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Everything You Always Wanted To Know About GAMEBOY
==============================================================================
Everything You Always Wanted To Know About GAMEBOY *
==============================================================================
* but were afraid to ask
Original information from Pan of Anthrox,
Marat Fayzullin, and Pascal Felber.
Expanded by Jeff Frohwein, 25-Mar-97
Updated at personal taste & by excellent hackwork, nocash 5/97
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 (Similiar to the Z80 processor.)
Main RAM: 8K Byte
Video RAM: 8K Byte
Screen Size 2.6"
Resolution: 160x144 (20x18)
Max # of sprites: 40
Max # sprites/line: 10
Max sprite size: 8x16
Min sprite size: 8x8
Clock Speed: 4.194304 MHz
Sound: 4 channels with stereo sound
Power: DC6V 0.7W
Processor
---------
The GameBoy uses a computer chip similiar 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 similiar to the Zilog Z80 processor. Compared to the
Z80, some instructions have been added and some have been
taken away.
The following are new instructions:
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
SWAP r ;rotate register by 4 bits
SWAP (HL) ;rotate (hl) by 4 bits
LD A,($FF00+nn)
LD A,($FF00+C)
LD ($FF00+nn),A
LD ($FF00+C),A
? STOP
? LD (nnnn),SP
ADD SP,dd ;dd=shortint
LD HL,SP+dd ;dd=shortint
The following instructions have been removed:
Any command that uses the IX or IY registers.
All input or output instructions.
All exchange instructions.
All commands prefixed by ED (except remapped reti).
All conditional jumps/calls/rets on parity/overflow and signflag.
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 Table (OAM)
--------------------------- FE00
Echo of 8kB Internal RAM
--------------------------- E000
8kB Internal RAM
--------------------------- C000
8kB switchable RAM bank
--------------------------- 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) and the ROM area from
6000-7FFF when using a ROM that is smaller than 32kB.
An output only port may be implemented anywhere between
6000-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.)
Cart Memory Info
----------------
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!!!)
0134-0143 Title of the game in ASCII. If it is less than
16 characters then the remaining bytes are
filled with 00's.
0144 Ascii hex digit, high nibble of licensee code.
0145 Ascii hex digit, low nibble of licensee code.
0146 SGB features (non-zero if SGB game)
Note: 0144-0146 are zero for non-super gameboy games.
0147 Cartridge type:
0 - ROM ONLY 3 - ROM+MBC1+RAM+BATTERY
1 - ROM+MBC1 5 - ROM+MBC2
2 - ROM+MBC1+RAM 6 - ROM+MBC2+BATTERY
0148 ROM size:
0 - 256kBit = 32kB = 2 banks
1 - 512kBit = 64kB = 4 banks
2 - 1MBit = 128kB = 8 banks
3 - 2MBit = 256kB = 16 banks
4 - 4MBit = 512kB = 32 banks
0149 RAM size:
0 - None
1 - 16kBit = 2kB = 1 bank
2 - 64kBit = 8kB = 1 bank
3 - 256kBit = 32kB = 4 banks
014A Country code:
0 - Japanese
1 - Non-Japanese
014B Licensee code:
33 - Super GameBoy game.
Check 0144/0145 for Licensee code.
79 - Accolade
A4 - Konami
014C Version number
014D Complement check (134..14C)
(PROGRAM WON'T RUN IF NOT CORRECT!!!)
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.)
Rom 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)
Writing a value 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.
Writing a value 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 0A into 0000-1FFF area*. To disable RAM bank
operations write a 00 into 0000-1FFF area. Disabling a
RAM bank probably protects that bank from false writes
during power down of the GameBoy.
* 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):
Same as MBC1 except RAM switching is not provided.
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 AF=$01B0, BC=$0013, DE=$00D8, HL=$014D and
Stack Pointer=$FFFE.
Video
-----
The main GameBoy screen buffer (aka 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 Table 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 Pattern 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 Pattern 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 Window Tile
Table in the same way as background tiles are stored in
the Background Tile Table. The tile patterns are taken
from the table at 8800-97FF and therefore have unsigned
numbers.
Both background and window can be disabled or enabled
separately via bits in the LCDC register.
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 -> 7Ch
22...22. 01111100 -> 7Ch
11...11. 22...22. -> 00000000 -> 00h
2222222. <-- digits 11000110 -> C6h
33...33. represent 11...11. -> 11000110 -> C6h
22...22. color 00000000 -> 00h
11...11. numbers 2222222. -> 00000000 -> 00h
........ 11111110 -> FEh
33...33. -> 11000110 -> C6h
11000110 -> C6h
22...22. -> 00000000 -> 00h
11000110 -> C6h
11...11. -> 11000110 -> C6h
00000000 -> 00h
........ -> 00000000 -> 00h
00000000 -> 00h
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.
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 therefore have unsigned numbers. Sprite
attributes reside in the Sprite Attribute Table (aka OAM)
at FE00-FE9F. OAM is divided into 40 4-byte blocks each
of which corresponds to a sprite. Blocks have the
following format:
Byte0 Y position on the screen, values beyond 16 will start out of screen
Byte1 X position on the screen, values beyond 8 will start out of screen
Byte2 Pattern number 0-255 [always unsigned, data at 8000-8FFF]
When sprite size 8x16 is selected the low-bit is ignored!
Byte3 Flags:
Bit7 Priority
If this bit is set to 1, sprite is displayed
behind the window and behind colors 1, 2,
and 3 of the background (i.e. sprite only
prevails over background color 0). Otherwise,
sprite is shown in front of the background and
in front or behind the window depending on the
window priority bit (see LCDC bit 1).
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.
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 pattern
White noise
These four sounds can be controlled independantly and
then mixed separately for each of the output terminals.
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,$ff
ldh (6),a ;Set TMA to divide clock by 1
ld a,4
ldh (7),a ;Set clock to 4096 Hertz
;This interval timer interrupts 65536 times per second
ld a,256-4
ldh (6),a ;Set TMA to divide clock by 4
ld a,5
ldh (7),a ;Set clock to 262144 Hertz
I/O Registers
-------------
FF00
Name - P1
Contents - Register for reading joy pad info
and determining system type. (R/W)
Bit 7 - Gameboy ID (0=super gameboy, 1=gameboy)
Bit 6 - Gameboy ID (0=super gameboy, 1=gameboy)
Bit 5 - P15 out port ---------+ (0=select)
Bit 4 - P14 out port --+ |
Bit 3 - P13 in port down start (0=pressed)
Bit 2 - P12 in port up select
Bit 1 - P11 in port left button-B
Bit 0 - P10 in port right button-A
To determine what type of GameBoy this is write an 03
to this register then read it back. An Fx indicates
GameBoy or GameBoy Pocket, 3x indicates Super GameBoy.
(The 'x' indicates a don't care value.)
LD A,$10 ;\turn on P15 (buttons)
LD ($FF00),A ;/
LD A,($FF00) ;
LD A,($FF00) ; wait a few cycles
AND $0F
SWAP A
LD B,A
LD A,$20 ;\turn on P14 (cursor)
LD ($FF00),A ;/
LD A,($FF00)
LD A,($FF00)
LD A,($FF00)
LD A,($FF00)
LD A,($FF00)
LD A,($FF00) ; wait a few MORE cycles
AND $0F
OR B
CPL ; invert (now 1 means pressed)
...
LD A,$30 ;\turn off P14 and P15
LD ($FF00),A ;/so keypress interrupt will watch no keylines
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
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
1: Internal Clock
FF04
Name - DIV
Contents - Divider Register (R/W)
FF05
Name - TIMA
Contents - Timer counter (R/W)
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
Bit 2 - Timer Stop
0: Stop Timer
1: Start Timer
Bits 1+0 - Input Clock Select
01: 262.144 khz
10: 65.536 khz
11: 16.384 khz
00: 4.096 khz
FF0F
Name - IF
Contents - Interrupt Flag (R/W)
Bit 4: New Value on Selected Joypad Keyline(s) (rst 60)
Bit 3: Serial I/O transfer end (rst 58)
Bit 2: Timer Overflow (rst 50)
Bit 1: LCD (see STAT) (rst 48)
Bit 0: V-Blank (rst 40)
The priority and jump address for the above 5 interrupts are:
Interrupt Priority Start Address
V-Blank 1 $0040
LCDC Status 2 $0048 - H-Blank, V-Blank, OAM-Access,
LYC=LY coincide (selectable)
Timer Overflow 3 $0050
Serial Transfer 4 $0058 - when transfer is complete
Joypad 5 $0060 - when P10..P13 changes
* 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 - CH1_ENT
Contents - Sound Channel 1, Tone Envelope (R/W)
Bit 6-4 - Sweep Time
Bit 3 - Sweep Increase/Decrease
0: Addition (frequency increases)
1: Subtraction (frequency increases)
Bit 2-0 - Number of sweep shift (# 0-7)
Sweep Time:
000: sweep off
001: 7.8 ms
010: 15.6 ms
011: 23.4 ms
100: 31.3 ms
101: 39.1 ms
110: 46.9 ms
111: 54.7 ms
FF11
Name - CH1_WAVE
Contents - Sound Channel 1, Wave pattern duty (R/W)
Bit 7-6 - Wave Pattern Duty
00: 12.5% ( _--------_--------_-------- )
01: 25% ( __-------__-------__------- )
10: 50% ( ____-----____-----____----- ) (default)
11: 75% ( ______---______---______--- )
Bit 5-0 were described as sound length but as nocash
diag.gmb shows, this is stupid, bit 5-0 have no function.
FF12
Name - CH1_ENV
Contents - Sound Channel 1, Volume Envelope (R/W)
Bit 7-4 - Initial volume (0-15)
Bit 3 - Attack/Release, ignored if Rate=0
0: Decrease volume to 0
1: Increase volume to 15
Bit 2-0 - Attack/Release Rate (0-7)
0 = off (stays at Initial Volume)
n = Increment/Decrement Volume each n*15.6ms
Example (Release):
Assume Initial Volume 5, Direction=Decrease, Rate=4.
The envelope will start at 5 and decrement by one each 62.4ms.
Past 312ms (5*62.4ms) the sound is completely faded out.
Example (Attack):
Initial Volume 5, Direction Increase, Rate=1 (fastest).
Envelope will start at 5, increments by one each 15.6ms.
Past 156ms ( (15-5)*15ms ) sound reaches maximum volume (15).
If EG (FF14.6) is xxx it will stay at 15. If EG is xxx it will
immediately went off past max volume was reached.
Example (Envelope off):
Initial Volume 10, Direction=Don't Care, Rate=0 (off).
Depending on EG the sound will stay at volume 10, or went
immediately off.
FF13
Name - CH1_FREQ_LO
Contents - Sound Channel 1, Frequency lo (W)
lower 8 bits of 11 bit frequency.
Next 3 bits stored in following register ($FF14)
freq = 133333 / (2048-xx) Hz
Therefore lowest supported frequency is 65.1 Hz,
highest is 133 kHz.
FF14
Name - CH2_FREQ_HI_KICK
Contents - Sound Channel 1, Frequency hi (R/W)
Only Bit 6 can be read.
Bit 7 - Kick, 1 = restart sound
Bit 6 - Stay, 1 = Turn sound off when envelope ends
Bit 2-0 - Frequency's higher 3 bits
FF16
Name - CH2_WAVE
Contents - Sound Channel 2, Wave Pattern Duty (R/W)
(See description for channel 1)
FF17
Name - CH2_ENV
Contents - Sound Channel 2, Volume Envelope (R/W)
(See description for channel 1)
FF18
Name - CH2_FREQ_LO
Contents - Sound Channel 2, frequency lo data (W)
(See description for channel 1)
FF19
Name - CH2_FREQ_HI_KICK
Contents - Sound Channel 2, frequency hi data (R/W)
(See description for channel 1)
FF1A
Name - CH3_ONOFF
Contents - Sound Channel 3, 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
FF1C
Name - CH3_VOLUME
Contents - Sound Channel 3, Select output level
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 - CH3_FREQ_LO
Contents - Sound Channel 3, frequency's lower data (W)
Lower 8 bits of an 11 bit frequency
FF1E
Name - CH3_FREQ_HI_KICK
Contents - Sound Channel 3 register, frequency's higher data (R/W)
Only bit 6 can be read.
Bit 7 - Kick, 1 = restart sound
Bit 6 - Stay, 1 = Turn sound off when WHAT? ends
Bit 2-0 - Frequency's higher 3 bits
FF21
Name - CH4_ENV
Contents - Sound Channel 4, Volume envelope (R/W)
(See description for channel 1)
FF22
Name - CH4_POLY
Contents - Sound Channel 4, 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 - CH4_KICK
Contents - Sound Channel 4, counter/consecutive; inital (R/W)
Only bit 6 can be read.
Bit 7 - Kick, 1 = restart sound
Bit 6 - Stay, 1 = Turn sound off when envelope ends
FF24
Name - SND_VIN
Contents - Voice Input (R/W)
Bit 7 - Vin->SO2 ON/OFF
Bit 6-4 - SO2 output level (volume) (# 0-7) (read only)
Bit 3 - Vin->SO1 ON/OFF
Bit 2-0 - SO1 output level (volume) (# 0-7) (read only)
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 - SND_STEREO
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 - SND_STAT
Contents - Sound on/off, Status (R/W)
Only Bit 7, 3-0 can be read.
Bit 7 - All sound on/off
0: stop all sound circuits
1: operate all sound circuits
Bit 3 - Sound 4 ON flag (read only)
Bit 2 - Sound 3 ON flag ("")
Bit 1 - Sound 2 ON flag ("")
Bit 0 - Sound 1 ON flag ("")
FF30 - FF3F
Name - Wave Pattern RAM
Contents - Waveform storage for arbitrary sound data
FF40
Name - LCDC
Contents - LCD Control (R/W)
Bit 7 - LCD Control Operation
0: Stop completely (no picture on screen)
1: operation
Bit 6 - Window Screen Display Data Select
0: $9800-$9BFF
1: $9C00-$9FFF
Bit 5 - Window Display
0: off
1: on
Bit 4 - BG Character Data Select
0: $8800-$97FF
1: $8000-$8FFF <- Same area as OBJ
Bit 3 - BG Screen Display Data Select
0: $9800-$9BFF
1: $9C00-$9FFF
Bit 2 - OBJ Construction
0: 8*8
1: 8*16
Bit 1 - Window priority bit
0: window overlaps all sprites
1: window only overlaps sprites whose
priority bit is set to 1
Bit 0 - BG Display
0: off
1: on
FF41
Name - STAT
Contents - LCDC Status (R/W)
Bits 6-3 - Interrupt Selection By LCDC Status
Bit 6 - LYC=LY Coincidence (1=Select)
Bit 5 - Mode 2: OAM-Search ("")
Bit 4 - Mode 1: V-Blank ("")
Bit 3 - Mode 0: H-Blank ("")
Bit 2 - Coincidence Flag
0: LYC not equal to LCDC LY
1: LYC = LCDC LY
Bit 1-0 - Mode Flag (Current status of the LCD controller)
0: During H-Blank. Entire Display Ram can be accessed.
1: During V-Blank. Entire Display Ram can be accessed.
2: During Searching OAM-RAM. OAM cannot be accessed.
3: During Transfering Data to LCD Driver. CPU cannot
access OAM and display RAM during this period.
The following are typical when the display is enabled:
Mode 0 000___000___000___000___000___000___000________________ H-Blank
Mode 1 _______________________________________11111111111111__ V-Blank
Mode 2 ___2_____2_____2_____2_____2_____2___________________2_ OAM
Mode 3 ____33____33____33____33____33____33__________________3 Transfer
The Mode Flag goes through the values 00, 02,
and 03 at a cycle of about 109uS. 00 is present
about 49uS, 02 about 20uS, and 03 about 40uS. This
is interrupted every 16.6ms by the VBlank (01).
The mode flag stays set at 01 for 1.1 ms.
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.
This is just a RASTER register. The current
line is thrown into here. But since there are
no RASTERS on an LCD display it's called the
LCDC Y-Coordinate.
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 152 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 - $FE9F) is not always available. A simple routine
that many games use to write data to Sprite memory is shown
below. Since it copies data to the sprite RAM at the appro-
priate times it removes that responsibility from the main
program. This routine is usually put into high RAM ($FF80 -
$FFFE) and called from a VBlank Interrupt:
Example program:
VBlank:
push af <- Save A reg & flags
ld a,BASE_ADRS <- transfer data from BASE_ADRS
ld ($ff46),a <- put A into DMA registers
ld a,28h <- loop length
Wait: <- We need to wait 152.5 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
The most likely reason that it is put into high RAM is so
that the BASE_ADRS may be changed dynamically by writing a
new value directly into RAM. This would serve to keep this
interrupt routine to an absolute minimum execution-time wise.
FF47
Name - BGP
Contents - BG Palette Data (W)
Bit 7-6 - Data for Dot Data 11
Bit 5-4 - Data for Dot Data 10
Bit 3-2 - Data for Dot Data 01
Bit 1-0 - Data for Dot Data 00
This selects the shade of gray you what for
your BG pixel. Since each pixel uses 2 bits,
the corresponding shade will be selected
from here. The Background Color (00) lies at
Bits 1-0, just put a value from 0-$3 to
change the color.
FF48
Name - OBP0
Contents - Object Palette 0 Data (W)
This selects the colors for sprite palette 0.
It works exactly as BGP ($FF47).
See BGP for details.
FF49
Name - OBP1
Contents - Object Palette 1 Data (W)
This Selects the colors for sprite palette 1.
It works exactly as BGP ($FF47).
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.
FF4B
Name - WX
Contents - Window X Position (plus seven) (R/W)
7 <= WX <= 166
WX must be greater than or equal to 7 and
must be less than or equal to 166.
Lets say WY = 70 and WX = 87 = 80+7.
The window would be positioned as so:
0 80 159
________________________
0 | | |
| | |
| | |
70 |-----------+------------|
| | Window |
| | Display |
| | Here |
|___________|____________|
OBJ Characters (Sprites) can still enter the
window. So can BG characters.
FFFF
Name - IE
Contents - Interrupt Enable (R/W)
Bit 4: New Value on Selected Joypad Keyline(s)
Bit 3: Serial I/O transfer end
Bit 2: Timer Overflow
Bit 1: LCDC (see STAT)
Bit 0: V-Blank
0: disable
1: enable
----------------------------------------------------------------------------
Gameboy opcodes (found in the net, no author-name included in de file, nocash)
00 NOP
01 LD BC,nnnn
02 LD (BC),A
03 INC BC
04 INC B
05 DEC B
06 LD B,nn
07 RLCA
08 LD (nnnn),SP ---- special (old ex af,af)
09 ADD HL,BC
0A LD A,(BC)
0B DEC BC
0C INC C
0D DEC C
0E LD C,nn
0F RRCA
10 00 STOP ---- special ??? (old djnz disp)
11 LD DE,nnnn
12 LD (DE),A
13 INC DE
14 INC D
15 DEC D
16 LD D,nn
17 RLA
18 JR disp
19 ADD HL,DE
1A LD A,(DE)
1B DEC DE
1C INC E
1D DEC E
1E LD E,nn
1F RRA
20 JR NZ,disp
21 LD HL,nnnn
22 LDI (HL),A ---- special (old ld (nnnn),hl)
23 INC HL
24 INC H
25 DEC H
26 LD H,nn
27 DAA
28 JR Z,disp
29 ADD HL,HL
2A LDI A,(HL) ---- special (old ld hl,(nnnn))
2B DEC HL
2C INC L
2D DEC L
2E LD L,nn
2F CPL
30 JR NC,disp
31 LD SP,nnnn
32 LDD (HL),A ---- special (old remapped ld (nnnn),a)
33 INC SP
34 INC (HL)
35 DEC (HL)
36 LD (HL),nn
37 SCF
38 JR C,disp
39 ADD HL,SP
3A LDD A,(HL) ---- special (old remapped ld a,(nnnn))
3B DEC SP
3C INC A
3D DEC A
3E LD A,nn
3F CCF
40 LD B,B 60 LD H,B
41 LD B,C 61 LD H,C
42 LD B,D 62 LD H,D
43 LD B,E 63 LD H,E
44 LD B,H 64 LD H,H
45 LD B,L 65 LD H,L
46 LD B,(HL) 66 LD H,(HL)
47 LD B,A 67 LD H,A
48 LD C,B 68 LD L,B
49 LD C,C 69 LD L,C
4A LD C,D 6A LD L,D
4B LD C,E 6B LD L,E
4C LD C,H 6C LD L,H
4D LD C,L 6D LD L,L
4E LD C,(HL) 6E LD L,(HL)
4F LD C,A 6F LD L,A
50 LD D,B 70 LD (HL),B
51 LD D,C 71 LD (HL),C
52 LD D,D 72 LD (HL),D
53 LD D,E 73 LD (HL),E
54 LD D,H 74 LD (HL),H
55 LD D,L 75 LD (HL),L
56 LD D,(HL) 76 HALT
57 LD D,A 77 LD (HL),A
58 LD E,B 78 LD A,B
59 LD E,C 79 LD A,C
5A LD E,D 7A LD A,D
5B LD E,E 7B LD A,E
5C LD E,H 7C LD A,H
5D LD E,L 7D LD A,L
5E LD E,(HL) 7E LD A,(HL)
5F LD E,A 7F LD A,A
80 ADD A,B A0 AND B
81 ADD A,C A1 AND C
82 ADD A,D A2 AND D
83 ADD A,E A3 AND E
84 ADD A,H A4 AND H
85 ADD A,L A5 AND L
86 ADD A,(HL) A6 AND (HL)
87 ADD A,A A7 AND A
88 ADC A,B A8 XOR B
89 ADC A,C A9 XOR C
8A ADC A,D AA XOR D
8B ADC A,E AB XOR E
8C ADC A,H AC XOR H
8D ADC A,L AD XOR L
8E ADC A,(HL) AE XOR (HL)
8F ADC A,A AF XOR A
90 SUB B B0 OR B
91 SUB C B1 OR C
92 SUB D B2 OR D
93 SUB E B3 OR E
94 SUB H B4 OR H
95 SUB L B5 OR L
96 SUB (HL) B6 OR (HL)
97 SUB A B7 OR A
98 SBC A,B B8 CP B
99 SBC A,C B9 CP C
9A SBC A,D BA CP D
9B SBC A,E BB CP E
9C SBC A,H BC CP H
9D SBC A,L BD CP L
9E SBC A,(HL) BE CP (HL)
9F SBC A,A BF CP A
C0 RET NZ
C1 POP BC
C2 JP NZ,nnnn
C3 JP nnnn
C4 CALL NZ,nnnn
C5 PUSH BC
C6 ADD A,nn
C7 RST 00H
C8 RET Z
C9 RET
CA JP Z,nnnn
CB nn ---(see beyond)---
CC CALL Z,nnnn
CD CALL nnnn
CE ADC A,nn
CF RST 8
D0 RET NC
D1 POP DE
D2 JP NC,nnnn
D3 - ---- ??? (old out (nn),a)
D4 CALL NC,nnnn
D5 PUSH DE
D6 SUB nn
D7 RST 10H
D8 RET C
D9 RETI ---- remapped (old exx)
DA JP C,nnnn
DB - ---- ??? (old in a,(nn))
DC CALL C,nnnn
DD - ---- ??? (old ix-commands)
DE SBC A,nn (nocash added, this opcode does existed, e.g. used by kwirk)
DF RST 18H
E0 LD ($FF00+nn),A ---- special (old ret po)
E1 POP HL
E2 LD ($FF00+C),A ---- special (old jp po,nnnn)
E3 - ---- ??? (old ex (sp),hl)
E4 - ---- ??? (old call po,nnnn)
E5 PUSH HL
E6 AND nn
E7 RST 20H
E8 ADD SP,dd ---- special (old ret pe) (nocash extended as shortint)
E9 JP (HL)
EA LD (nnnn),A ---- special (old jp pe,nnnn)
EB - ---- ??? (old ex de,hl)
EC - ---- ??? (old call pe,nnnn)
ED - ---- ??? (old ed-commands)
EE XOR nn
EF RST 28H
F0 LD A,($FF00+nn) ---- special (old ret p)
F1 POP AF
F2 LD A,(C) ---- special (old jp p,nnnn)
F3 DI
F4 - ---- ??? (old call p,nnnn)
F5 PUSH AF
F6 OR nn
F7 RST 30H
F8 LD HL,SP+dd ---- special (old ret m) (nocash corrected)
F9 LD SP,HL
FA LD A,(nnnn) ---- special (old jp m,nnnn)
FB EI
FC - ---- ??? (old call m,nnnn)
FD - ---- ??? (old iy-commands)
FE CP nn
FF RST 38H
CB 00 RLC B
CB 01 RLC C
CB 02 RLC D
CB 03 RLC E
CB 04 RLC H
CB 05 RLC L
CB 06 RLC (HL)
CB 07 RLC A
CB 08 RRC B
CB 09 RRC C
CB 0A RRC D
CB 0B RRC E
CB 0C RRC H
CB 0D RRC L
CB 0E RRC (HL)
CB 0F RRC A
CB 10 RL B
CB 11 RL C
CB 12 RL D
CB 13 RL E
CB 14 RL H
CB 15 RL L
CB 16 RL (HL)
CB 17 RL A
CB 18 RR B
CB 19 RR C
CB 1A RR D
CB 1B RR E
CB 1C RR H
CB 1D RR L
CB 1E RR (HL)
CB 1F RR A
CB 20 SLA B
CB 21 SLA C
CB 22 SLA D
CB 23 SLA E
CB 24 SLA H
CB 25 SLA L
CB 26 SLA (HL)
CB 27 SLA A
CB 28 SRA B
CB 29 SRA C
CB 2A SRA D
CB 2B SRA E
CB 2C SRA H
CB 2D SRA L
CB 2E SRA (HL)
CB 2F SRA A
CB 30 SWAP B ---- special (old sll)
CB 31 SWAP C ---- special ""
CB 32 SWAP D ---- special ""
CB 33 SWAP E ---- special ""
CB 34 SWAP H ---- special ""
CB 35 SWAP L ---- special ""
CB 36 SWAP (HL) ---- special ""
CB 37 SWAP A ---- special ""
CB 38 SRL B
CB 39 SRL C
CB 3A SRL D
CB 3B SRL E
CB 3C SRL H
CB 3D SRL L
CB 3E SRL (HL)
CB 3F SRL A
CB 40+n*38 BIT n,B
CB 41+n*38 BIT n,C
CB 42+n*38 BIT n,D
CB 43+n*38 BIT n,E
CB 44+n*38 BIT n,H
CB 45+n*38 BIT n,L
CB 46+n*38 BIT n,(HL)
CB 47+n*38 BIT n,A
CB 80+n*38 RES n,B
CB 81+n*38 RES n,C
CB 82+n*38 RES n,D
CB 83+n*38 RES n,E
CB 84+n*38 RES n,H
CB 85+n*38 RES n,L
CB 86+n*38 RES n,(HL)
CB 87+n*38 RES n,A
CB C0+n*38 SET n,B
CB C1+n*38 SET n,C
CB C2+n*38 SET n,D
CB C3+n*38 SET n,E
CB C4+n*38 SET n,H
CB C5+n*38 SET n,L
CB C6+n*38 SET n,(HL)
CB C7+n*38 SET n,A
----------------------------
nocash hackwork and additions
- rst 60h called on all changes on selected joypad keylines (not only hi2lo)
- joypad keylines are selecting by writing a 0 to de keyline bit (not a 1)
- added key names right at joypad bit description
- removed overkilled joypad description blablabla
- call,jump,ret on parity/overflow,sign-flags were removed
- ed-prefixed commands were removed (except remapped reti)
- new instructions are ...
- added opcode list
- added char resolution (gameboy specs)
- (0149)=0 -> ram size 0 Byte or 512 Byte
- added sprite origin (x=8, y=16)
- added sprites of 8x16 ignore low-bit of sprite number (castlevania adv. etc)
- fixed the wx=80 (instead of 87) bug in window schematic
- made lcd stat description less confusing (mode 2 same than 10 means what?)
- think FF25.0 will enable channel 1 (not 0)
- lcdcont sounded like continue, replaced as lcdc (lcdctrl would be fine, too)
- wave duty described as (__--) instead of 50% only
- snd freq means 133333Hz/(2048-xx)
- snd len (FF11.0-5) is stupid, bits are unused
- described FF24.volume as read only, described FF26.0-3 as read only
- named sound registers as CH1_ENV etc. instead of NR10 etc.
- renamed initial to kick. renamed count/consec. to stay and added description
- named sound channels as channel 1..4 (not as mode 1..4)
----------------------------
nocash hackwork and additions (opcode list)
- added undefined opcodes
- corrected ld hl,sp+dd (not ld hl,(sp+dd)
- extended * to nn, nnnn or dd/disp (especially add sp,dd (not add sp,*))
- commented all changes to z80
- made it look friendlier
