Optimizing For GBDK

    Global variables
    Use as many global variables as you can; the Game Boy has a lot of RAM compared to other platforms such as the NES, but is slow at using the stack. 
    Thus, minimizing the number of local variables, especially in heavily-called functions, will reduce the time spent manipulating the stack.
    Optimized code
    SDCC doesn't seem to have optimization passes, so you should write code as efficient as possible. There will be a readability tradeoff after some point, 
    so I recommend you get the comment machine gun out and put some everywhere.
    Inlining
    Avoid using functions if you can inline them, which skips passing all arguments to the stack, mostly. Macros will be your friends there.
    NEVER use recursive functions
    NEVER use printf
    printf clobbers a sizeable chunk of VRAM with unnecessary text tiles (unless you absolutely need hebrew in your game, I s'pose?). Instead, you should 
    sprintf to a buffer in WRAM, then put that on the screen. Bonus: this lets you use a custom font!
    Geometry funcs
    Avoid the functions that draw geometry on-screen (lines, rectangles, etc.). The Game Boy isn't designed for this kind of drawing method, and you 
    will have a hard time mixing this with, say, background art. Plus, the functions are super slow.
    const (very important!)
    Declaring something as const severely reduces its size in the ROM, RAM usage, and CPU usage.
    The technical reason behind that is that non-const values, especially arrays, are loaded to RAM from ROM in an extremely inefficient way. 
    This takes up a LOT more ROM, and copies the value(s) to RAM when it's unneeded. (And the GB does not have enough RAM for that to be viable.)
    Don't use MBC1
    MBC1 is often assumed to be the simplest of all MBCs... but it has a quirk that adds some overhead every time ROM or SRAM bank switches are performed. 
    MBC3 and MBC5 don't have this quirk, and don't add any complexity. Using MBC1 has no real use. (Let's not talk about MBC2, either.)

============================================================================
            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
 
                      Last update 9-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.
 
 
Game Boy Specs
--------------
 
 CPU: 8-bit (Similar to the Z80 processor.)
 Main RAM: 8K Byte
 Video RAM: 8K Byte
 Screen Size 2.6"
 Resolution: 160x144 (20x18 tiles)
 Max # of sprites: 40
 Max # sprites/line: 10
 Max sprite size: 8x16
 Min sprite size: 8x8
 Clock Speed: 4.194304 MHz (4.295454 MHz for Super GB)
 Horiz Sync: 9198 KHz (9420 KHz for Super GB)
 Vert Sync: 59.73 Hz (61.17 Hz for Super GB)
 Sound: 4 channels with stereo sound
 Power: DC6V 0.7W (DC3V 0.7W for GB Pocket)
 
 
Processor
---------
 
  The GameBoy uses a computer chip similar to an Intel 8080.
 It contains all of the instructions of an 8080 except there
 are no exchange instructions. In many ways the processor is
 more similar to the Zilog Z80 processor. Compared to the
 Z80, some instructions have been added and some have been
 taken away.
 
 The following are added instructions:
 
  ADD  SP,nn             ;nn = signed byte
  LDI  (HL),A            ;Write A to (HL) and increment HL
  LDD  (HL),A            ;Write A to (HL) and decrement HL
  LDI  A,(HL)            ;Write (HL) to A and increment HL
  LDD  A,(HL)            ;Write (HL) to A and decrement HL
  LD  A,($FF00%20nn)
  LD  A,($FF00%20C)
  LD  ($FF00%20nn),A
  LD  ($FF00%20C),A
  LD  (nnnn),SP
  LD  HL,(SP%20nn)         ;nn = signed byte
  STOP                   ;?
  SWAP r                 ;Rotate register r by 4 bits
 
 The following instructions have been removed:
 
  Any command that uses the IX or IY registers.
  All IN/OUT instructions.
  All exchange instructions.
  All commands prefixed by ED (except remapped RETI).
  All conditional jumps/calls/rets on parity/overflow and sign flag.
 
 The following instructions have different opcodes:
 
  LD  A,[nnnn]
  LD  [nnnn],A
  RETI
 
 
General Memory Map*                  Hardware Write Registers
------------------                   ------------------------
 
  Interrupt Enable Register
 --------------------------- FFFF
  Internal RAM
 --------------------------- FF80
  Empty but unusable for I/O
 --------------------------- FF4C
  I/O ports
 --------------------------- FF00
  Empty but unusable for I/O
 --------------------------- FEA0
  Sprite Attrib Memory (OAM)
 --------------------------- FE00
  Echo of 8kB Internal RAM
 --------------------------- E000
  8kB Internal RAM
 --------------------------- C000       -------------------------
  8kB switchable RAM bank              /      MBC1 ROM/RAM Select
 --------------------------- A000     /  ------------------------
  8kB Video RAM                      /  /     RAM Bank Select
 --------------------------- 8000 --/  /  -----------------------
  16kB switchable ROM bank   6000 ----/  /    ROM Bank Select
 --------------------------- 4000 ------/  ----------------------
  16kB ROM bank #0           2000 --------/   RAM Bank enable
 --------------------------- 0000 -------------------------------
 
 * NOTE: b = bit, B = byte
 
 
Echo of 8kB Internal RAM
------------------------
 
 The addresses E000-FE00 appear to access the internal RAM
the same as C000-DE00. (i.e. If you write a byte to address
E000 it will appear at C000 and E000. Similarly, writing a
byte to C000 will appear at C000 and E000.)
 
 
User I/O
--------
 
 There are no empty spaces in the memory map for
implementing input ports except the switchable RAM bank
area (not an option on the Super Smart Card since it's
RAM bank is always enabled).
 
 An output only port may be implemented anywhere between
A000-FDFF. If implemented in a RAM area care should be
taken to use an area of RAM not used for anything else.
(FE00 and above can't be used because the CPU doesn't
generate an external /WR for these locations.)
 
 If you have a cart with an MBC1, a ROM 4Mbit or smaller,
and a RAM 8Kbyte or smaller (or no RAM) then you can use
pins 6 & 7 of the MBC1 for 2 digital output pins for
whatever purpose you wish. To use them you must first
put the MBC1 into 4MbitROM/32KbyteRAM mode by writing
01 to 6000. The two least significant bits you write
to 4000 will then be output to these pins.
 
 
Reserved Memory Locations
-------------------------
 
0000       Restart $00 Address (RST $00 calls this address.)
 
0008       Restart $08 Address (RST $08 calls this address.)
 
0010       Restart $10 Address (RST $10 calls this address.)
 
0018       Restart $18 Address (RST $18 calls this address.)
 
0020       Restart $20 Address (RST $20 calls this address.)
 
0028       Restart $28 Address (RST $28 calls this address.)
 
0030       Restart $30 Address (RST $30 calls this address.)
 
0038       Restart $38 Address (RST $38 calls this address.)
 
0040       Vertical Blank Interrupt Start Address
 
0048       LCDC Status Interrupt Start Address
 
0050       Timer Overflow Interrupt Start Address
 
0058       Serial Transfer Completion Interrupt Start Address
 
0060       High-to-Low of P10-P13 Interrupt Start Address
 
An internal information area is located at 0100-014F in
each cartridge. It contains the following values:
 
0100-0103  This is the begin code execution point in a
           cart. Usually there is a NOP and a JP
           instruction here but not always.
 
0104-0133  Scrolling Nintendo graphic:
           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
           ( PROGRAM WON'T RUN IF CHANGED!!!)
 
0134-0142  Title of the game in UPPER CASE ASCII. If it
           is less than 16 characters then the remaining
           bytes are filled with 00's.
 
0143       $80 = Color GB, $00 or other = not Color GB
 
0144       Ascii hex digit, high nibble of licensee code (new).
0145       Ascii hex digit, low nibble of licensee code (new).
           (These are normally $00 if [$014B] <> $33.)
 
0146       GB/SGB Indicator (00 = GameBoy, 03 = Super GameBoy functions)
           (Super GameBoy functions won't work if <> $03.)
 
0147       Cartridge type:
           0 - ROM ONLY                12 - ROM%20MBC3%20RAM
           1 - ROM%20MBC1                13 - ROM%20MBC3%20RAM%20BATT
           2 - ROM%20MBC1%20RAM            19 - ROM%20MBC5
           3 - ROM%20MBC1%20RAM%20BATT       1A - ROM%20MBC5%20RAM
           5 - ROM%20MBC                 1B - ROM%20MBC5%20RAM%20BATT
           6 - ROM%20MBC2%20BATTERY        1C - ROM%20MBC5%20RUMBLE
           8 - ROM%20RAM                 1D - ROM%20MBC5%20RUMBLE%20SRAM
           9 - ROM%20RAM%20BATTERY         1E - ROM%20MBC5%20RUMBLE%20SRAM%20BATT
           B - ROM%20MMM01               1F - Pocket Camera
           C - ROM%20MMM01%20SRAM          FD - Bandai TAMA5
           D - ROM%20MMM01%20SRAM%20BATT     FE - Hudson HuC-3
           F - ROM%20MBC3%20TIMER%20BATT     FF - Hudson HuC-1
          10 - ROM%20MBC3%20TIMER%20RAM%20BATT
          11 - ROM%20MBC3
 
0148       ROM size:
             0 - 256Kbit =  32KByte =   2 banks
             1 - 512Kbit =  64KByte =   4 banks
             2 -   1Mbit = 128KByte =   8 banks
             3 -   2Mbit = 256KByte =  16 banks
             4 -   4Mbit = 512KByte =  32 banks
             5 -   8Mbit =   1MByte =  64 banks
             6 -  16Mbit =   2MByte = 128 banks
           $52 -   9Mbit = 1.1MByte =  72 banks
           $53 -  10Mbit = 1.2MByte =  80 banks
           $54 -  12Mbit = 1.5MByte =  96 banks
 
0149       RAM size:
           0 - None
           1 -  16kBit =  2kB = 1 bank
           2 -  64kBit =  8kB = 1 bank
           3 - 256kBit = 32kB = 4 banks
           4 -   1MBit =128kB =16 banks
 
014A       Destination code:
           0 - Japanese
           1 - Non-Japanese
 
014B       Licensee code (old):
           33 - Check 0144/0145 for Licensee code.
           79 - Accolade
           A4 - Konami
           (Super GameBoy function won't work if <> $33.)
 
014C       Mask ROM Version number (Usually $00)
 
014D       Complement check
           (PROGRAM WON'T RUN ON GB IF NOT CORRECT!!!)
           (It will run on Super GB, however, if incorrect.)
 
014E-014F  Checksum (higher byte first) produced by
           adding all bytes of a cartridge except for two
           checksum bytes and taking two lower bytes of
           the result. (GameBoy ignores this value.)
 
 
Cartridge Types
---------------
 
The following define the byte at cart location 0147:
 
 ROM ONLY
  This is a 32kB (256kb) ROM and occupies 0000-7FFF.
 
 MBC1 (Memory Bank Controller 1)
   MBC1 has two different maximum memory modes:
  16Mbit ROM/8KByte RAM  or  4Mbit ROM/32KByte RAM.
 
   The MBC1 defaults to 16Mbit ROM/8KByte RAM mode
  on power up. Writing a value (XXXXXXXS - X = Don't
  care, S = Memory model select) into 6000-7FFF area
  will select the memory model to use. S = 0 selects
  16/8 mode. S = 1 selects 4/32 mode.
 
   Writing a value (XXXBBBBB - X = Don't cares, B =
  bank select bits) into 2000-3FFF area will select an
  appropriate ROM bank at 4000-7FFF. Values of 0 and 1
  do the same thing and point to ROM bank 1. Rom bank 0
  is not accessible from 4000-7FFF and can only be read
  from 0000-3FFF.
 
  If memory model is set to 4/32:
    Writing a value (XXXXXXBB - X = Don't care, B =
   bank select bits) into 4000-5FFF area will select an
   appropriate RAM bank at A000-C000. Before you can
   read or write to a RAM bank you have to enable it by
   writing a XXXX1010 into 0000-1FFF area*. To disable
   RAM bank operations write any value but XXXX1010
   into 0000-1FFF area. Disabling a RAM bank probably
   protects that bank from false writes during power
   down of the GameBoy. (NOTE: Nintendo suggests values
   $0A to enable and $00 to disable RAM bank!!)
 
  If memory model is set to 16/8 mode:
    Writing a value (XXXXXXBB - X = Don't care, B =
   bank select bits) into 4000-5FFF area will set the
   two most significant ROM address lines.
 
  * NOTE: The Super Smart Card doesn't require this
   operation because it's RAM bank is ALWAYS enabled.
   Include this operation anyway to allow your code
   to work with both.
 
 MBC2 (Memory Bank Controller 2):
   This memory controller works much like the MBC1
  controller with the following exceptions:
 
   MBC2 will work with ROM sizes up to 2Mbit.
 
   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.
 
   RAM switching is not provided. Unlike the MBC1 which
  uses external RAM, MBC2 has 512 x 4 bits of RAM which
  is in the controller itself. It still requires an
  external battery to save data during power-off though.
 
   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.
 
   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.
 
 HuC1 (Memory Bank / 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.
  The Japanese cart "Fighting Phoenix" (internal cart
  name: SUPER B DAMAN) is known to contain this chip.
 
 
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.
 
 GB & GB Pocket:
      Next, 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.
 
 Super GB:
      Even though the GB & GBP check the memory locations
      from $134 to $14d, the SGB doesn't.
 
  If the above checks pass then the internal ROM is
 disabled and cartridge program execution begins at
 location $100 with the following register values:
 
   AF=$01-GB/SGB, $FF-GBP, $11-GBC
   F =$B0
   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.
 
 
Low-Power Mode
--------------
 
  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.
 
 WARNING: On the Color GB (and possibly on the other
 versions of GB as well) sometimes the instruction
 immediately following the HALT instruction is skipped.
 As a result, always put a NOP after the HALT instruction.
 
  Below is suggested code for GameBoy programs:
 
  ; ** Main Game Loop **
  Main:
        halt                    ; stop system clock
                                ; return from halt when interrupted
        nop                     ; (See WARNING above.)
 
        ld      a,(VblnkFlag)
        or      a               ; V-Blank interrupt ?
        jr      z,Main          ; No, some other interrupt
 
        xor     a
        ld      (VblnkFlag),a   ; Clear V-Blank flag
 
        call    Controls        ; button inputs
        call    Game            ; game operation
 
        jr      Main
 
  ; ** V-Blank Interrupt Routine **
  Vblnk:
        push    af
        push    bc
        push    de
        push    hl
 
        call    SpriteDma       ; Do sprite updates
 
        ld      a,1
        ld      (VblnkFlag),a
 
        pop     hl
        pop     de
        pop     bc
        pop     af
        reti
 
 
Video
-----
 
  The main GameBoy screen buffer (background) consists
 of 256x256 pixels or 32x32 tiles (8x8 pixels each). Only
 160x144 pixels can be displayed on the screen. Registers
 SCROLLX and SCROLLY hold the coordinates of background to
 be displayed in the left upper corner of the screen.
 Background wraps around the screen (i.e. when part of it
 goes off the screen, it appears on the opposite side.)
 
  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.
 
  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
 WNDPOSX and WNDPOSY registers. Screen coordinates of the
 top left corner of a window are WNDPOSX-7,WNDPOSY. The
 tile numbers for the window are stored in the Tile Data
 Table. None of the windows tiles are ever transparent.
 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.
 
  If the window is used and a scan line interrupt disables
 it (either by writing to LCDC or by setting WX > 166)
 and a scan line interrupt a little later on enables it
 then the window will resume appearing on the screen at the
 exact position of the window where it left off earlier.
 This way, even if there are only 16 lines of useful graphics
 in the window, you could display the first 8 lines at the
 top of the screen and the next 8 lines at the bottom if
 you wanted to do so.
 
  WX may be changed during a scan line interrupt (to either
 cause a graphic distortion effect or to disable the window
 (WX>166) ) but changes to WY are not dynamic and won't
 be noticed until the next screen redraw.
 
  The tile images are stored in the Tile Pattern Tables.
 Each 8x8 image occupies 16 bytes, where each 2 bytes
 represent a line:
 
  Tile:                                     Image:
 
  .33333..                     .33333.. -> 01111100 -> $7C
  22...22.                                 01111100 -> $7C
  11...11.                     22...22. -> 00000000 -> $00
  2222222. <-- digits                      11000110 -> $C6
  33...33.     represent       11...11. -> 11000110 -> $C6
  22...22.     color                       00000000 -> $00
  11...11.     numbers         2222222. -> 00000000 -> $00
  ........                                 11111110 -> $FE
                               33...33. -> 11000110 -> $C6
                                           11000110 -> $C6
                               22...22. -> 00000000 -> $00
                                           11000110 -> $C6
                               11...11. -> 11000110 -> $C6
                                           00000000 -> $00
                               ........ -> 00000000 -> $00
                                           00000000 -> $00
 
  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, the background, and the window display. 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.
 
 
Sprites
------
 
  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 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. OAM is divided
 into 40 4-byte blocks each of which corresponds to a sprite.
 
  In 8x16 sprite mode, the least significant bit of the
 sprite pattern number is ignored and treated as 0.
 
  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.
 
  When sprites with the same x coordinate values overlap,
 they have priority according to table ordering. (i.e.
 $FE00 - highest, $FE04 - next highest, etc.)
 
  Please note that Sprite X=0, Y=0 hides a sprite. To
 display a sprite use the following formulas:
 
 SpriteScreenPositionX(Upper left corner of sprite) = SpriteX - 8
 SpriteScreenPositionY(Upper left corner of sprite) = SpriteY - 16
 
  To display a sprite in the upper left corner of the
 screen set sprite X=8, Y=16.
 
  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%2016. Just setting the X
 coordinate to X=0 or X=>160%208 on a sprite will hide it
 but it will still affect other sprites sharing the same
 lines.
 
 Blocks have the following
 format:
 
  Byte0  Y position on the screen
  Byte1  X position on the screen
  Byte2  Pattern number 0-255 (Unlike some tile
         numbers, sprite pattern numbers are unsigned.
         LSB is ignored (treated as 0) in 8x16 mode.)
  Byte3  Flags:
 
         Bit7  Priority
               If this bit is set to 0, sprite is displayed
               on top of background & window. If this bit
               is set to 1, then sprite will be hidden behind
               colors 1, 2, and 3 of the background & window.
               (Sprite only prevails over color 0 of BG & win.)
         Bit6  Y flip
               Sprite pattern is flipped vertically if
               this bit is set to 1.
         Bit5  X flip
               Sprite pattern is flipped horizontally if
               this bit is set to 1.
         Bit4  Palette number
               Sprite colors are taken from OBJ1PAL if
               this bit is set to 1 and from OBJ0PAL
               otherwise.
 
 
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 xx     (xx = bc,de, or hl)
  dec xx
 
  ldi a,(hl)
  ldd a,(hl)
 
  ldi (hl),a
  ldd (hl),a
 
  Only sprites 1 & 2 ($FE00 & $FE04) are not affected
 by these instructions.
 
 
Sound
-----
 
  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.
 
  When setting the initial value of the envelope and
 restarting the length counter, set the initial flag to 1
 and initialize the data.
 
  Under the following situations the Sound ON flag is
 reset and the sound output stops:
 
  1. When the sound output is stopped by the length counter.
  2. When overflow occurs at the addition mode while sweep
     is operating at sound 1.
 
  When the Sound OFF flag for sound 3 (bit 7 of NR30) is
 set at 0, the cancellation of the OFF mode must be done
 by setting the sound OFF flag to 1. By initializing
 sound 3, it starts it's function.
 
  When the All Sound OFF flag (bit 7 of NR52) is set to 0,
 the mode registers for sounds 1,2,3, and 4 are reset and
 the sound output stops. (NOTE: The setting of each sounds
 mode register must be done after the All Sound OFF mode
 is cancelled. During the All Sound OFF mode, each sound
 mode register cannot be set.)
 
  NOTE: DURING THE ALL SOUND OFF MODE, GB POWER CONSUMPTION
 DROPS BY 16% OR MORE! WHILE YOUR PROGRAMS AREN'T USING
 SOUND THEN SET THE ALL SOUND OFF FLAG TO 0. IT DEFAULTS
 TO 1 ON RESET.
 
  These tend to be the two most important equations in
 converting between Hertz and GB frequency registers:
 (Sounds will have a 2.4% higher frequency on Super GB.)
 
     gb = 2048 - (131072 / Hz)
 
     Hz  = 131072 / (2048 - gb)
 
 
Timer
-----
 
  Sometimes it's useful to have a timer that interrupts at
 regular intervals for routines that require periodic or
 percise updates. The timer in the GameBoy has a selectable
 frequency of 4096, 16384, 65536, or 262144 Hertz. This
 frequency increments the Timer Counter (TIMA). When it
 overflows, it generates an interrupt. It is then loaded
 with the contents of Timer Modulo (TMA). The following
 are examples:
 
 ;This interval timer interrupts 4096 times per second
 
     ld  a,-1
     ld  ($FF06),a     ;Set TMA to divide clock by 1
     ld  a,4
     ld  ($FF07),a     ;Set clock to 4096 Hertz
 
 ;This interval timer interrupts 65536 times per second
 
     ld  a,-4
     ld  ($FF06),a     ;Set TMA to divide clock by 4
     ld  a,5
     ld  ($FF07),a     ;Set clock to 262144 Hertz
 
 
Serial I/O
----------
 
  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
 so the programmer must be more creative when using this port.
 
  During a transfer, a byte is shifted in at the same time
 that a byte is shifted out. The rate of the shift is deter-
 mined by whether the clock source is internal or external.
 If internal, the bits are shifted out at a rate of 8192Hz
 (122 microseconds) per bit. 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.
 
  A programmer initates a serial transfer by setting bit 7
 of $FF02. This bit may be read and is automatically set
 to 0 at the completion of transfer. After this bit is set,
 an interrupt will then occur eight bit clocks later if the
 serial interrupt is enabled.
  If internal clock is selected and serial interrupt is
 enabled, this interrupt occurs 122*8 microseconds later.
  If external clock is selected and serial interrupt is
 enabled, an interrupt will occur eight bit clocks later.
 
  Initiating a serial transfer with external clock will
 wait forever if no external clock is present. This allows
 a certain amount of synchronization with each serial port.
 
  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
 
 
Interrupt Procedure
-------------------
 
  The IME (interrupt master enable) flag is reset by DI and
 prohibits all interrupts. It is set by EI and acknowledges
 the interrupt setting by the IE register.
 
 1. When an interrupt is generated, the IF flag will be set.
 2. If the IME flag is set & the corresponding IE flag is
    set, the following 3 steps are performed.
 3. Reset the IME flag and prevent all interrupts.
 4. The PC (program counter) is pushed onto the stack.
 5. Jump to the starting address of the interrupt.
 
  Resetting of the IF register, which was the cause of the
 interrupt, is done by hardware.
 
  During the interrupt, pushing of registers to be used
 should be performed by the interrupt routine.
 
  Once the interrupt service is in progress, all the
 interrupts will be prohibited. However, if the IME flag
 and the IE flag are controlled, a number of interrupt
 services can be made possible by nesting.
 
  Return from an interrupt routine can be performed by
 either RETI or RET instruction.
 
  The RETI instruction enables interrupts after doing a
 return operation.
 
  If a RET is used as the final instruction in an interrupt
 routine, interrupts will remain disabled unless a EI was
 used in the interrupt routine or is used at a later time.
 
  The interrupt will be acknowledged during opcode fetch
 period of each instruction.
 
 
Interrupt Descriptions
----------------------
 
  The following interrupts only occur if they have been
 enabled in the Interrupt Enable register ($FFFF) and
 if the interrupts have actually been enabled using the
 EI instruction.
 
  V-Blank -
 
   The V-Blank interrupt occurs ~59.7 times a second
   on a regular GB and ~61.1 times a second on a Super
   GB (SGB). This interrupt occurs at the beginning of
   the V-Blank period. During this period video hardware
   is not using video ram so it may be freely accessed.
   This period lasts approximately 1.1 milliseconds.
 
  LCDC Status -
 
   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.
 
  Timer Overflow -
 
   This interrupt occurs when the TIMA register ($FF05)
   changes from $FF to $00.
 
  Serial Transfer Completion -
 
   This interrupt occurs when a serial transfer has
   completed on the game link port.
 
  High-to-Low of P10-P13 -
 
   This interrupt occurs on a transition of any of the
   keypad input lines from high to low. Due to the fact
   that keypad "bounce"* is virtually always present,
   software should expect this interrupt to occur one
   or more times for every button press and one or more
   times for every button release.
 
   * - Bounce tends to be a side effect of any button
      making or breaking a connection. During these
      periods, it is very common for a small amount of
      oscillation between high & low states to take place.
 
I/O Registers
-------------
 
FF00
   Name     - P1
   Contents - Register for reading joy pad info
              and determining system type.    (R/W)
 
           Bit 7 - Not used
           Bit 6 - Not used
           Bit 5 - P15 out port
           Bit 4 - P14 out port
           Bit 3 - P13 in port
           Bit 2 - P12 in port
           Bit 1 - P11 in port
           Bit 0 - P10 in port
 
         This is the matrix layout for register $FF00:
 
 
                 P14        P15
                  |          |
        P10-------O-Right----O-A
                  |          |
        P11-------O-Left-----O-B
                  |          |
        P12-------O-Up-------O-Select
                  |          |
        P13-------O-Down-----O-Start
                  |          |
 
       Example code:
 
          Game: Ms. Pacman
          Address: $3b1
 
        LD A,$20       <- bit 5 = $20
        LD ($FF00),A   <- select P14 by setting it low
        LD A,($FF00)
        LD A,($FF00)   <- wait a few cycles
        CPL            <- complement A
        AND $0F        <- get only first 4 bits
        SWAP A         <- swap it
        LD B,A         <- store A in B
        LD A,$10
        LD ($FF00),A   <- select P15 by setting it low
        LD A,($FF00)
        LD A,($FF00)
        LD A,($FF00)
        LD A,($FF00)
        LD A,($FF00)
        LD A,($FF00)   <- Wait a few MORE cycles
        CPL            <- complement (invert)
        AND $0F        <- get first 4 bits
        OR B           <- put A and B together
 
        LD B,A         <- store A in D
        LD A,($FF8B)   <- read old joy data from ram
        XOR B          <- toggle w/current button bit
        AND B          <- get current button bit back
        LD ($FF8C),A   <- save in new Joydata storage
        LD A,B         <- put original value in A
        LD ($FF8B),A   <- store it as old joy data
 
 
        LD A,$30       <- deselect P14 and P15
        LD ($FF00),A   <- RESET Joypad
        RET            <- Return from Subroutine
 
          The button values using the above method are such:
          $80 - Start             $8 - Down
          $40 - Select            $4 - Up
          $20 - B                 $2 - Left
          $10 - A                 $1 - Right
 
          Let's say we held down A, Start, and Up.
          The value returned in accumulator A would be $94
 
 
FF01
   Name     - SB
   Contents - Serial transfer data (R/W)
 
              8 Bits of data to be read/written
 
FF02
   Name     - SC
   Contents - SIO control  (R/W)
 
              Bit 7 - Transfer Start Flag
                      0: Non transfer
                      1: Start transfer
 
              Bit 0 - Shift Clock
                      0: External Clock (500KHz Max.)
                      1: Internal Clock (8192Hz)
 
               Transfer is initiated by setting the
              Transfer Start Flag. This bit may be read
              and is automatically set to 0 at the end of
              Transfer.
 
               Transmitting and receiving serial data is
              done simultaneously. The received data is
              automatically stored in SB.
 
FF04
   Name     - DIV
   Contents - Divider Register (R/W)
 
              This register is incremented 16384 (~16779
              on SGB) times a second. Writing any value
              sets it to $00.
FF05
   Name     - TIMA
   Contents - Timer counter (R/W)
 
              This timer is incremented by a clock frequency
              specified by the TAC register ($FF07). The timer
              generates an interrupt when it overflows.
 
FF06
   Name     - TMA
   Contents - Timer Modulo (R/W)
 
              When the TIMA overflows, this data will be loaded.
 
FF07
   Name     - TAC
   Contents - Timer Control (R/W)
 
              Bit 2 - Timer Stop
                      0: Stop Timer
                      1: Start Timer
 
              Bits 1%200 - Input Clock Select
                         00: 4.096 KHz    (~4.194 KHz SGB)
                         01: 262.144 KHz  (~268.4 KHz SGB)
                         10: 65.536 KHz   (~67.11 KHz SGB)
                         11: 16.384 KHz   (~16.78 KHz SGB)
 
FF0F
   Name     - IF
   Contents - Interrupt Flag (R/W)
 
              Bit 4: Transition from High to Low of Pin number P10-P13
              Bit 3: Serial I/O transfer complete
              Bit 2: Timer Overflow
              Bit 1: LCDC (see STAT)
              Bit 0: V-Blank
 
   The priority and jump address for the above 5 interrupts are:
 
    Interrupt        Priority        Start Address
 
    V-Blank             1              $0040
    LCDC Status         2              $0048 - Modes 0, 1, 2
                                               LYC=LY coincide (selectable)
    Timer Overflow      3              $0050
    Serial Transfer     4              $0058 - when transfer is complete
    Hi-Lo of P10-P13    5              $0060
 
    * When more than 1 interrupts occur at the same time
      only the interrupt with the highest priority can be
      acknowledged. When an interrupt is used a '0' should
      be stored in the IF register before the IE register
      is set.
 
 
FF10
   Name     - NR 10
   Contents - Sound Mode 1 register, 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) %20/- X(t-1)/2^n
 
FF11
   Name     - NR 11
   Contents - Sound Mode 1 register, Sound length/Wave pattern duty (R/W)
 
              Only Bits 7-6 can be read.
 
              Bit 7-6 - Wave Pattern Duty
              Bit 5-0 - Sound length data (t1: 0-63)
 
              Wave Duty: 00: 12.5% ( _--------_--------_-------- )
                         01: 25%   ( __-------__-------__------- )
                         10: 50%   ( ____-----____-----____----- ) (default)
                         11: 75%   ( ______---______---______--- )
 
              Sound Length = (64-t1)*(1/256) seconds
FF12
   Name     - NR 12
   Contents - Sound Mode 1 register, Envelope (R/W)
 
              Bit 7-4 - Initial volume of envelope
              Bit 3 -   Envelope UP/DOWN
                         0: Attenuate
                         1: Amplify
              Bit 2-0 - Number of envelope sweep (n: 0-7)
                        (If zero, stop envelope operation.)
 
              Initial volume of envelope is from 0 to $F.
              Zero being no sound.
 
              Length of 1 step = n*(1/64) seconds
 
FF13
   Name     - NR 13
   Contents - Sound Mode 1 register, Frequency lo (W)
 
              Lower 8 bits of 11 bit frequency (x).
              Next 3 bit are in NR 14 ($FF14)
 
FF14
   Name     - NR 14
   Contents - Sound Mode 1 register, Frequency hi (R/W)
 
              Only Bit 6 can be read.
 
              Bit 7 - Initial (when set, sound restarts)
              Bit 6 - Counter/consecutive selection
              Bit 2-0 - Frequency's higher 3 bits (x)
 
              Frequency = 4194304/(32*(2048-x)) Hz
                        = 131072/(2048-x) Hz
 
              Counter/consecutive Selection
               0 = Regardless of the length data in NR11
                   sound can be produced consecutively.
               1 = Sound is generated during the time period
                   set by the length data in NR11. After this
                   period the sound 1 ON flag (bit 0 of NR52)
                   is reset.
 
FF16
   Name     - NR 21
   Contents - Sound Mode 2 register, Sound Length; Wave Pattern Duty (R/W)
 
              Only bits 7-6 can be read.
 
              Bit 7-6 - Wave pattern duty
              Bit 5-0 - Sound length data (t1: 0-63)
 
              Wave Duty: 00: 12.5% ( _--------_--------_-------- )
                         01: 25%   ( __-------__-------__------- )
                         10: 50%   ( ____-----____-----____----- ) (default)
                         11: 75%   ( ______---______---______--- )
 
              Sound Length = (64-t1)*(1/256) seconds
 
FF17
   Name     - NR 22
   Contents - Sound Mode 2 register, envelope (R/W)
 
              Bit 7-4 - Initial volume of envelope
              Bit 3 -   Envelope UP/DOWN
                         0: Attenuate
                         1: Amplify
              Bit 2-0 - Number of envelope sweep (n: 0-7)
                        (If zero, stop envelope operation.)
 
              Initial volume of envelope is from 0 to $F.
              Zero being no sound.
 
              Length of 1 step = n*(1/64) seconds
 
FF18
   Name     - NR 23
   Contents - Sound Mode 2 register, frequency lo data (W)
 
              Frequency's lower 8 bits of 11 bit data (x).
              Next 3 bits are in NR 14 ($FF19).
 
FF19
   Name     - NR 24
   Contents - Sound Mode 2 register, frequency hi data (R/W)
 
              Only bit 6 can be read.
 
              Bit 7 - Initial (when set, sound restarts)
              Bit 6 - Counter/consecutive selection
              Bit 2-0 - Frequency's higher 3 bits (x)
 
              Frequency = 4194304/(32*(2048-x)) Hz
                        = 131072/(2048-x) Hz
 
              Counter/consecutive Selection
               0 = Regardless of the length data in NR21
                   sound can be produced consecutively.
               1 = Sound is generated during the time period
                   set by the length data in NR21. After this
                   period the sound 2 ON flag (bit 1 of NR52)
                   is reset.
 
FF1A
   Name     - NR 30
   Contents - Sound Mode 3 register, Sound on/off (R/W)
 
              Only bit 7 can be read
 
              Bit 7 - Sound OFF
                      0: Sound 3 output stop
                      1: Sound 3 output OK
 
FF1B
   Name     - NR 31
   Contents - Sound Mode 3 register, sound length (R/W)
 
              Bit 7-0 - Sound length (t1: 0 - 255)
 
              Sound Length = (256-t1)*(1/2) seconds
 
FF1C
   Name     - NR 32
   Contents - Sound Mode 3 register, Select output level (R/W)
 
              Only bits 6-5 can be read
 
              Bit 6-5 - Select output level
                        00: Mute
                        01: Produce Wave Pattern RAM Data as it is
                            (4 bit length)
                        10: Produce Wave Pattern RAM data shifted once
                            to the RIGHT (1/2)  (4 bit length)
                        11: Produce Wave Pattern RAM data shifted twice
                            to the RIGHT (1/4)  (4 bit length)
 
       * - Wave Pattern RAM is located from $FF30-$FF3f.
 
FF1D
   Name     - NR 33
   Contents - Sound Mode 3 register, frequency's lower data (W)
 
              Lower 8 bits of an 11 bit frequency (x).
 
FF1E
   Name     - NR 34
   Contents - Sound Mode 3 register, frequency's higher data (R/W)
 
              Only bit 6 can be read.
 
              Bit 7 - Initial (when set, sound restarts)
              Bit 6 - Counter/consecutive flag
              Bit 2-0 - Frequency's higher 3 bits (x).
 
              Frequency = 4194304/(64*(2048-x)) Hz
                        = 65536/(2048-x) Hz
 
              Counter/consecutive Selection
               0 = Regardless of the length data in NR31
                   sound can be produced consecutively.
               1 = Sound is generated during the time period
                   set by the length data in NR31. After this
                   period the sound 3 ON flag (bit 2 of NR52)
                   is reset.
 
FF20
   Name     - NR 41
   Contents - Sound Mode 4 register, sound length (R/W)
 
              Bit 5-0 - Sound length data (t1: 0-63)
 
              Sound Length = (64-t1)*(1/256) seconds
 
FF21
   Name     - NR 42
   Contents - Sound Mode 4 register, envelope (R/W)
 
              Bit 7-4 - Initial volume of envelope
              Bit 3 -   Envelope UP/DOWN
                         0: Attenuate
                         1: Amplify
              Bit 2-0 - Number of envelope sweep (n: 0-7)
                        (If zero, stop envelope operation.)
 
              Initial volume of envelope is from 0 to $F.
              Zero being no sound.
 
              Length of 1 step = n*(1/64) seconds
 
FF22
   Name     - NR 43
   Contents - Sound Mode 4 register, polynomial counter (R/W)
 
              Bit 7-4 - Selection of the shift clock frequency of the
                        polynomial counter
              Bit 3   - Selection of the polynomial counter's step
              Bit 2-0 - Selection of the dividing ratio of frequencies
 
              Selection of the dividing ratio of frequencies:
              000: f * 1/2^3 * 2
              001: f * 1/2^3 * 1
              010: f * 1/2^3 * 1/2
              011: f * 1/2^3 * 1/3
              100: f * 1/2^3 * 1/4
              101: f * 1/2^3 * 1/5
              110: f * 1/2^3 * 1/6
              111: f * 1/2^3 * 1/7           f = 4.194304 Mhz
 
              Selection of the polynomial counter step:
              0: 15 steps
              1: 7 steps
 
              Selection of the shift clock frequency of the polynomial
              counter:
 
              0000: dividing ratio of frequencies * 1/2
              0001: dividing ratio of frequencies * 1/2^2
              0010: dividing ratio of frequencies * 1/2^3
              0011: dividing ratio of frequencies * 1/2^4
                    :                          :
                    :                          :
                    :                          :
              0101: dividing ratio of frequencies * 1/2^14
              1110: prohibited code
              1111: prohibited code
 
FF23
   Name     - NR 44
   Contents - Sound Mode 4 register, counter/consecutive; inital (R/W)
 
              Only bit 6 can be read.
 
              Bit 7 - Initial (when set, sound restarts)
              Bit 6 - Counter/consecutive selection
 
              Counter/consecutive Selection
               0 = Regardless of the length data in NR41
                   sound can be produced consecutively.
               1 = Sound is generated during the time period
                   set by the length data in NR41. After this
                   period the sound 4 ON flag (bit 3 of NR52)
                   is reset.
 
FF24
   Name     - NR 50
   Contents - Channel control / ON-OFF / Volume (R/W)
 
              Bit 7 - Vin->SO2 ON/OFF
              Bit 6-4 - SO2 output level (volume) (# 0-7)
              Bit 3 - Vin->SO1 ON/OFF
              Bit 2-0 - SO1 output level (volume) (# 0-7)
 
              Vin->SO1 (Vin->SO2)
 
              By synthesizing the sound from sound 1
              through 4, the voice input from Vin
              terminal is put out.
              0: no output
              1: output OK
 
FF25
    Name     - NR 51
    Contents - 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
    Name     - NR 52  (Value at reset: $F1-GB, $F0-SGB)
    Contents - Sound on/off (R/W)
 
               Bit 7 - All sound on/off
                       0: stop all sound circuits
                       1: operate all sound circuits
               Bit 3 - Sound 4 ON flag
               Bit 2 - Sound 3 ON flag
               Bit 1 - Sound 2 ON flag
               Bit 0 - Sound 1 ON flag
 
                Bits 0 - 3 of this register are meant to
               be status bits to be read. Writing to these
               bits does NOT enable/disable sound.
 
                If your GB programs don't use sound then
               write $00 to this register to save 16% or
               more on GB power consumption.
FF30 - FF3F
   Name     - 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.
 
FF40
   Name     - LCDC  (value $91 at reset)
   Contents - LCD Control (R/W)
 
              Bit 7 - LCD Control Operation *
                      0: Stop completely (no picture on screen)
                      1: operation
 
              Bit 6 - Window Tile Map Display Select
                      0: $9800-$9BFF
                      1: $9C00-$9FFF
 
              Bit 5 - Window Display
                      0: off
                      1: on
 
              Bit 4 - BG & Window Tile Data Select
                      0: $8800-$97FF
                      1: $8000-$8FFF <- Same area as OBJ
 
              Bit 3 - BG Tile Map Display Select
                      0: $9800-$9BFF
                      1: $9C00-$9FFF
 
              Bit 2 - OBJ (Sprite) Size
                      0: 8*8
                      1: 8*16 (width*height)
 
              Bit 1 - OBJ (Sprite) Display
                      0: off
                      1: on
 
              Bit 0 - BG Display
                      0: off
                      1: on
 
       * - Stopping LCD operation (bit 7 from 1 to 0)
           must be performed during V-blank to work
           properly. V-blank can be confirmed when the
           value of LY is greater than or equal to 144.
 
FF41
   Name     - STAT
   Contents - LCDC Status   (R/W)
 
              Bits 6-3 - Interrupt Selection By LCDC Status
 
              Bit 6 - LYC=LY Coincidence (Selectable)
              Bit 5 - Mode 10
              Bit 4 - Mode 01
              Bit 3 - Mode 00
                      0: Non Selection
                      1: Selection
 
              Bit 2 - Coincidence Flag
                      0: LYC not equal to LCDC LY
                      1: LYC = LCDC LY
 
              Bit 1-0 - Mode Flag
                        00: During H-Blank
                        01: During V-Blank
                        10: During Searching OAM-RAM
                        11: During Transfering Data to LCD Driver
 
     STAT shows the current status of the LCD controller.
     Mode 00: When the flag is 00 it is the H-Blank period
              and the CPU can access the display RAM
              ($8000-$9FFF).
 
     Mode 01: When the flag is 01 it is the V-Blank period
              and the CPU can access the display RAM
              ($8000-$9FFF).
 
     Mode 10: When the flag is 10 then the OAM is being
              used ($FE00-$FE9F). The CPU cannot access
              the OAM during this period
 
     Mode 11: When the flag is 11 both the OAM and display
              RAM are being used. The CPU cannot access
              either during this period.
 
 
     The following are typical when the display is enabled:
 
Mode 0  000___000___000___000___000___000___000________________
Mode 1  _______________________________________11111111111111__
Mode 2  ___2_____2_____2_____2_____2_____2___________________2_
Mode 3  ____33____33____33____33____33____33__________________3
 
 
       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.)
 
FF42
   Name     - SCY
   Contents - Scroll Y   (R/W)
 
              8 Bit value $00-$FF to scroll BG Y screen
              position.
 
FF43
   Name     - SCX
   Contents - Scroll X   (R/W)
 
              8 Bit value $00-$FF to scroll BG X screen
              position.
 
FF44
   Name     - LY
   Contents - 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
   Name     - LYC
   Contents - LY Compare  (R/W)
 
            The LYC compares itself with the LY. If the
            values are the same it causes the STAT to set
            the coincident flag.
 
FF46
   Name     - DMA
   Contents - DMA Transfer and Start Address (W)
 
   The DMA Transfer (40*28 bit) from internal ROM or RAM
   ($0000-$F19F) to the OAM (address $FE00-$FE9F) can be
   performed. It takes 160 microseconds for the transfer.
 
   40*28 bit = #140  or #$8C.  As you can see, it only
   transfers $8C bytes of data. OAM data is $A0 bytes
   long, from $0-$9F.
 
   But if you examine the OAM data you see that 4 bits are
   not in use.
 
   40*32 bit = #$A0, but since 4 bits for each OAM is not
   used it's 40*28 bit.
 
   It transfers all the OAM data to OAM RAM.
 
   The DMA transfer start address can be designated every
   $100 from address $0000-$F100. That means $0000, $0100,
   $0200, $0300....
 
    As can be seen by looking at register $FF41 Sprite RAM
   ($FE00 - $FE9F) is not always available. A simple routine
   that many games use to write data to Sprite memory is shown
   below. Since it copies data to the sprite RAM at the appro-
   priate times it removes that responsibility from the main
   program.
    All of the memory space, except high ram ($FF80-$FFFE),
   is not accessible during DMA. Because of this, the routine
   below must be copied & executed in high ram. It is usually
   called from a V-blank Interrupt.
 
   Example program:
 
      org $40
      jp VBlank
 
      org $ff80
VBlank:
      push af        <- Save A reg & flags
      ld a,BASE_ADRS <- transfer data from BASE_ADRS
      ld ($ff46),a   <- put A into DMA registers
      ld a,28h       <- loop length
Wait:                <- We need to wait 160 microseconds.
      dec a          <-  4 cycles - decrease A by 1
      jr nz,Wait     <- 12 cycles - branch if Not Zero to Wait
      pop af         <- Restore A reg & flags
      reti           <- Return from interrupt
 
 
FF47
   Name     - BGP
   Contents - BG & Window Palette Data  (R/W)
 
              Bit 7-6 - Data for Dot Data 11 (Normally darkest color)
              Bit 5-4 - Data for Dot Data 10
              Bit 3-2 - Data for Dot Data 01
              Bit 1-0 - Data for Dot Data 00 (Normally lightest color)
 
              This selects the shade of grays to use for
              the background (BG) & window pixels. Since
              each pixel uses 2 bits, the corresponding
              shade will be selected from here.
 
FF48
   Name     - OBP0
   Contents - Object Palette 0 Data (R/W)
 
              This selects the colors for sprite palette 0.
              It works exactly as BGP ($FF47) except each
              each value of 0 is transparent.
 
FF49
   Name     - OBP1
   Contents - Object Palette 1 Data (R/W)
 
              This Selects the colors for sprite palette 1.
              It works exactly as OBP0 ($FF48).
              See BGP for details.
 
FF4A
   Name     - WY
   Contents - Window Y Position  (R/W)
 
              0 <= WY <= 143
 
              WY must be greater than or equal to 0 and
              must be less than or equal to 143 for
              window to be visible.
 
FF4B
   Name     - WX
   Contents - Window X Position  (R/W)
 
              0 <= WX <= 166
 
              WX must be greater than or equal to 0 and
              must be less than or equal to 166 for
              window to be visible.
 
              WX is offset from absolute screen coordinates
              by 7. Setting the window to WX=7, WY=0 will
              put the upper left corner of the window at
              absolute screen coordinates 0,0.
 
 
              Lets say WY = 70 and WX = 87.
              The window would be positioned as so:
 
               0                  80               159
               ______________________________________
            0 |                                      |
              |                   |                  |
              |                                      |
              |         Background Display           |
              |               Here                   |
              |                                      |
              |                                      |
           70 |         -         %20------------------|
              |                   | 80,70            |
              |                   |                  |
              |                   |  Window Display  |
              |                   |       Here       |
              |                   |                  |
              |                   |                  |
          143 |___________________|__________________|
 
 
          OBJ Characters (Sprites) can still enter the
          window. None of the window colors are
          transparent so any background tiles under the
          window are hidden.
 
FFFF
   Name     - IE
   Contents - Interrupt Enable (R/W)
 
              Bit 4: Transition from High to Low of Pin
                     number P10-P13.
              Bit 3: Serial I/O transfer complete
              Bit 2: Timer Overflow
              Bit 1: LCDC (see STAT)
              Bit 0: V-Blank
 
              0: disable
              1: enable