer overflows (ie. when TIMA gets bigger than FFh), then
an interrupt is requested by setting Bit 2 in the IF Register (FF0F). When
that interrupt is enabled, then the CPU will execute it by calling the timer
interrupt vector at 0050h.

Note
The above described Timer is the built-in timer in the gameboy. It has nothing
to do with the MBC3s battery buffered Real Time Clock - that's a completely
different thing, described in the chapter about Memory Banking Controllers.

Interrupts
----------

IME - Interrupt Master Enable Flag (Write Only)
  0 - Disable all Interrupts
  1 - Enable all Interrupts that are enabled in IE Register (FFFF)
The IME flag is used to disable all interrupts, overriding any enabled bits in
the IE Register. It isn't possible to access the IME flag by using a I/O
address, instead IME is accessed directly from the CPU, by the following
opcodes/operations:
  EI     ;Enable Interrupts  (ie. IME=1)
  DI     ;Disable Interrupts (ie. IME=0)
  RETI   ;Enable Ints & Return (same as the opcode combination EI, RET)
    ;Disable Ints & Call to Interrupt Vector
Whereas  means the operation which is automatically executed by the CPU
when it executes an interrupt.

FFFF - IE - Interrupt Enable (R/W)
  Bit 0: V-Blank  Interrupt Enable  (INT 40h)  (1=Enable)
  Bit 1: LCD STAT Interrupt Enable  (INT 48h)  (1=Enable)
  Bit 2: Timer    Interrupt Enable  (INT 50h)  (1=Enable)
  Bit 3: Serial   Interrupt Enable  (INT 58h)  (1=Enable)
  Bit 4: Joypad   Interrupt Enable  (INT 60h)  (1=Enable)

FF0F - IF - Interrupt Flag (R/W)
  Bit 0: V-Blank  Interrupt Request (INT 40h)  (1=Request)
  Bit 1: LCD STAT Interrupt Request (INT 48h)  (1=Request)
  Bit 2: Timer    Interrupt Request (INT 50h)  (1=Request)
  Bit 3: Serial   Interrupt Request (INT 58h)  (1=Request)
  Bit 4: Joypad   Interrupt Request (INT 60h)  (1=Request)
When an interrupt signal changes from low to high, then the corresponding bit
in the IF register becomes set. For example, Bit 0 becomes set when the LCD
controller enters into the V-Blank period.

Interrupt Requests
Any set bits in the IF register are only  an interrupt to be
executed. The actual  happens only if both the IME flag, and the
corresponding bit in the IE register are set, otherwise the interrupt 'waits'
until both IME and IE allow its execution.

Interrupt Execution
When an interrupt gets executed, the corresponding bit in the IF register
becomes automatically reset by the CPU, and the IME flag becomes cleared
(disabeling any further interrupts until the program re-enables the
interrupts, typically by using the RETI instruction), and the corresponding
Interrupt Vector (that are the addresses in range 0040h-0060h, as shown in IE
and IF register decriptions above) becomes called.

Manually Requesting/Discarding Interrupts
As the CPU automatically sets and cleares the bits in the IF register it is
usually not required to write to the IF register. However, the user may still
do that in order to manually request (or discard) interrupts. As for real
interrupts, a manually requested interrupt isn't executed unless/until IME and
IE allow its execution.

Interrupt Priorities
In the following three situations it might happen that more than 1 bit in the
IF register are set, requesting more than one interrupt at once:
  1) More than one interrupt signal changed from Low
     to High at the same time.
  2) Several interrupts have been requested during a
     time in which IME/IE didn't allow these interrupts
     to be executed directly.
  3) The user has written a value with several "1" bits
     (for example 1Fh) to the IF register.
Provided that IME and IE allow the execution of more than one of the requested
interrupts, then the interrupt with the highest priority becomes executed
first. The priorities are ordered as the bits in the IE and IF registers, Bit
0 (V-Blank) having the highest priority, and Bit 4 (Joypad) having the lowest
priority.

Nested Interrupts
The CPU automatically disables all other interrupts by setting IME=0 when it
executes an interrupt. Usually IME remains zero until the interrupt procedure
returns (and sets IME=1 by the RETI instruction). However, if you want any
other interrupts of lower or higher (or same) priority to be allowed to be
executed from inside of the interrupt procedure, then you can place an EI
instruction into the interrupt procedure.

CGB Registers
-------------

Forward
This chapter describes only CGB (Color Gameboy) registers that didn't fit into
normal categories - most CGB registers are described in the chapter about
Video Display (Color Palettes, VRAM Bank, VRAM DMA Transfers, and changed
meaning of Bit 0 of LCDC Control register). Also, a changed bit is noted in
the chapter about the Serial/Link port.

Unlocking CGB functions
When using any CGB registers (including those in the Video/Link chapters), you
must first unlock CGB features by changing byte 0143h in the cartridge header.
Typically use a value of 80h for games which support both CGB and monochrome
gameboys, and C0h for games which work on CGBs only. Otherwise, the CGB will
operate in monochrome "Non CGB" compatibility mode.

Detecting CGB (and GBA) functions
CGB hardware can be detected by examing the CPU accumulator (A-register)
directly after startup. A value of 11h indicates CGB (or GBA) hardware, if so,
CGB functions can be used (if unlocked, see above).
When A=11h, you may also examine Bit 0 of the CPUs B-Register to separate
between CGB (bit cleared) and GBA (bit set), by that detection it is possible
to use 'repaired' color palette data matching for GBA displays.

FF4D - KEY1 - CGB Mode Only - Prepare Speed Switch
  Bit 7: Current Speed     (0=Normal, 1=Double) (Read Only)
  Bit 0: Prepare Speed Switch (0=No, 1=Prepare) (Read/Write)
This register is used to prepare the gameboy to switch between CGB Double
Speed Mode and Normal Speed Mode. The actual speed switch is performed by
executing a STOP command after Bit 0 has been set. After that Bit 0 will be
cleared automatically, and the gameboy will operate at the 'other' speed. The
recommended speed switching procedure in pseudo code would be:
  IF KEY1_BIT7 <> DESIRED_SPEED THEN
    IE=00H       ;(FFFF)=00h
    JOYP=30H     ;(FF00)=30h
    KEY1=01H     ;(FF4D)=01h
    STOP         ;STOP
  ENDIF
The CGB is operating in Normal Speed Mode when it is turned on. Note that
using the Double Speed Mode increases the power consumption, it would be
recommended to use Single Speed whenever possible. However, the display will
flicker (white) for a moment during speed switches, so this cannot be done
permanentely.
In Double Speed Mode the following will operate twice as fast as normal:
  The CPU (2.10 MHz, 1 Cycle = approx. 0.5us)
  Timer and Divider Registers
  Serial Port (Link Cable)
  DMA Transfer to OAM
And the following will keep operating as usual:
  LCD Video Controller
  HDMA Transfer to VRAM
  All Sound Timings and Frequencies

FF56 - RP - CGB Mode Only - Infrared Communications Port
This register allows to input and output data through the CGBs built-in
Infrared Port. When reading data, bit 6 and 7 must be set (and obviously Bit 0
must be cleared - if you don't want to receive your own gameboys IR signal).
After sending or receiving data you should reset the register to 00h to reduce
battery power consumption again.
  Bit 0:   Write Data   (0=LED Off, 1=LED On)             (Read/Write)
  Bit 1:   Read Data    (0=Receiving IR Signal, 1=Normal) (Read Only)
  Bit 6-7: Data Read Enable (0=Disable, 3=Enable)         (Read/Write)
Note that the receiver will adapt itself to the normal level of IR pollution
in the air, so if you would send a LED ON signal for a longer period, then the
receiver would treat that as normal (=OFF) after a while. For example, a
Philips TV Remote Control sends a series of 32 LED ON/OFF pulses (length 10us
ON, 17.5us OFF each) instead of a permanent 880us LED ON signal.
Even though being generally CGB compatible, the GBA does not include an
infra-red port.

FF70 - SVBK - CGB Mode Only - WRAM Bank
In CGB Mode 32 KBytes internal RAM are available. This memory is divided into
8 banks of 4 KBytes each. Bank 0 is always available in memory at C000-CFFF,
Bank 1-7 can be selected into the address space at D000-DFFF.
  Bit 0-2  Select WRAM Bank (Read/Write)
Writing a value of 01h-07h will select Bank 1-7, writing a value of 00h will
select Bank 1 either.

FF6C - Undocumented (FEh) - Bit 0   (Read/Write) - CGB Mode Only
FF72 - Undocumented (00h) - Bit 0-7 (Read/Write)
FF73 - Undocumented (00h) - Bit 0-7 (Read/Write)
FF74 - Undocumented (00h) - Bit 0-7 (Read/Write) - CGB Mode Only
FF75 - Undocumented (8Fh) - Bit 4-6 (Read/Write)
FF76 - Undocumented (00h) - Always 00h (Read Only)
FF77 - Undocumented (00h) - Always 00h (Read Only)
These are undocumented CGB Registers. The numbers in brackets () indicate the
initial values. Purpose of these registers is unknown (if any). Registers FF6C
and FF74 are always FFh if the CGB is in Non CGB Mode.

SGB Functions
-------------

General Information
--> SGB Description
--> SGB Unlocking and Detecting SGB Functions
--> SGB Command Packet Transfers
--> SGB VRAM Transfers
--> SGB Command Summary
--> SGB Color Palettes Overview

SGB Commands
--> SGB Palette Commands
--> SGB Color Attribute Commands
--> SGB Sound Functions
--> SGB System Control Commands
--> SGB Multiplayer Command
--> SGB Border and OBJ Commands

SGB Description
---------------

General Description
Basically, the SGB (Super Gameboy) is an adapter cartridge that allows to play
gameboy games on a SNES (Super Nintendo Entertainment System) gaming console.
In detail, you plug the gameboy cartridge into the SGB cartridge, then plug
the SGB cartridge into the SNES, and then connect the SNES to your TV Set. In
result, games can be played and viewed on the TV Set, and are controlled by
using the SNES joypad(s).

More Technical Description
The SGB cartridge just contains a normal gameboy CPU and normal gameboy video
controller. Normally the video signal from this controller would be sent to
the LCD screen, however, in this special case the SNES read out the video
signal and displays it on the TV set by using a special SNES BIOS ROM which is
located in the SGB cartridge. Also, normal gameboy sound output is forwared to
the SNES and output to the TV Set, vice versa, joypad input is forwared from
the SNES controller(s) to the gameboy joypad inputs.

Normal Monochrome Games
Any gameboy games which have been designed for normal monochrome handheld
gameboys will work with the SGB hardware as well. The SGB will apply a four
color palette to these games by replacing the normal four grayshades. The
160x144 pixel gamescreen is displayed in the middle of the 256x224 pixel SNES
screen (the unused area is filled by a screen border bitmap). The user may
access built-in menues, allowing to change color palette data, to select
between several pre-defined borders, etc.

Games that have been designed to support SGB functions may also access the
following additional features:

Colorized Game Screen
There's limited ability to colorize the gamescreen by assigning custom color
palettes to each 20x18 display characters, however, this works mainly for
static display data such like title screens or status bars, the 20x18 color
attribute map is non-scrollable, and it is not possible to assign separate
colors to moveable foreground sprites (OBJs), so that animated screen regions
will be typically restricted to using a single palette of four colors only.

SNES Foreground Sprites
Up to 24 foreground sprites (OBJs) of 8x8 or 16x16 pixels, 16 colors can be
displayed. When replacing (or just overlaying) the normal gameboy OBJs by SNES
OBJs it'd be thus possible to display OBJs with other colors than normal
background area. This method doesn't appear to be very popular, even though it
appears to be quite easy to implement, however, the bottommost character line
of the gamescreen will be masked out because this area is used to transfer OAM
data to the SNES.

The SGB Border
The possibly most popular and most impressive feature is to replace the
default SGB screen border by a custom bitmap which is stored in the game
cartridge.

Multiple Joypads
Up to four joypads can be conected to the SNES, and SGB software may read-out
each of these joypads separately, allowing up to four players to play the same
game simultaneously. Unlike for multiplayer handheld games, this requires only
one game cartridge and only one SGB/SNES, and no link cables are required, the
downside is that all players must share the same display screen.

Sound Functions
Beside for normal gameboy sound, a number of digital sound effects is
pre-defined in the SNES BIOS, these effects may be accessed quite easily.
Programmers whom are familiar with SNES sounds may also access the SNES sound
chip, or use the SNES MIDI engine directly in order to produce other sound
effects or music.

Taking Control of the SNES CPU
Finally, it is possible to write program code or data into SNES memory, and to
execute such program code by using the SNES CPU.

SGB System Clock
Because the SGB is synchronized to the SNES CPU, the gameboy system clock is
directly chained to the SNES system clock. In result, the gameboy CPU, video
controller, timers, and sound frequencies will be all operated approx 2.4%
faster as by normal gameboys.
Basically, this should be no problem, and the game will just run a little bit
faster. However sensitive musicians may notice that sound frequencies are a
bit too high, programs that support SGB functions may avoid this effect by
reducing frequencies of gameboy sounds when having detected SGB hardware.
Also, I think that I've heard that SNES models which use a 50Hz display
refresh rate (rather than 60Hz) are resulting in respectively slower
SGB/gameboy timings ???

SGB Unlocking and Detecting SGB Functions
-----------------------------------------

Cartridge Header
SGB games are required to have a cartridge header with Nintendo and proper
checksum just as normal gameboy games. Also, two special entries must be set
in order to unlock SGB functions:
  146h - SGB Flag - Must be set to 03h for SGB games
  14Bh - Old Licensee Code - Must be set 33h for SGB games
When these entries aren't set, the game will still work just like all
'monochrome' gameboy games, but it cannot access any of the special SGB
functions.

Detecting SGB hardware
The recommended detection method is to send a MLT_REQ command which enables
two (or four) joypads. A normal handheld gameboy will ignore this command, a
SGB will now return incrementing joypad IDs each time when deselecting
keyboard lines (see MLT_REQ description for details).
Now read-out joypad state/IDs several times, and if the ID-numbers are
changing, then it is a SGB (a normal gameboy would typically always return 0Fh
as ID). Finally, when not intending to use more than one joypad, send another
MLT_REQ command in order to re-disable the multi-controller mode.
Detection works regardless of whether and how many joypads are physically
connected to the SNES. However, detection works only when having unlocked SGB
functions in the cartridge header, as described above. 

Separating between SGB and SGB2
It is also possible to separate between SGB and SGB2 models by examining the
inital value of the accumulator (A-register) directly after startup.
  01h  SGB or Normal Gameboy (DMG)
  FFh  SGB2 or Pocket Gameboy
  11h  CGB or GBA
Because values 01h and FFh are shared for both handhelds and SGBs, it is still
required to use the above MLT_REQ detection procedure. As far as I know the
SGB2 doesn't have any extra features which'd require separate SGB2 detection
except for curiosity purposes, for example, the game "Tetris DX" chooses to
display an alternate SGB border on SGB2s.

Reportedly, some SGB models include link ports (just like handheld gameboy)
(my own SGB does not have such an port), possibly this feature is available in
SGB2-type models only ???

SGB Command Packet Transfers
----------------------------

Command packets (aka Register Files) are transferred from the gameboy to the
SNES by using P14 and P15 output lines of the JOYPAD register (FF00h), these
lines are normally used to select the two rows in the gameboy keyboard matrix
(which still works).

Transferring Bits
A command packet transfer must be initiated by setting both P14 and P15 to
LOW, this will reset and start the SNES packet receiving program. Data is then
transferred (LSB first), setting P14=LOW will indicate a "0" bit, and setting
P15=LOW will indicate a "1" bit. For example:
       RESET 0   0   1   1   0   1   0
  P14  --_---_---_-----------_-------_--...
  P15  --_-----------_---_-------_------...
Data and reset pulses must be kept LOW for at least 5us. P14 and P15 must be
kept both HIGH for at least 15us between any pulses.
Obviously, it'd be no good idea to access the JOYPAD register during the
transfer, for example, in case that your VBlank interrupt procedure reads-out
joypad states each frame, be sure to disable that interrupt during the
transfer (or disable only the joypad procedure by using a software flag).

Transferring Packets
Each packet is invoked by a RESET pulse, then 128 bits of data are transferred
(16 bytes, LSB of first byte first), and finally, a "0"-bit must be
transferred as stop bit. The structure of normal packets is:
   1 PULSE Reset
   1 BYTE  Command Code*8+Length
  15 BYTES Parameter Data
   1 BIT   Stop Bit (0)
The above 'Length' indicates the total number of packets (1-7, including the
first packet) which will be sent, ie. if more than 15 parameter bytes are
used, then further packet(s) will follow, as such:
   1 PULSE Reset
  16 BYTES Parameter Data
   1 BIT   Stop Bit (0)
By using all 7 packets, up to 111 data bytes (15+16*6) may be sent.
Unused bytes at the end of the last packet must be set to zero.
A 60ms (4 frames) delay should be invoked between each packet transfer.

SGB VRAM Transfers
------------------

Overview
Beside for the packet transfer method, larger data blocks of 4KBytes can be
transferred by using the video signal. These transfers are invoked by first
sending one of the commands with the ending _TRN (by using normal packet
transfer), the 4K data block is then read-out by the SNES from gameboy display
memory during the next frame.

Transfer Data
Normally, transfer data should be stored at 8000h-8FFFh in gameboy VRAM,
even though the SNES receives the data in from display scanlines, it will
automatically re-produce the same ordering of bits and bytes, as being
originally stored at 8000h-8FFFh in gameboy memory.

Preparing the Display
The above method works only when recursing the following things: BG Map must
display unsigned characters 00h-FFh on the screen; 00h..13h in first line,
14h..27h in next line, etc. The gameboy display must be enabled, the display
may not be scrolled, OBJ sprites should not overlap the background tiles, the
BGP palette register must be set to E4h.

Transfer Time
Note that the transfer data should be prepared in VRAM  sending the
transfer command packet. The actual transfer starts at the beginning of the
next frame after the command has been sent, and the transfer ends at the end
of the 5th frame after the command has been sent (not counting the frame in
which the command has been sent).

Avoiding Screen Garbage
The display will contain 'garbage' during the transfer, this dirt-effect can
be avoided by freezing the screen (in the state which has been displayed
before the transfer) by using the MASK_EN command.
Of course, this works only when actually executing the game on a SGB (and not
on normal handheld gameboys), it'd be thus required to detect the presence of
SGB hardware before blindly sending VRAM data.

SGB Command Summary
-------------------

SGB System Command Table
  Code Name      Expl.
  00   PAL01     Set SGB Palette 0,1 Data
  01   PAL23     Set SGB Palette 2,3 Data
  02   PAL03     Set SGB Palette 0,3 Data
  03   PAL12     Set SGB Palette 1,2 Data
  04   ATTR_BLK  "Block" Area Designation Mode
  05   ATTR_LIN  "Line" Area Designation Mode
  06   ATTR_DIV  "Divide" Area Designation Mode
  07   ATTR_CHR  "1CHR" Area Designation Mode
  08   SOUND     Sound On/Off
  09   SOU_TRN   Transfer Sound PRG/DATA
  0A   PAL_SET   Set SGB Palette Indirect
  0B   PAL_TRN   Set System Color Palette Data
  0C   ATRC_EN   Enable/disable Attraction Mode
  0D   TEST_EN   Speed Function
  0E   ICON_EN   SGB Function
  0F   DATA_SND  SUPER NES WRAM Transfer 1
  10   DATA_TRN  SUPER NES WRAM Transfer 2
  11   MLT_REG   Controller 2 Request
  12   JUMP      Set SNES Program Counter
  13   CHR_TRN   Transfer Character Font Data
  14   PCT_TRN   Set Screen Data Color Data
  15   ATTR_TRN  Set Attribute from ATF
  16   ATTR_SET  Set Data to ATF
  17   MASK_EN   Game Boy Window Mask
  18   OBJ_TRN   Super NES OBJ Mode

SGB Color Palettes Overview
---------------------------

Available SNES Palettes
The SGB/SNES provides 8 palettes of 16 colors each, each color may be defined
out of a selection of 34768 colors (15 bit). Palettes 0-3 are used to colorize
the gamescreen, only the first four colors of each of these palettes are used.
Palettes 4-7 are used for the SGB Border, all 16 colors of each of these
palettes may be used.

Color 0 Restriction
Color 0 of each of the eight palettes is transparent, causing the backdrop
color to be displayed instead. The backdrop color is typically defined by the
most recently color being assigned to Color 0 (regardless of the palette
number being used for that operation).
Effectively, gamescreen palettes can have only three custom colors each, and
SGB border palettes only 15 colors each, additionally, color 0 can be used for
for all palettes, which will then all share the same color though.

Translation of Grayshades into Colors
Because the SGB/SNES reads out the gameboy video controllers display signal,
it translates the different grayshades from the signal into SNES colors as
such:
  White       -->  Color 0
  Light Gray  -->  Color 1
  Dark Gray   -->  Color 2
  Black       -->  Color 3
Note that gameboy colors 0-3 are assigned to user-selectable grayshades by the
gameboys BGP, OBP1, and OBP2 registers. There is thus no fixed relationship
between gameboy colors 0-3 and SNES colors 0-3.

Using Gameboy BGP/OBP Registers
A direct translation of color 0-3 into color 0-3 may be produced by setting
BGP/OBP registers to a value of 0E4h each. However, in case that your program
uses black background for example, then you may internally assign background
as "White" at the gameboy side by BGP/OBP registers (which is then interpreted
as SNES color 0, which is shared for all SNES palettes). The advantage is that
you may define Color 0 as Black at the SNES side, and may assign custom colors
for Colors 1-3 of each SNES palette.

System Color Palette Memory
Beside for the actually visible palettes, up to 512 palettes of 4 colors each
may be defined in SNES RAM. Basically, this is completely irrelevant because
the palettes are just stored in RAM whithout any relationship to the displayed
picture, anyways, these pre-defined colors may be transferred to actually
visible palettes slightly faster as when transferring palette data by separate
command packets.

SGB Palette Commands
--------------------

SGB Command 00h - PAL01
Transmit color data for SGB palette 0, color 0-3, and for SGB palette 1, color
1-3 (without separate color 0).
  Byte  Content
  0     Command*8+Length (fixed length=01h)
  1-E   Color Data for 7 colors of 2 bytes (16bit) each:
          Bit 0-4   - Red Intensity   (0-31)
          Bit 5-9   - Green Intensity (0-31)
          Bit 10-14 - Blue Intensity  (0-31)
          Bit 15    - Not used (zero)
  F     Not used (00h)
The value transferred as color 0 will be applied for all eight palettes.

SGB Command 01h - PAL23
Same as above PAL01, but for Palettes 2 and 3 respectively.

SGB Command 02h - PAL03
Same as above PAL01, but for Palettes 0 and 3 respectively.

SGB Command 03h - PAL12
Same as above PAL01, but for Palettes 1 and 2 respectively.

SGB Command 0Ah - PAL_SET
Used to copy pre-defined palette data from SGB system color palette to actual
SGB palette.
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1-2   System Palette number for SGB Color Palette 0 (0-511)
  3-4   System Palette number for SGB Color Palette 1 (0-511)
  5-6   System Palette number for SGB Color Palette 2 (0-511)
  7-8   System Palette number for SGB Color Palette 3 (0-511)
  9     Attribute File
          Bit 0-5 - Attribute File Number (00h-2Ch) (Used only if Bit7=1)
          Bit 6   - Cancel Mask           (0=No change, 1=Yes)
          Bit 7   - Use Attribute File    (0=No, 1=Apply above ATF Number)
  A-F   Not used (zero)
Before using this function, System Palette data should be initialized by
PAL_TRN command, and (when used) Attribute File data should be initialized by
ATTR_TRN.

SGB Command 0Bh - PAL_TRN
Used to initialize SGB system color palettes in SNES RAM.
System color palette memory contains 512 pre-defined palettes, these palettes
do not directly affect the display, however, the PAL_SET command may be later
used to transfer four of these 'logical' palettes to actual visible 'physical'
SGB palettes. Also, the OBJ_TRN function will use groups of 4 System Color
Palettes (4*4 colors) for SNES OBJ palettes (16 colors).
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1-F   Not used (zero)
The palette data is sent by VRAM-Transfer (4 KBytes).
  000-FFF  Data for System Color Palette 0-511
Each Palette consists of four 16bit-color definitions (8 bytes).
Note: The data is stored at 3000h-3FFFh in SNES memory.

SGB Color Attribute Commands
----------------------------

SGB Command 04h - ATTR_BLK
Used to specify color attributes for the inside or outside of one or more
rectangular screen regions.
  Byte  Content
  0     Command*8+Length (length=1..7)
  1     Number of Data Sets (01h..12h)
  2-7   Data Set #1
          Byte 0 - Control Code (0-7)
            Bit 0 - Change Colors inside of surrounded area     (1=Yes)
            Bit 1 - Change Colors of surrounding character line (1=Yes)
            Bit 2 - Change Colors outside of surrounded area    (1=Yes)
            Bit 3-7 - Not used (zero)
            Exception: When changing only the Inside or Outside, then the
            Surrounding line becomes automatically changed to same color.
          Byte 1 - Color Palette Designation
            Bit 0-1 - Palette Number for inside of surrounded area
            Bit 2-3 - Palette Number for surrounding character line
            Bit 4-5 - Palette Number for outside of surrounded area
            Bit 6-7 - Not used (zero)
          Data Set Byte 2 - Coordinate X1 (left)
          Data Set Byte 3 - Coordinate Y1 (upper)
          Data Set Byte 4 - Coordinate X2 (right)
          Data Set Byte 5 - Coordinate Y2 (lower)
            Specifies the coordinates of the surrounding rectangle.
  8-D   Data Set #2 (if any)
  E-F   Data Set #3 (continued at 0-3 in next packet) (if any)
When sending three or more data sets, data is continued in further packet(s).
Unused bytes at the end of the last packet should be set to zero. The format
of the separate Data Sets is described below.

SGB Command 05h - ATTR_LIN
Used to specify color attributes of one or more horizontal or vertical
character lines.
  Byte  Content
  0     Command*8+Length (length=1..7)
  1     Number of Data Sets (01h..6Eh) (one byte each)
  2     Data Set #1
          Bit 0-4 - Line Number    (X- or Y-coordinate, depending on bit 7)
          Bit 5-6 - Palette Number (0-3)
          Bit 7   - H/V Mode Bit   (0=Vertical line, 1=Horizontal Line)
  3     Data Set #2 (if any)
  4     Data Set #3 (if any)
  etc.
When sending 15 or more data sets, data is continued in further packet(s).
Unused bytes at the end of the last packet should be set to zero. The format
of the separate Data Sets (one byte each) is described below.
The length of each line reaches from one end of the screen to the other end.
In case that some lines overlap each other, then lines from lastmost data sets
will overwrite lines from previous data sets.

SGB Command 06h - ATTR_DIV
Used to split the screen into two halfes, and to assign separate color
attributes to each half, and to the division line between them.
  Byte  Content
  0     Command*8+Length   (fixed length=1)
  1     Color Palette Numbers and H/V Mode Bit
          Bit 0-1  Palette Number below/right of division line
          Bit 2-3  Palette Number above/left of division line
          Bit 4-5  Palette Number for division line
          Bit 6    H/V Mode Bit  (0=split left/right, 1=split above/below)
  2     X- or Y-Coordinate (depending on H/V bit)
  3-F   Not used (zero)

SGB Command 07h - ATTR_CHR
Used to specify color attributes for separate characters.
  Byte  Content
  0     Command*8+Length (length=1..6)
  1     Beginning X-Coordinate
  2     Beginning Y-Coordinate
  3-4   Number of Data Sets (1-360)
  5     Writing Style   (0=Left to Right, 1=Top to Bottom)
  6     Data Sets 1-4   (Set 1 in MSBs, Set 4 in LSBs)
  7     Data Sets 5-8   (if any)
  8     Data Sets 9-12  (if any)
  etc.
When sending 41 or more data sets, data is continued in further packet(s).
Unused bytes at the end of the last packet should be set to zero. Each data
set consists of two bits, indicating the palette number for one character.
Depending on the writing style, data sets are written from left to right, or
from top to bottom. In either case the function wraps to the next row/column
when reaching the end of the screen.

SGB Command 15h - ATTR_TRN
Used to initialize Attribute Files (ATFs) in SNES RAM. Each ATF consists of
20x18 color attributes for the gameboy screen. This function does not directly
affect display attributes. Instead, one of the defined ATFs may be copied to
actual display memory at a later time by using ATTR_SET or PAL_SET functions.
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1-F   Not used (zero)
The ATF data is sent by VRAM-Transfer (4 KBytes).
  000-FD1  Data for ATF0 through ATF44 (4050 bytes)
  FD2-FFF  Not used
Each ATF consists of 90 bytes, that are 5 bytes (20x2bits) for each of the 18
character lines of the gameboy window. The two most significant bits of the
first byte define the color attribute (0-3) for the first character of the
first line, the next two bits the next character, and so on.

SGB Command 16h - ATTR_SET
Used to transfer attributes from Attribute File (ATF) to gameboy window.
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1     Attribute File Number (00-2Ch), Bit 6=Cancel Mask
  2-F   Not used (zero)
When above Bit 6 is set, the gameboy screen becomes re-enabled after the
transfer (in case it has been disabled/frozen by MASK_EN command).
Note: The same functions may be (optionally) also included in PAL_SET
commands, as described in the chapter about Color Palette Commands.

SGB Sound Functions
-------------------

SGB Command 08h - SOUND
Used to start/stop internal sound effect, start/stop sound using internal tone
data.
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1     Sound Effect A (Port 1) Decrescendo 8bit Sound Code
  2     Sound Effect B (Port 2) Sustain     8bit Sound Code
  3     Sound Effect Attributes
          Bit 0-1 - Sound Effect A Pitch  (0..3=Low..High)
          Bit 2-3 - Sound Effect A Volume (0..2=High..Low, 3=Mute on)
          Bit 4-5 - Sound Effect B Pitch  (0..3=Low..High)
          Bit 6-7 - Sound Effect B Volume (0..2=High..Low, 3=Not used)
  4     Music Score Code (must be zero if not used)
  5-F   Not used (zero)
See Sound Effect Tables below for a list of available pre-defined effects.
"Notes"
1) Mute is only active when both bits D2 and D3 are 1.
2) When the volume is set for either Sound Effect A or Sound Effect B, mute is
turned off.
3) When Mute on/off has been executed, the sound fades out/fades in.
4) Mute on/off operates on the (BGM) which is reproduced by Sound Effect A,
Sound Effect B, and the Super NES APU. A "mute off" flag does not exist by
itself. When mute flag is set, volume and pitch of Sound Effect A (port 1) and
Sound Effect B (port 2) must be set.

SGB Command 09h - SOU_TRN
Used to transfer sound code or data to SNES Audio Processing Unit memory
(APU-RAM).
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1-F   Not used (zero)
The sound code/data is sent by VRAM-Transfer (4 KBytes).
  000      One (or two ???) 16bit expression(s ???) indicating the
           transfer destination address and transfer length.
  ...-...  Transfer Data
  ...-FFF  Remaining bytes not used
Possible destinations in APU-RAM are:
  0400h-2AFFh  APU-RAM Program Area (9.75KBytes)
  2B00h-4AFFh  APU-RAM Sound Score Area (8Kbytes)
  4DB0h-EEFFh  APU-RAM Sampling Data Area (40.25 Kbytes)
This function may be used to take control of the SNES sound chip, and/or to
access the SNES MIDI engine. In either case it requires deeper knowledge of
SNES sound programming.

SGB Sound Effect A/B Tables
Below lists the digital sound effects that are pre-defined in the SGB/SNES
BIOS, and which can be used with the SGB "SOUND" Command.
Effect A and B may be simultaneously reproduced.
The P-column indicates the recommended Pitch value, the V-column indicates the
numbers of Voices used. Sound Effect A uses voices 6,7. Sound Effect B uses
voices 0,1,4,5. Effects that use less voices will use only the upper voices
(eg. 4,5 for Effect B with only two voices).

Sound Effect A Flag Table
  Code Description             P V     Code Description             P V
  00  Dummy flag, re-trigger   - 2     18  Fast Jump                3 1
  80  Effect A, stop/silent    - 2     19  Jet (rocket) takeoff     0 1
  01  Nintendo                 3 1     1A  Jet (rocket) landing     0 1
  02  Game Over                3 2     1B  Cup breaking             2 2
  03  Drop                     3 1     1C  Glass breaking           1 2
  04  OK ... A                 3 2     1D  Level UP                 2 2
  05  OK ... B                 3 2     1E  Insert air               1 1
  06  Select...A               3 2     1F  Sword swing              1 1
  07  Select...B               3 1     20  Water falling            2 1
  08  Select...C               2 2     21  Fire                     1 1
  09  Mistake...Buzzer         2 1     22  Wall collapsing          1 2
  0A  Catch Item               2 2     23  Cancel                   1 2
  0B  Gate squeaks 1 time      2 2     24  Walking                  1 2
  0C  Explosion...small        1 2     25  Blocking strike          1 2
  0D  Explosion...medium       1 2     26  Picture floats on & off  3 2
  0E  Explosion...large        1 2     27  Fade in                  0 2
  0F  Attacked...A             3 1     28  Fade out                 0 2
  10  Attacked...B             3 2     29  Window being opened      1 2
  11  Hit (punch)...A          0 2     2A  Window being closed      0 2
  12  Hit (punch)...B          0 2     2B  Big Laser                3 2
  13  Breath in air            3 2     2C  Stone gate closes/opens  0 2
  14  Rocket Projectile...A    3 2     2D  Teleportation            3 1
  15  Rocket Projectile...B    3 2     2E  Lightning                0 2
  16  Escaping Bubble          2 1     2F  Earthquake               0 2
  17  Jump                     3 1     30  Small Laser              2 2
Sound effect A is used for formanto sounds (percussion sounds).

Sound Effect B Flag Table
  Code Description             P V     Code Description             P V
  00  Dummy flag, re-trigger   - 4     0D  Waterfall                2 2
  80  Effect B, stop/silent    - 4     0E  Small character running  3 1
  01  Applause...small group   2 1     0F  Horse running            3 1
  02  Applause...medium group  2 2     10  Warning sound            1 1
  03  Applause...large group   2 4     11  Approaching car          0 1
  04  Wind                     1 2     12  Jet flying               1 1
  05  Rain                     1 1     13  UFO flying               2 1
  06  Storm                    1 3     14  Electromagnetic waves    0 1
  07  Storm with wind/thunder  2 4     15  Score UP                 3 1
  08  Lightning                0 2     16  Fire                     2 1
  09  Earthquake               0 2     17  Camera shutter, formanto 3 4
  0A  Avalanche                0 2     18  Write, formanto          0 1
  0B  Wave                     0 1     19  Show up title, formanto  0 1
  0C  River                    3 2
Sound effect B is mainly used for looping sounds (sustained sounds).

SGB System Control Commands
---------------------------

SGB Command 17h - MASK_EN
Used to mask the gameboy window, among others this can be used to freeze the
gameboy screen before transferring data through VRAM (the SNES then keeps
displaying the gameboy screen, even though VRAM doesn't contain meaningful
display information during the transfer).
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1     Gameboy Screen Mask (0-3)
          0  Cancel Mask   (Display activated)
          1  Freeze Screen (Keep displaying current picture)
          2  Blank Screen  (Black)
          3  Blank Screen  (Color 0)
  2-F   Not used (zero)
Freezing works only if the SNES has stored a picture, ie. if necessary wait
one or two frames before freezing (rather than freezing directly after having
displayed the picture).
The Cancel Mask function may be also invoked (optionally) by completion of
PAL_SET and ATTR_SET commands.

SGB Command 0Ch - ATRC_EN
Used to enable/disable Attraction mode. It is totally unclear what an
attraction mode is ???, but it is enabled by default.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     Attraction Disable  (0=Enable, 1=Disable)
  2-F   Not used (zero)

SGB Command 0Dh - TEST_EN
Used to enable/disable test mode for "SGB-CPU variable clock speed function".
This function is disabled by default.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     Test Mode Enable    (0=Disable, 1=Enable)
  2-F   Not used (zero)
Maybe intended to determine whether SNES operates at 50Hz or 60Hz display
refresh rate ??? Possibly result can be read-out from joypad register ???

SGB Command 0Eh - ICON_EN
Used to enable/disable ICON function. Possibly meant to enable/disable
SGB/SNES popup menues which might otherwise activated during gameboy game
play. By default all functions are enabled (0).
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     Disable Bits
          Bit 0 - Use of SGB-Built-in Color Palettes    (1=Disable)
          Bit 1 - Controller Set-up Screen    (0=Enable, 1=Disable)
          Bit 2 - SGB Register File Transfer (0=Receive, 1=Disable)
          Bit 3-6 - Not used (zero)
  2-F   Not used (zero)
Above Bit 2 will suppress all further packets/commands when set, this might be
useful when starting a monochrome game from inside of the SGB-menu of a
multi-gamepak which contains a collection of different games.

SGB Command 0Fh - DATA_SND
Used to write one or more bytes directly into SNES Work RAM.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     SNES Destination Address, low
  2     SNES Destination Address, high
  3     SNES Destination Address, bank number
  4     Number of bytes to write (01h-0Bh)
  5     Data Byte #1
  6     Data Byte #2 (if any)
  7     Data Byte #3 (if any)
  etc.
Unused bytes at the end of the packet should be set to zero, this function is
restricted to a single packet, so that not more than 11 bytes can be defined
at once.
Free Addresses in SNES memory are Bank 0 1800h-1FFFh, Bank 7Fh 0000h-FFFFh.

SGB Command 10h - DATA_TRN
Used to transfer binary code or data directly into SNES RAM.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     SNES Destination Address, low
  2     SNES Destination Address, high
  3     SNES Destination Address, bank number
  4-F   Not used (zero)
The data is sent by VRAM-Transfer (4 KBytes).
  000-FFF  Data
Free Addresses in SNES memory are Bank 0 1800h-1FFFh, Bank 7Fh 0000h-FFFFh.
The transfer length is fixed at 4KBytes ???, so that directly writing to the
free 2KBytes at 0:1800h would be a not so good idea ???

SGB Command 12h - JUMP
Used to set the SNES program counter to a specified address. Optionally, it
may be used to set a new address for the SNES NMI handler, the NMI handler
remains unchanged if all bytes 4-6 are zero.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     SNES Program Counter, low
  2     SNES Program Counter, high
  3     SNES Program Counter, bank number
  4     SNES NMI Handler, low
  5     SNES NMI Handler, high
  6     SNES NMI Handler, bank number
  7-F   Not used, zero
Note: The game "Space Invaders 94" uses this function when selecting "Arcade
mode" to execute SNES program code which has been previously transferred from
the SGB to the SNES. The type of the CPU which is used in the SNES is unknown
???

SGB Multiplayer Command
-----------------------

SGB Command 11h - MLT_REQ
Used to request multiplayer mode (ie. input from more than one joypad).
Because this function provides feedback from the SGB/SNES to the gameboy
program, it is also used to detect SGB hardware.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     Multiplayer Control (0-3) (Bit0=Enable, Bit1=Two/Four Players)
          0 = One player
          1 = Two players
          3 = Four players
  2-F   Not used (zero)
In one player mode, the second joypad (if any) is used for the SGB system
program. In two player mode, both joypads are used for the game. Because SNES
have only two joypad sockets, four player mode requires an external
"Multiplayer 5" adapter.

Reading Multiple Controllers (Joypads)
When having enabled multiple controllers by MLT_REQ, data for each joypad can
be read out through JOYPAD register (FF00) as follows: First set P14 and P15
both HIGH (deselect both Buttons and Cursor keys), you can now read the lower
4bits of FF00 which indicate the joypad ID for the following joypad input:
  0Fh  Joypad 1
  0Eh  Joypad 2
  0Dh  Joypad 3
  0Ch  Joypad 4
Next, set P14 and P15 low (one after each other) to select Buttons and Cursor
lines, and read-out joypad state as normally. When completed, set P14 and P15
back HIGH, this automatically increments the joypad number (or restarts
counting once reached the lastmost joypad). Repeat the procedure until you
have read-out states for all two (or four) joypads.

SGB Border and OBJ Commands
---------------------------

SGB Command 13h - CHR_TRN
Used to transfer tile data (characters) to SNES Tile memory in VRAM. This
normally used to define BG tiles for the SGB Border (see PCT_TRN), but might
be also used to define moveable SNES foreground sprites (see OBJ_TRN).
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1     Tile Transfer Destination
          Bit 0   - Tile Numbers   (0=Tiles 00h-7Fh, 1=Tiles 80h-FFh)
          Bit 1   - Tile Type      (0=BG Tiles, 1=OBJ Tiles)
          Bit 2-7 - Not used (zero)
  2-F   Not used (zero)
The tile data is sent by VRAM-Transfer (4 KBytes).
  000-FFF  Bitmap data for 128 Tiles
Each tile occupies 16bytes (8x8 pixels, 16 colors each).
When intending to transfer more than 128 tiles, call this function twice (once
for tiles 00h-7Fh, and once for tiles 80h-FFh). Note: The BG/OBJ Bit seems to
have no effect and writes to the same VRAM addresses for both BG and OBJ ???

SGB Command 14h - PCT_TRN
Used to transfer tile map data and palette data to SNES BG Map memory in VRAM
to be used for the SGB border. The actual tiles must be separately transferred
by using the CHR_TRN function.
  Byte  Content
  0     Command*8+Length    (fixed length=1)
  1-F   Not used (zero)
The map data is sent by VRAM-Transfer (4 KBytes).
  000-7FF  BG Map 32x32 Entries of 16bit each (2048 bytes)
  800-87F  BG Palette Data (Palettes 4-7, each 16 colors of 16bits each)
  880-FFF  Not used, don't care
Each BG Map Entry consists of a 16bit value as such:
  Bit 0-9   - Character Number (use only 00h-FFh, upper 2 bits zero)
  Bit 10-12 - Palette Number   (use only 4-7, officially use only 4-6)
  Bit 13    - BG Priority      (use only 0)
  Bit 14    - X-Flip           (0=Normal, 1=Mirror horizontally)
  Bit 15    - Y-Flip           (0=Normal, 1=Mirror vertically)
Even though 32x32 map entries are transferred, only upper 32x28 are actually
used (256x224 pixels, SNES screen size). The 20x18 entries in the center of
the 32x28 area should be set to 0000h as transparent space for the gameboy
window to be displayed inside. Reportedly, non-transparent border data will
cover the gameboy window.

SGB Command 18h - OBJ_TRN
Used to transfer OBJ attributes to SNES OAM memory. Unlike all other functions
with the ending _TRN, this function does not use the usual one-shot 4KBytes
VRAM transfer method.
Instead, when enabled (below execute bit set), data is permanently (each
frame) read out from the lower character line of the gameboy screen. To
suppress garbage on the display, the lower line is masked, and only the upper
20x17 characters of the gameboy window are used - the masking method is
unknwon - frozen, black, or recommended to be covered by the SGB border, or
else ??? Also, when the function is enabled, "system attract mode is not
performed" - whatever that means ???
  Byte  Content
  0     Command*8+Length (fixed length=1)
  1     Control Bits
          Bit 0   - SNES OBJ Mode enable (0=Cancel, 1=Enable)
          Bit 1   - Change OBJ Color     (0=No, 1=Use definitions below)
          Bit 2-7 - Not used (zero)
  2-3   System Color Palette Number for OBJ Palette 4 (0-511)
  4-5   System Color Palette Number for OBJ Palette 5 (0-511)
  6-7   System Color Palette Number for OBJ Palette 6 (0-511)
  8-9   System Color Palette Number for OBJ Palette 7 (0-511)
          These color entries are ignored if above Control Bit 1 is zero.
          Because each OBJ palette consists of 16 colors, four system
          palette entries (of 4 colors each) are transferred into each
          OBJ palette. The system palette numbers are not required to be
          aligned to a multiple of four, and will wrap to palette number
          0 when exceeding 511. For example, a value of 511 would copy
          system palettes 511, 0, 1, 2 to the SNES OBJ palette.
  A-F   Not used (zero)
The recommended method is to "display" gameboy BG tiles F9h..FFh from left to
right as first 7 characters of the bottom-most character line of the gameboy
screen. As for normal 4KByte VRAM transfers, this area should not be scrolled,
should not be overlapped by gameboy OBJs, and the gameboy BGP palette register
should be set up properly. By following that method, SNES OAM data can be
defined in the 70h bytes of the gameboy BG tile memory at following addresses:
  8F90-8FEF  SNES OAM, 24 Entries of 4 bytes each (96 bytes)
  8FF0-8FF5  SNES OAM MSBs, 24 Entries of 2 bits each (6 bytes)
  8FF6-8FFF  Not used, don't care (10 bytes)
The format of SNES OAM Entries is:
  Byte 0  OBJ X-Position (0-511, MSB is separately stored, see below)
  Byte 1  OBJ Y-Position (0-255)
  Byte 2-3  Attributes (16bit)
    Bit 0-8    Tile Number     (use only 00h-FFh, upper bit zero)
    Bit 9-11   Palette Number  (use only 4-7)
    Bit 12-13  OBJ Priority    (use only 3)
    Bit 14     X-Flip          (0=Normal, 1=Mirror horizontally)
    Bit 15     Y-Flip          (0=Normal, 1=Mirror vertically)
The format of SNES OAM MSB Entries is:
  Actually, the format is unknown ??? However, 2 bits are used per entry:
  One bit is the most significant bit of the OBJ X-Position.
  The other bit specifies the OBJ size (8x8 or 16x16 pixels).

CPU Registers and Flags
-----------------------

Registers
  16bit Hi   Lo   Name/Function
  AF    A    -    Accumulator & Flags
  BC    B    C    BC
  DE    D    E    DE
  HL    H    L    HL
  SP    -    -    Stack Pointer
  PC    -    -    Program Counter/Pointer
As shown above, most registers can be accessed either as one 16bit register,
or as two separate 8bit registers.

The Flag Register (lower 8bit of AF register)
  Bit  Name  Set Clr  Expl.
  7    zf    Z   NZ   Zero Flag
  6    n     -   -    Add/Sub-Flag (BCD)
  5    h     -   -    Half Carry Flag (BCD)
  4    cy    C   NC   Carry Flag
  3-0  -     -   -    Not used (always zero)
Conatins the result from the recent instruction which has affected flags.

The Zero Flag (Z)
This bit becomes set (1) if the result of an operation has been zero (0). Used
for conditional jumps.

The Carry Flag (C, or Cy)
Becomes set when the result of an addition became bigger than FFh (8bit) or
FFFFh (16bit). Or when the result of a subtraction or comparision became less
than zero (much as for Z80 and 80x86 CPUs, but unlike as for 65XX and ARM
CPUs). Also the flag becomes set when a rotate/shift operation has shifted-out
a "1"-bit.
Used for conditional jumps, and for instructions such like ADC, SBC, RL, RLA,
etc.

The BCD Flags (N, H)
These flags are (rarely) used for the DAA instruction only, N Indicates
whether the previous instruction has been an addition or subtraction, and H
indicates carry for lower 4bits of the result, also for DAA, the C flag must
indicate carry for upper 8bits.
After adding/subtracting two BCD numbers, DAA is intended to convert the
result into BCD format; BCD numbers are ranged from 00h to 99h rather than 00h
to FFh.
Because C and H flags must contain carry-outs for each digit, DAA cannot be
used for 16bit operations (which have 4 digits), or for INC/DEC operations
(which do not affect C-flag).

CPU Instruction Set
-------------------

Tables below specify the mnemonic, opcode bytes, clock cycles, affected flags
(ordered as znhc), and explanatation.
The timings assume a CPU clock frequency of 4.194304 MHz (or 8.4
MHz for CGB in double speed mode), as all gameboy timings are divideable
by 4, many people specify timings and clock frequency divided by 4.

GMB 8bit-Loadcommands
  ld   r,r         xx         4 ---- r=r
  ld   r,n         xx nn      8 ---- r=n
  ld   r,(HL)      xx         8 ---- r=(HL)
  ld   (HL),r      7x         8 ---- (HL)=r
  ld   (HL),n      36 nn     12 ----
  ld   A,(BC)      0A         8 ----
  ld   A,(DE)      1A         8 ----
  ld   A,(nn)      FA        16 ----
  ld   (BC),A      02         8 ----
  ld   (DE),A      12         8 ----
  ld   (nn),A      EA        16 ----
  ld   A,(FF00+n)  F0 nn     12 ---- read from io-port n (memory FF00+n)
  ld   (FF00+n),A  E0 nn     12 ---- write to io-port n (memory FF00+n)
  ld   A,(FF00+C)  F2         8 ---- read from io-port C (memory FF00+C)
  ld   (FF00+C),A  E2         8 ---- write to io-port C (memory FF00+C)
  ldi  (HL),A      22         8 ---- (HL)=A, HL=HL+1
  ldi  A,(HL)      2A         8 ---- A=(HL), HL=HL+1
  ldd  (HL),A      32         8 ---- (HL)=A, HL=HL-1
  ldd  A,(HL)      3A         8 ---- A=(HL), HL=HL-1

GMB 16bit-Loadcommands
  ld   rr,nn       x1 nn nn  12 ---- rr=nn (rr may be BC,DE,HL or SP)
  ld   SP,HL       F9         8 ---- SP=HL
  push rr          x5        16 ---- SP=SP-2  (SP)=rr   (rr may be
BC,DE,HL,AF)
  pop  rr          x1        12 (AF) rr=(SP)  SP=SP+2   (rr may be
BC,DE,HL,AF)

GMB 8bit-Arithmetic/logical Commands
  add  A,r         8x         4 z0hc A=A+r
  add  A,n         C6 nn      8 z0hc A=A+n
  add  A,(HL)      86         8 z0hc A=A+(HL)
  adc  A,r         8x         4 z0hc A=A+r+cy
  adc  A,n         CE nn      8 z0hc A=A+n+cy
  adc  A,(HL)      8E         8 z0hc A=A+(HL)+cy
  sub  r           9x         4 z1hc A=A-r
  sub  n           D6 nn      8 z1hc A=A-n
  sub  (HL)        96         8 z1hc A=A-(HL)
  sbc  A,r         9x         4 z1hc A=A-r-cy
  sbc  A,n         DE nn      8 z1hc A=A-n-cy
  sbc  A,(HL)      9E         8 z1hc A=A-(HL)-cy
  and  r           Ax         4 z010 A=A & r
  and  n           E6 nn      8 z010 A=A & n
  and  (HL)        A6         8 z010 A=A & (HL)
  xor  r           Ax         4 z000
  xor  n           EE nn      8 z000
  xor  (HL)        AE         8 z000
  or   r           Bx         4 z000 A=A | r
  or   n           F6 nn      8 z000 A=A | n
  or   (HL)        B6         8 z000 A=A | (HL)
  cp   r           Bx         4 z1hc compare A-r
  cp   n           FE nn      8 z1hc compare A-n
  cp   (HL)        BE         8 z1hc compare A-(HL)
  inc  r           xx         4 z0h- r=r+1
  inc  (HL)        34        12 z0h- (HL)=(HL)+1
  dec  r           xx         4 z1h- r=r-1
  dec  (HL)        35        12 z1h- (HL)=(HL)-1
  daa              27         4 z-0x decimal adjust akku
  cpl              2F         4 -11- A = A xor FF

GMB 16bit-Arithmetic/logical Commands
  add  HL,rr     x9           8 -0hc HL = HL+rr     ;rr may be BC,DE,HL,SP
  inc  rr        x3           8 ---- rr = rr+1      ;rr may be BC,DE,HL,SP
  dec  rr        xB           8 ---- rr = rr-1      ;rr may be BC,DE,HL,SP
  add  SP,dd     E8          16 00hc SP = SP +/- dd ;dd is 8bit signed number
  ld   HL,SP+dd  F8          12 00hc HL = SP +/- dd ;dd is 8bit signed number

GMB Rotate- und Shift-Commands
  rlca           07           4 000c rotate akku left
  rla            17           4 000c rotate akku left through carry
  rrca           0F           4 000c rotate akku right
  rra            1F           4 000c rotate akku right through carry
  rlc  r         CB 0x        8 z00c rotate left
  rlc  (HL)      CB 06       16 z00c rotate left
  rl   r         CB 1x        8 z00c rotate left through carry
  rl   (HL)      CB 16       16 z00c rotate left through carry
  rrc  r         CB 0x        8 z00c rotate right
  rrc  (HL)      CB 0E       16 z00c rotate right
  rr   r         CB 1x        8 z00c rotate right through carry
  rr   (HL)      CB 1E       16 z00c rotate right through carry
  sla  r         CB 2x        8 z00c shift left arithmetic (b0=0)
  sla  (HL)      CB 26       16 z00c shift left arithmetic (b0=0)
  swap r         CB 3x        8 z000 exchange low/hi-nibble
  swap (HL)      CB 36       16 z000 exchange low/hi-nibble
  sra  r         CB 2x        8 z00c shift right arithmetic (b7=b7)
  sra  (HL)      CB 2E       16 z00c shift right arithmetic (b7=b7)
  srl  r         CB 3x        8 z00c shift right logical (b7=0)
  srl  (HL)      CB 3E       16 z00c shift right logical (b7=0)

GMB Singlebit Operation Commands
  bit  n,r       CB xx        8 z01- test bit n
  bit  n,(HL)    CB xx       12 z01- test bit n
  set  n,r       CB xx        8 ---- set bit n
  set  n,(HL)    CB xx       16 ---- set bit n
  res  n,r       CB xx        8 ---- reset bit n
  res  n,(HL)    CB xx       16 ---- reset bit n

GMB CPU-Controlcommands
  ccf            3F           4 -00c cy=cy xor 1
  scf            37           4 -001 cy=1
  nop            00           4 ---- no operation
  halt           76         N*4 ---- halt until interrupt occurs (low power)
  stop           10 00        ? ---- low power standby mode (VERY low power)
  di             F3           4 ---- disable interrupts, IME=0
  ei             FB           4 ---- enable interrupts, IME=1

GMB Jumpcommands
  jp   nn        C3 nn nn    16 ---- jump to nn, PC=nn
  jp   HL        E9           4 ---- jump to HL, PC=HL
  jp   f,nn      xx nn nn 16;12 ---- conditional jump if nz,z,nc,c
  jr   PC+dd     18 dd       12 ---- relative jump to nn (PC=PC+/-7bit)
  jr   f,PC+dd   xx dd     12;8 ---- conditional relative jump if nz,z,nc,c
  call nn        CD nn nn    24 ---- call to nn, SP=SP-2, (SP)=PC, PC=nn
  call f,nn      xx nn nn 24;12 ---- conditional call if nz,z,nc,c
  ret            C9          16 ---- return, PC=(SP), SP=SP+2
  ret  f         xx        20;8 ---- conditional return if nz,z,nc,c
  reti           D9          16 ---- return and enable interrupts (IME=1)
  rst  n         xx          16 ---- call to 00,08,10,18,20,28,30,38

CPU Comparision with Z80
------------------------

Comparision with 8080
Basically, the gameboy CPU works more like an older 8080 CPU rather than like
a more powerful Z80 CPU. It is, however, supporting CB-prefixed instructions.
Also, all known gameboy assemblers using the more obvious Z80-style syntax,
rather than the chaotic 8080-style syntax.

Comparision with Z80
Any DD-, ED-, and FD-prefixed instructions are missing, that means no IX-,
IY-registers, no block commands, and some other missing commands.
All exchange instructions have been removed (including total absence of second
register set), 16bit memory accesses are mostly missing, and 16bit arithmetic
functions are heavily cut-down.
The gameboy has no IN/OUT instructions, instead I/O ports are accessed
directly by normal LD instructions, or by special LD (FF00+n) opcodes.
The sign and parity/overflow flags have been removed.
The gameboy operates approximately as fast as a 4MHz Z80 (8MHz in CGB double
speed mode), execution time of all instructions has been rounded up to a
multiple of 4 cycles though.

Moved, Removed, and Added Opcodes
  Opcode  Z80             GMB
  ---------------------------------------
  08      EX   AF,AF      LD   (nn),SP
  10      DJNZ PC+dd      STOP
  22      LD   (nn),HL    LDI  (HL),A
  2A      LD   HL,(nn)    LDI  A,(HL)
  32      LD   (nn),A     LDD  (HL),A
  3A      LD   A,(nn)     LDD  A,(HL)
  D3      OUT  (n),A      -
  D9      EXX             RETI
  DB      IN   A,(n)      -
  DD                  -
  E0      RET  PO         LD   (FF00+n),A
  E2      JP   PO,nn      LD   (FF00+C),A
  E3      EX   (SP),HL    -
  E4      CALL P0,nn      -
  E8      RET  PE         ADD  SP,dd
  EA      JP   PE,nn      LD   (nn),A
  EB      EX   DE,HL      -
  EC      CALL PE,nn      -
  ED                -
  F0      RET  P          LD   A,(FF00+n)
  F2      JP   P,nn       LD   A,(FF00+C)
  F4      CALL P,nn       -
  F8      RET  M          LD   HL,SP+dd
  FA      JP   M,nn       LD   A,(nn)
  FC      CALL M,nn       -
  FD                  -
  CB3X    SLL  r/(HL)     SWAP r/(HL)
Note: The unused (-) opcodes will lock-up the gameboy CPU when used.

The Cartridge Header
--------------------

An internal information area is located at 0100-014F in
each cartridge. It contains the following values:

0100-0103 - Entry Point
After displaying the Nintendo Logo, the built-in boot procedure jumps to this
address (100h), which should then jump to the actual main program in the
cartridge. Usually this 4 byte area contains a NOP instruction, followed by a
JP 0150h instruction. But not always.

0104-0133 - Nintendo Logo
These bytes define the bitmap of the Nintendo logo that is displayed when the
gameboy gets turned on. The hexdump of this bitmap is:
  CE ED 66 66 CC 0D 00 0B 03 73 00 83 00 0C 00 0D
  00 08 11 1F 88 89 00 0E DC CC 6E E6 DD DD D9 99
  BB BB 67 63 6E 0E EC CC DD DC 99 9F BB B9 33 3E
The gameboys boot procedure verifies the content of this bitmap (after it has
displayed it), and LOCKS ITSELF UP if these bytes are incorrect. A CGB
verifies only the first 18h bytes of the bitmap, but others (for example a
pocket gameboy) verify all 30h bytes.

0134-0143 - Title
Title of the game in UPPER CASE ASCII. If it is less than 16 characters then
the remaining bytes are filled with 00's. When inventing the CGB, Nintendo has
reduced the length of this area to 15 characters, and some months later they
had the fantastic idea to reduce it to 11 characters only. The new meaning of
the ex-title bytes is described below.

013F-0142 - Manufacturer Code
In older cartridges this area has been part of the Title (see above), in newer
cartridges this area contains an 4 character uppercase manufacturer code.
Purpose and Deeper Meaning unknown.

0143 - CGB Flag
In older cartridges this byte has been part of the Title (see above). In CGB
cartridges the upper bit is used to enable CGB functions. This is required,
otherwise the CGB switches itself into Non-CGB-Mode. Typical values are:
  80h - Game supports CGB functions, but works on old gameboys also.
  C0h - Game works on CGB only (physically the same as 80h).
Values with Bit 7 set, and either Bit 2 or 3 set, will switch the gameboy into
a special non-CGB-mode with uninitialized palettes. Purpose unknown,
eventually this has been supposed to be used to colorize monochrome games that
include fixed palette data at a special location in ROM.

0144-0145 - New Licensee Code
Specifies a two character ASCII licensee code, indicating the company or
publisher of the game. These two bytes are used in newer games only (games
that have been released after the SGB has been invented). Older games are
using the header entry at 014B instead.

0146 - SGB Flag
Specifies whether the game supports SGB functions, common values are:
  00h = No SGB functions (Normal Gameboy or CGB only game)
  03h = Game supports SGB functions
The SGB disables its SGB functions if this byte is set to another value than
03h.

0147 - Cartridge Type
Specifies which Memory Bank Controller (if any) is used in the cartridge, and
if further external hardware exists in the cartridge.
  00h  ROM ONLY                 13h  MBC3+RAM+BATTERY
  01h  MBC1                     15h  MBC4
  02h  MBC1+RAM                 16h  MBC4+RAM
  03h  MBC1+RAM+BATTERY         17h  MBC4+RAM+BATTERY
  05h  MBC2                     19h  MBC5
  06h  MBC2+BATTERY             1Ah  MBC5+RAM
  08h  ROM+RAM                  1Bh  MBC5+RAM+BATTERY
  09h  ROM+RAM+BATTERY          1Ch  MBC5+RUMBLE
  0Bh  MMM01                    1Dh  MBC5+RUMBLE+RAM
  0Ch  MMM01+RAM                1Eh  MBC5+RUMBLE+RAM+BATTERY
  0Dh  MMM01+RAM+BATTERY        FCh  POCKET CAMERA
  0Fh  MBC3+TIMER+BATTERY       FDh  BANDAI TAMA5
  10h  MBC3+TIMER+RAM+BATTERY   FEh  HuC3
  11h  MBC3                     FFh  HuC1+RAM+BATTERY
  12h  MBC3+RAM

0148 - ROM Size
Specifies the ROM Size of the cartridge. Typically calculated as "32KB shl N".
  00h -  32KByte (no ROM banking)
  01h -  64KByte (4 banks)
  02h - 128KByte (8 banks)
  03h - 256KByte (16 banks)
  04h - 512KByte (32 banks)
  05h -   1MByte (64 banks)  - only 63 banks used by MBC1
  06h -   2MByte (128 banks) - only 125 banks used by MBC1
  07h -   4MByte (256 banks)
  52h - 1.1MByte (72 banks)
  53h - 1.2MByte (80 banks)
  54h - 1.5MByte (96 banks)

0149 - RAM Size
Specifies the size of the external RAM in the cartridge (if any).
  00h - None
  01h - 2 KBytes
  02h - 8 Kbytes
  03h - 32 KBytes (4 banks of 8KBytes each)
When using a MBC2 chip 00h must be specified in this entry, even though the
MBC2 includes a built-in RAM of 512 x 4 bits.

014A - Destination Code
Specifies if this version of the game is supposed to be sold in japan, or
anywhere else. Only two values are defined.
  00h - Japanese
  01h - Non-Japanese

014B - Old Licensee Code
Specifies the games company/publisher code in range 00-FFh. A value of 33h
signalizes that the New License Code in header bytes 0144-0145 is used
instead.
(Super GameBoy functions won't work if <> $33.)

014C - Mask ROM Version number
Specifies the version number of the game. That is usually 00h.

014D - Header Checksum
Contains an 8 bit checksum across the cartridge header bytes 0134-014C. The
checksum is calculated as follows:
  x=0:FOR i=0134h TO 014Ch:x=x-MEM[i]-1:NEXT
The lower 8 bits of the result must be the same than the value in this entry.
The GAME WON'T WORK if this checksum is incorrect.

014E-014F - Global Checksum
Contains a 16 bit checksum (upper byte first) across the whole cartridge ROM.
Produced by adding all bytes of the cartridge (except for the two checksum
bytes). The Gameboy doesn't verify this checksum.

Memory Bank Controllers
-----------------------

As the gameboys 16 bit address bus offers only limited space for ROM and RAM
addressing, many games are using Memory Bank Controllers (MBCs) to expand the
available address space by bank switching. These MBC chips are located in the
game cartridge (ie. not in the gameboy itself), several different MBC types
are available:

--> None (32KByte ROM only)
--> MBC1 (max 2MByte ROM and/or 32KByte RAM)
--> MBC2 (max 256KByte ROM and 512x4 bits RAM)
--> MBC3 (max 2MByte ROM and/or 32KByte RAM and Timer)
--> HuC1 (MBC with Infrared Controller)

--> MBC Timing Issues

In each cartridge, the required (or preferred) MBC type should be specified in
byte at 0147h of the ROM. (As described in the chapter about The Cartridge
Header.)

None (32KByte ROM only)
-----------------------

Small games of not more than 32KBytes ROM do not require a MBC chip for ROM
banking. The ROM is directly mapped to memory at 0000-7FFFh. Optionally up to
8KByte of RAM could be connected at A000-BFFF, even though that could require
a tiny MBC-like circuit, but no real MBC chip.

MBC1 (max 2MByte ROM and/or 32KByte RAM)
----------------------------------------

This is the first MBC chip for the gameboy. Any newer MBC chips are working
similiar, so that is relative easy to upgrade a program from one MBC chip to
another - or even to make it compatible to several different types of MBCs.

Note that the memory in range 0000-7FFF is used for both reading from ROM, and
for writing to the MBCs Control Registers.

0000-3FFF - ROM Bank 00 (Read Only)
This area always contains the first 16KBytes of the cartridge ROM.

4000-7FFF - ROM Bank 01-7F (Read Only)
This area may contain any of the further 16KByte banks of the ROM, allowing to
address up to 125 ROM Banks (almost 2MByte). As described below, bank numbers
20h, 40h, and 60h cannot be used, resulting in the odd amount of 125 banks.

A000-BFFF - RAM Bank 00-03, if any (Read/Write)
This area is used to address external RAM in the cartridge (if any). External
RAM is often battery buffered, allowing to store game positions or high score
tables, even if the gameboy is turned off, or if the cartridge is removed from
the gameboy. Available RAM sizes are: 2KByte (at A000-A7FF), 8KByte (at
A000-BFFF), and 32KByte (in form of four 8K banks at A000-BFFF).

0000-1FFF - RAM Enable (Write Only)
Before external RAM can be read or written, it must be enabled by writing to
this address space. It is recommended to disable external RAM after accessing
it, in order to protect its contents from damage during power down of the
gameboy. Usually the following values are used:
  00h  Disable RAM (default)
  0Ah  Enable RAM
Practically any value with 0Ah in the lower 4 bits enables RAM, and any other
value disables RAM.

2000-3FFF - ROM Bank Number (Write Only)
Writing to this address space selects the lower 5 bits of the ROM Bank Number
(in range 01-1Fh). When 00h is written, the MBC translates that to bank 01h
also. That doesn't harm so far, because ROM Bank 00h can be always directly
accessed by reading from 0000-3FFF.
But (when using the register below to specify the upper ROM Bank bits), the
same happens for Bank 20h, 40h, and 60h. Any attempt to address these ROM
Banks will select Bank 21h, 41h, and 61h instead.

4000-5FFF - RAM Bank Number - or - Upper Bits of ROM Bank Number (Write Only)
This 2bit register can be used to select a RAM Bank in range from 00-03h, or
to specify the upper two bits (Bit 5-6) of the ROM Bank number, depending on
the current ROM/RAM Mode. (See below.)

6000-7FFF - ROM/RAM Mode Select (Write Only)
This 1bit Register selects whether the two bits of the above register should
be used as upper two bits of the ROM Bank, or as RAM Bank Number.
  00h = ROM Banking Mode (up to 8KByte RAM, 2MByte ROM) (default)
  01h = RAM Banking Mode (up to 32KByte RAM, 512KByte ROM)
The program may freely switch between both modes, the only limitiation is that
only RAM Bank 00h can be used during Mode 0, and only ROM Banks 00-1Fh can be
used during Mode 1.

MBC2 (max 256KByte ROM and 512x4 bits RAM)
------------------------------------------

0000-3FFF - ROM Bank 00 (Read Only)
Same as for MBC1.

4000-7FFF - ROM Bank 01-0F (Read Only)
Same as for MBC1, but only a total of 16 ROM banks is supported.

A000-A1FF - 512x4bits RAM, built-in into the MBC2 chip (Read/Write)
The MBC2 doesn't support external RAM, instead it includes 512x4 bits of
built-in RAM (in the MBC2 chip itself). It still requires an external battery
to save data during power-off though.
As the data consists of 4bit values, only the lower 4 bits of the "bytes" in
this memory area are used.

0000-1FFF - RAM Enable (Write Only)
The least significant bit of the upper address byte must be zero to
enable/disable cart RAM. For example the following addresses can be used to
enable/disable cart RAM: 0000-00FF, 0200-02FF, 0400-04FF, ..., 1E00-1EFF.
The suggested address range to use for MBC2 ram enable/disable is 0000-00FF.

2000-3FFF - ROM Bank Number (Write Only)
Writing a value (XXXXBBBB - X = Don't cares, B = bank select bits) into
2000-3FFF area will select an appropriate ROM bank at 4000-7FFF.

The least significant bit of the upper address byte must be one to select a
ROM bank. For example the following addresses can be used to select a ROM
bank: 2100-21FF, 2300-23FF, 2500-25FF, ..., 3F00-3FFF.
The suggested address range to use for MBC2 rom bank selection is 2100-21FF.

MBC3 (max 2MByte ROM and/or 32KByte RAM and Timer)
--------------------------------------------------

Beside for the ability to access up to 2MB ROM (128 banks), and 32KB RAM (4
banks), the MBC3 also includes a built-in Real Time Clock (RTC). The RTC
requires an external 32.768 kHz Quartz Oscillator, and an external battery (if
it should continue to tick when the gameboy is turned off).

0000-3FFF - ROM Bank 00 (Read Only)
Same as for MBC1.

4000-7FFF - ROM Bank 01-7F (Read Only)
Same as for MBC1, except that accessing banks 20h, 40h, and 60h is supported
now.

A000-BFFF - RAM Bank 00-03, if any (Read/Write)
A000-BFFF - RTC Register 08-0C (Read/Write)
Depending on the current Bank Number/RTC Register selection (see below), this
memory space is used to access an 8KByte external RAM Bank, or a single RTC
Register.

0000-1FFF - RAM and Timer Enable (Write Only)
Mostly the same as for MBC1, a value of 0Ah will enable reading and writing to
external RAM - and to the RTC Registers! A value of 00h will disable either.

2000-3FFF - ROM Bank Number (Write Only)
Same as for MBC1, except that the whole 7 bits of the RAM Bank Number are
written directly to this address. As for the MBC1, writing a value of 00h,
will select Bank 01h instead. All other values 01-7Fh select the corresponding
ROM Banks.

4000-5FFF - RAM Bank Number - or - RTC Register Select (Write Only)
As for the MBC1s RAM Banking Mode, writing a value in range for 00h-03h maps
the corresponding external RAM Bank (if any) into memory at A000-BFFF.
When writing a value of 08h-0Ch, this will map the corresponding RTC register
into memory at A000-BFFF. That register could then be read/written by
accessing any address in that area, typically that is done by using address
A000.

6000-7FFF - Latch Clock Data (Write Only)
When writing 00h, and then 01h to this register, the current time becomes
latched into the RTC registers. The latched data will not change until it
becomes latched again, by repeating the write 00h->01h procedure.
This is supposed for  from the RTC registers. It is proof to read the
latched (frozen) time from the RTC registers, while the clock itself continues
to tick in background.

The Clock Counter Registers
  08h  RTC S   Seconds   0-59 (0-3Bh)
  09h  RTC M   Minutes   0-59 (0-3Bh)
  0Ah  RTC H   Hours     0-23 (0-17h)
  0Bh  RTC DL  Lower 8 bits of Day Counter (0-FFh)
  0Ch  RTC DH  Upper 1 bit of Day Counter, Carry Bit, Halt Flag
        Bit 0  Most significant bit of Day Counter (Bit 8)
        Bit 6  Halt (0=Active, 1=Stop Timer)
        Bit 7  Day Counter Carry Bit (1=Counter Overflow)
The Halt Flag is supposed to be set before  to the RTC Registers.

The Day Counter
The total 9 bits of the Day Counter allow to count days in range from 0-511
(0-1FFh). The Day Counter Carry Bit becomes set when this value overflows. In
that case the Carry Bit remains set until the program does reset it.
Note that you can store an offset to the Day Counter in battery RAM. For
example, every time you read a non-zero Day Counter, add this Counter to the
offset in RAM, and reset the Counter to zero. This method allows to count any
number of days, making your program Year-10000-Proof, provided that the
cartridge gets used at least every 511 days.

Delays
When accessing the RTC Registers it is recommended to execute a 4ms delay (4
Cycles in Normal Speed Mode) between the separate accesses.

HuC1 (MBC with Infrared Controller)
------------------------------------

This controller (made by Hudson Soft) appears to be very similar to an MBC1
with the main difference being that it supports infrared LED input / output.
(Similiar to the infrared port that has been later invented in CGBs.)

The Japanese cart "Fighting Phoenix" (internal cart name: SUPER B DAMAN) is
known to contain this chip.

MBC Timing Issues
-----------------

Using MBCs with CGB Double Speed Mode
The MBC5 has been designed to support CGB Double Speed Mode.
There have been rumours that older MBCs (like MBC1-3) wouldn't be fast enough
in that mode. If so, it might be nethertheless possible to use Double Speed
during periods which use only code and data which is located in internal RAM.
However, despite of the above, my own good old selfmade MBC1-EPROM card
appears to work stable and fine even in Double Speed Mode though.

Gamegenie/Shark Cheats
----------------------

Game Shark and Gamegenie are external cartridge adapters that can be plugged
between the gameboy and the actual game cartridge. Hexadecimal codes can be
then entered for specific games, typically providing things like Infinite Sex,
255 Cigarettes, or Starting directly in Wonderland Level PRO, etc.

Gamegenie (ROM patches)
Gamegenie codes consist of nine-digit hex numbers, formatted as ABC-DEF-GHI,
the meaning of the separate digits is:
  AB    New data
  FCDE  Memory address, XORed by 0F000h
  GI    Old data, XORed by 0BAh and rotated left by two
  H     Don't know, maybe checksum and/or else
The address should be located in ROM area 0000h-7FFFh, the adapter permanently
compares address/old data with address/data being read by the game, and
replaces that data by new data if necessary. That method (more or less)
prohibits unwanted patching of wrong memory banks. Eventually it is also
possible to patch external RAM ?
Newer devices reportedly allow to specify only the first six digits
(optionally). As far as I rememeber, around three or four codes can be used
simultaneously.

Game Shark (RAM patches)
Game Shark codes consist of eight-digit hex numbers, formatted as ABCDEFGH,
the meaning of the separate digits is:
  AB    External RAM bank number
  CD    New Data
  GHEF  Memory Address (internal or external RAM, A000-DFFF)
As far as I understand, patching is implement by hooking the original VBlank
interrupt handler, and re-writing RAM values each frame. The downside is that
this method steals some CPU time, also, it cannot be used to patch program
code in ROM.
As far as I rememeber, somewhat 10-25 codes can be used simultaneously.

Power Up Sequence
-----------------

When the GameBoy is powered up, a 256 byte program starting at memory location
0 is executed. This program is located in a ROM inside the GameBoy. The first
thing the program does is read the cartridge locations from $104 to $133 and
place this graphic of a Nintendo logo on the screen at the top. This image is
then scrolled until it is in the middle of the screen. Two musical notes are
then played on the internal speaker. Again, the cartridge locations $104 to
$133 are read but this time they are compared with a table in the internal
rom. If any byte fails to compare, then the GameBoy stops comparing bytes and
simply halts all operations. If all locations compare the same, then the
GameBoy starts adding all of the bytes in the cartridge from $134 to $14d. A
value of 25 decimal is added to this total. If the least significant byte of
the result is a not a zero, then the GameBoy will stop doing anything. If it
is a zero, then the internal ROM is disabled and cartridge program execution
begins at location $100 with the following register values:

   AF=$01B0
   BC=$0013
   DE=$00D8
   HL=$014D
   Stack Pointer=$FFFE
   [$FF05] = $00   ; TIMA
   [$FF06] = $00   ; TMA
   [$FF07] = $00   ; TAC
   [$FF10] = $80   ; NR10
   [$FF11] = $BF   ; NR11
   [$FF12] = $F3   ; NR12
   [$FF14] = $BF   ; NR14
   [$FF16] = $3F   ; NR21
   [$FF17] = $00   ; NR22
   [$FF19] = $BF   ; NR24
   [$FF1A] = $7F   ; NR30
   [$FF1B] = $FF   ; NR31
   [$FF1C] = $9F   ; NR32
   [$FF1E] = $BF   ; NR33
   [$FF20] = $FF   ; NR41
   [$FF21] = $00   ; NR42
   [$FF22] = $00   ; NR43
   [$FF23] = $BF   ; NR30
   [$FF24] = $77   ; NR50
   [$FF25] = $F3   ; NR51
   [$FF26] = $F1-GB, $F0-SGB ; NR52
   [$FF40] = $91   ; LCDC
   [$FF42] = $00   ; SCY
   [$FF43] = $00   ; SCX
   [$FF45] = $00   ; LYC
   [$FF47] = $FC   ; BGP
   [$FF48] = $FF   ; OBP0
   [$FF49] = $FF   ; OBP1
   [$FF4A] = $00   ; WY
   [$FF4B] = $00   ; WX
   [$FFFF] = $00   ; IE

It is not a good idea to assume the above values will always exist. A later
version GameBoy could contain different values than these at reset. Always set
these registers on reset rather than assume they are as above.

Please note that GameBoy internal RAM on power up contains random data. All of
the GameBoy emulators tend to set all RAM to value $00 on entry.

Cart RAM the first time it is accessed on a real GameBoy contains random data.
It will only contain known data if the GameBoy code initializes it to some
value.

Reducing Power Consumption
--------------------------

The following can be used to recude the power consumption of the gameboy, and
to extend the life of the batteries.

--> PWR Using the HALT Instruction
--> PWR Using the STOP Instruction
--> PWR Disabeling the Sound Controller
--> PWR Not using CGB Double Speed Mode
--> PWR Using the Skills

PWR Using the HALT Instruction
------------------------------

It is recommended that the HALT instruction be used whenever possible to
reduce power consumption & extend the life of the batteries. This command
stops the system clock reducing the power consumption of both the CPU and ROM.

The CPU will remain suspended until an interrupt occurs at which point the
interrupt is serviced and then the instruction immediately following the HALT
is executed.

Depending on how much CPU time is required by a game, the HALT instruction can
extend battery life anywhere from 5 to 50% or possibly more.

When waiting for a vblank event, this would be a BAD example:
  @@wait:
   ld   a,(0FF44h)      ;LY
   cp   a,144
   jr   nz,@@wait

A better example would be a procedure as shown below. In this case the vblank
interrupt must be enabled, and your vblank interrupt procedure must set
vblank_flag to a non-zero value.
   ld   hl,vblank_flag  ;hl=pointer to vblank_flag
   xor  a               ;a=0
  @@wait:               ;wait...
   halt                 ;suspend CPU - wait for ANY interrupt
   cp   a,(hl)          ;vblank flag still zero?
   jr   z,@@wait        ;wait more if zero
   ld   (hl),a          ;set vblank_flag back to zero
The vblank_flag is used to determine whether the HALT period has been
terminated by a vblank interrupt, or by another interrupt. In case that your
program has all other interrupts disabled, then it would be proof to replace
the above procedure by a single HALT instruction.

PWR Using the STOP Instruction
------------------------------

The STOP instruction is intended to switch the gameboy into VERY low power
standby mode. For example, a program may use this feature when it hasn't
sensed keyboard input for a longer period (assuming that somebody forgot to
turn off the gameboy).

Before invoking STOP, it might be required to disable Sound and Video manually
(as well as IR-link port in CGB). Much like HALT, the STOP state is terminated
by interrupt events - in this case this would be commonly a joypad interrupt.
The joypad register might be required to be prepared for STOP either.

PWR Disabeling the Sound Controller
-----------------------------------

If your programs doesn't use sound at all (or during some periods) then write
00h to register FF26 to save 16% or more on GB power consumption.
Sound can be turned back on by writing 80h to the same register, all sound
registers must be then re-initialized.
When the gameboy becomes turned on, sound is enabled by default, and must be
turned off manually when not used.

PWR Not using CGB Double Speed Mode
-----------------------------------

Because CGB Double Speed mode consumes more power, it'd be recommended to use
normal speed when possible.
There's limited ability to switch between both speeds, for example, a game
might use normal speed in the title screen, and double speed in the game, or
vice versa.
However, during speed switch the display collapses for a short moment, so that
it'd be no good idea to alter speeds within active game or title screen
periods.

PWR Using the Skills
--------------------

Most of the above power saving methods will produce best results when using
efficient and tight assembler code which requires as less CPU power as
possible. Thus, experienced old-school programmers will (hopefully) produce
lower power consumption, as than HLL-programming teenagers, for example.

Sprite RAM Bug
--------------

There is a flaw in the GameBoy hardware that causes trash to be written to OAM
RAM if the following commands are used while their 16-bit content is in the
range of $FE00 to $FEFF:
  inc rr        dec rr          ;rr = bc,de, or hl
  ldi a,(hl)    ldd a,(hl)
  ldi (hl),a    ldd (hl),a
Only sprites 1 & 2 ($FE00 & $FE04) are not affected by these instructions.

External Connectors
-------------------

Cartridge Slot
  Pin   Name    Expl.
  1     VDD     Power Supply +5V DC
  2     PHI     System Clock
  3     /WR     Write
  4     /RD     Read
  5     /CS     Chip Select
  6-21  A0-A15  Address Lines
  22-29 D0-D7   Data Lines
  30    /RES    Reset signal
  31    VIN     External Sound Input
  32    GND     Ground

Link Port
Pin numbers are arranged as 2,4,6 in upper row, 1,3,5 un lower row; outside
view of gameboy socket; flat side of socket upside.
Colors as used in most or all standard link cables, because SIN and SOUT are
crossed, colors Red and Orange are exchanged at one cable end.
  Pin Name Color  Expl.
  1   VCC  -      +5V DC
  2   SOUT red    Data Out
  3   SIN  orange Data In
  4   P14  -      Not used
  5   SCK  green  Shift Clock
  6   GND  blue   Ground
Note: The original gameboy used larger plugs (unlike pocket gameboys and
newer), linking between older/newer gameboys is possible by using cables with
one large and one small plug though.

Stereo Sound Connector (3.5mm, female)
  Pin     Expl.
  Tip     Sound Left
  Middle  Sound Right
  Base    Ground

External Power Supply
...



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