- 1 SGB Description
- 2 SGB Unlocking and Detecting SGB Functions
- 3 SGB Command Packet Transfers
- 4 SGB VRAM Transfers
- 5 SGB Command Summary
- 6 SGB Color Palettes Overview
- 7 SGB Palette Commands
- 8 SGB Color Attribute Commands
- 9 SGB Sound Functions
- 10 SGB System Control Commands
- 11 SGB Multiplayer Command
- 12 SGB Border and OBJ Commands
Basically, the SGB (Super Gameboy) is an adapter cartridge that allows to play gameboy games on a SNES (Super Nintendo Entertainment System) gaming console. In detail, you plug the gameboy cartridge into the SGB cartridge, then plug the SGB cartridge into the SNES, and then connect the SNES to your TV Set. In result, games can be played and viewed on the TV Set, and are controlled by using the SNES joypad(s).
More Technical Description
The SGB cartridge just contains a normal gameboy CPU and normal gameboy video controller. Normally the video signal from this controller would be sent to the LCD screen, however, in this special case the SNES read out the video signal and displays it on the TV set by using a special SNES BIOS ROM which is located in the SGB cartridge. Also, normal gameboy sound output is forwared to the SNES and output to the TV Set, vice versa, joypad input is forwared from the SNES controller(s) to the gameboy joypad inputs.
Normal Monochrome Games
Any gameboy games which have been designed for normal monochrome handheld gameboys will work with the SGB hardware as well. The SGB will apply a four color palette to these games by replacing the normal four grayshades. The 160x144 pixel gamescreen is displayed in the middle of the 256x224 pixel SNES screen (the unused area is filled by a screen border bitmap). The user may access built-in menues, allowing to change color palette data, to select between several pre-defined borders, etc.
Games that have been designed to support SGB functions may also access the following additional features:
Colorized Game Screen
There's limited ability to colorize the gamescreen by assigning custom color palettes to each 20x18 display characters, however, this works mainly for static display data such like title screens or status bars, the 20x18 color attribute map is non-scrollable, and it is not possible to assign separate colors to moveable foreground sprites (OBJs), so that animated screen regions will be typically restricted to using a single palette of four colors only.
SNES Foreground Sprites
Up to 24 foreground sprites (OBJs) of 8x8 or 16x16 pixels, 16 colors can be displayed. When replacing (or just overlaying) the normal gameboy OBJs by SNES OBJs it'd be thus possible to display OBJs with other colors than normal background area. This method doesn't appear to be very popular, even though it appears to be quite easy to implement, however, the bottommost character line of the gamescreen will be masked out because this area is used to transfer OAM data to the SNES.
The SGB Border
The possibly most popular and most impressive feature is to replace the default SGB screen border by a custom bitmap which is stored in the game cartridge.
Up to four joypads can be conected to the SNES, and SGB software may read-out each of these joypads separately, allowing up to four players to play the same game simultaneously. Unlike for multiplayer handheld games, this requires only one game cartridge and only one SGB/SNES, and no link cables are required, the downside is that all players must share the same display screen.
Beside for normal gameboy sound, a number of digital sound effects is pre-defined in the SNES BIOS, these effects may be accessed quite easily. Programmers whom are familiar with SNES sounds may also access the SNES sound chip, or use the SNES MIDI engine directly in order to produce other sound effects or music.
Taking Control of the SNES CPU
Finally, it is possible to write program code or data into SNES memory, and to execute such program code by using the SNES CPU.
SGB System Clock
Because the SGB is synchronized to the SNES CPU, the gameboy system clock is directly chained to the SNES system clock. In result, the gameboy CPU, video controller, timers, and sound frequencies will be all operated approx 2.4% faster as by normal gameboys. Basically, this should be no problem, and the game will just run a little bit faster. However sensitive musicians may notice that sound frequencies are a bit too high, programs that support SGB functions may avoid this effect by reducing frequencies of gameboy sounds when having detected SGB hardware. Also, I think that I've heard that SNES models which use a 50Hz display refresh rate (rather than 60Hz) are resulting in respectively slower SGB/gameboy timings ???
SGB Unlocking and Detecting SGB Functions
SGB games are required to have a cartridge header with Nintendo and proper checksum just as normal gameboy games. Also, two special entries must be set in order to unlock SGB functions:
146h - SGB Flag - Must be set to 03h for SGB games 14Bh - Old Licensee Code - Must be set 33h for SGB games
When these entries aren't set, the game will still work just like all 'monochrome' gameboy games, but it cannot access any of the special SGB functions.
Detecting SGB hardware
The recommended detection method is to send a MLT_REQ command which enables two (or four) joypads. A normal handheld gameboy will ignore this command, a SGB will now return incrementing joypad IDs each time when deselecting keyboard lines (see MLT_REQ description for details). Now read-out joypad state/IDs several times, and if the ID-numbers are changing, then it is a SGB (a normal gameboy would typically always return 0Fh as ID). Finally, when not intending to use more than one joypad, send another MLT_REQ command in order to re-disable the multi-controller mode. Detection works regardless of whether and how many joypads are physically connected to the SNES. However, detection works only when having unlocked SGB functions in the cartridge header, as described above.
Separating between SGB and SGB2
It is also possible to separate between SGB and SGB2 models by examining the inital value of the accumulator (A-register) directly after startup.
01h SGB or Normal Gameboy (DMG) FFh SGB2 or Pocket Gameboy 11h CGB or GBA
Because values 01h and FFh are shared for both handhelds and SGBs, it is still required to use the above MLT_REQ detection procedure. As far as I know the SGB2 doesn't have any extra features which'd require separate SGB2 detection except for curiosity purposes, for example, the game "Tetris DX" chooses to display an alternate SGB border on SGB2s.
Reportedly, some SGB models include link ports (just like handheld gameboy) (my own SGB does not have such an port), possibly this feature is available in SGB2-type models only ???
SGB Command Packet Transfers
Command packets (aka Register Files) are transferred from the gameboy to the SNES by using P14 and P15 output lines of the JOYPAD register (FF00h), these lines are normally used to select the two rows in the gameboy keyboard matrix (which still works).
A command packet transfer must be initiated by setting both P14 and P15 to LOW, this will reset and start the SNES packet receiving program. Data is then transferred (LSB first), setting P14=LOW will indicate a "0" bit, and setting P15=LOW will indicate a "1" bit. For example:
RESET 0 0 1 1 0 1 0 P14 --_---_---_-----------_-------_--... P15 --_-----------_---_-------_------...
Data and reset pulses must be kept LOW for at least 5us. P14 and P15 must be kept both HIGH for at least 15us between any pulses. Obviously, it'd be no good idea to access the JOYPAD register during the transfer, for example, in case that your VBlank interrupt procedure reads-out joypad states each frame, be sure to disable that interrupt during the transfer (or disable only the joypad procedure by using a software flag).
Each packet is invoked by a RESET pulse, then 128 bits of data are transferred (16 bytes, LSB of first byte first), and finally, a "0"-bit must be transferred as stop bit. The structure of normal packets is:
1 PULSE Reset 1 BYTE Command Code*8+Length 15 BYTES Parameter Data 1 BIT Stop Bit (0)
The above 'Length' indicates the total number of packets (1-7, including the first packet) which will be sent, ie. if more than 15 parameter bytes are used, then further packet(s) will follow, as such:
1 PULSE Reset 16 BYTES Parameter Data 1 BIT Stop Bit (0)
By using all 7 packets, up to 111 data bytes (15+16*6) may be sent. Unused bytes at the end of the last packet must be set to zero. A 60ms (4 frames) delay should be invoked between each packet transfer.
SGB VRAM Transfers
Beside for the packet transfer method, larger data blocks of 4KBytes can be transferred by using the video signal. These transfers are invoked by first sending one of the commands with the ending _TRN (by using normal packet transfer), the 4K data block is then read-out by the SNES from gameboy display memory during the next frame.
Normally, transfer data should be stored at 8000h-8FFFh in gameboy VRAM, even though the SNES receives the data in from display scanlines, it will automatically re-produce the same ordering of bits and bytes, as being originally stored at 8000h-8FFFh in gameboy memory.
Preparing the Display
The above method works only when recursing the following things: BG Map must display unsigned characters 00h-FFh on the screen; 00h..13h in first line, 14h..27h in next line, etc. The gameboy display must be enabled, the display may not be scrolled, OBJ sprites should not overlap the background tiles, the BGP palette register must be set to E4h.
Note that the transfer data should be prepared in VRAM <before> sending the transfer command packet. The actual transfer starts at the beginning of the next frame after the command has been sent, and the transfer ends at the end of the 5th frame after the command has been sent (not counting the frame in which the command has been sent).
Avoiding Screen Garbage The display will contain 'garbage' during the transfer, this dirt-effect can be avoided by freezing the screen (in the state which has been displayed before the transfer) by using the MASK_EN command. Of course, this works only when actually executing the game on a SGB (and not on normal handheld gameboys), it'd be thus required to detect the presence of SGB hardware before blindly sending VRAM data.
SGB Command Summary
SGB System Command Table
Code Name Expl. 00 PAL01 Set SGB Palette 0,1 Data 01 PAL23 Set SGB Palette 2,3 Data 02 PAL03 Set SGB Palette 0,3 Data 03 PAL12 Set SGB Palette 1,2 Data 04 ATTR_BLK "Block" Area Designation Mode 05 ATTR_LIN "Line" Area Designation Mode 06 ATTR_DIV "Divide" Area Designation Mode 07 ATTR_CHR "1CHR" Area Designation Mode 08 SOUND Sound On/Off 09 SOU_TRN Transfer Sound PRG/DATA 0A PAL_SET Set SGB Palette Indirect 0B PAL_TRN Set System Color Palette Data 0C ATRC_EN Enable/disable Attraction Mode 0D TEST_EN Speed Function 0E ICON_EN SGB Function 0F DATA_SND SUPER NES WRAM Transfer 1 10 DATA_TRN SUPER NES WRAM Transfer 2 11 MLT_REG Controller 2 Request 12 JUMP Set SNES Program Counter 13 CHR_TRN Transfer Character Font Data 14 PCT_TRN Set Screen Data Color Data 15 ATTR_TRN Set Attribute from ATF 16 ATTR_SET Set Data to ATF 17 MASK_EN Game Boy Window Mask 18 OBJ_TRN Super NES OBJ Mode
SGB Color Palettes Overview
Available SNES Palettes
The SGB/SNES provides 8 palettes of 16 colors each, each color may be defined out of a selection of 34768 colors (15 bit). Palettes 0-3 are used to colorize the gamescreen, only the first four colors of each of these palettes are used. Palettes 4-7 are used for the SGB Border, all 16 colors of each of these palettes may be used.
Color 0 Restriction
Color 0 of each of the eight palettes is transparent, causing the backdrop color to be displayed instead. The backdrop color is typically defined by the most recently color being assigned to Color 0 (regardless of the palette number being used for that operation). Effectively, gamescreen palettes can have only three custom colors each, and SGB border palettes only 15 colors each, additionally, color 0 can be used for for all palettes, which will then all share the same color though.
Translation of Grayshades into Colors
Because the SGB/SNES reads out the gameboy video controllers display signal, it translates the different grayshades from the signal into SNES colors as such:
White --> Color 0 Light Gray --> Color 1 Dark Gray --> Color 2 Black --> Color 3
Note that gameboy colors 0-3 are assigned to user-selectable grayshades by the gameboys BGP, OBP1, and OBP2 registers. There is thus no fixed relationship between gameboy colors 0-3 and SNES colors 0-3.
Using Gameboy BGP/OBP Registers
A direct translation of color 0-3 into color 0-3 may be produced by setting BGP/OBP registers to a value of 0E4h each. However, in case that your program uses black background for example, then you may internally assign background as "White" at the gameboy side by BGP/OBP registers (which is then interpreted as SNES color 0, which is shared for all SNES palettes). The advantage is that you may define Color 0 as Black at the SNES side, and may assign custom colors for Colors 1-3 of each SNES palette.
System Color Palette Memory
Beside for the actually visible palettes, up to 512 palettes of 4 colors each may be defined in SNES RAM. Basically, this is completely irrelevant because the palettes are just stored in RAM whithout any relationship to the displayed picture, anyways, these pre-defined colors may be transferred to actually visible palettes slightly faster as when transferring palette data by separate command packets.
SGB Palette Commands
SGB Command 00h - PAL01
Transmit color data for SGB palette 0, color 0-3, and for SGB palette 1, color 1-3 (without separate color 0).
Byte Content 0 Command*8+Length (fixed length=01h) 1-E Color Data for 7 colors of 2 bytes (16bit) each: Bit 0-4 - Red Intensity (0-31) Bit 5-9 - Green Intensity (0-31) Bit 10-14 - Blue Intensity (0-31) Bit 15 - Not used (zero) F Not used (00h)
The value transferred as color 0 will be applied for all eight palettes.
SGB Command 01h - PAL23
Same as above PAL01, but for Palettes 2 and 3 respectively.
SGB Command 02h - PAL03
Same as above PAL01, but for Palettes 0 and 3 respectively.
SGB Command 03h - PAL12
Same as above PAL01, but for Palettes 1 and 2 respectively.
SGB Command 0Ah - PAL_SET
Used to copy pre-defined palette data from SGB system color palette to actual SGB palette.
Byte Content 0 Command*8+Length (fixed length=1) 1-2 System Palette number for SGB Color Palette 0 (0-511) 3-4 System Palette number for SGB Color Palette 1 (0-511) 5-6 System Palette number for SGB Color Palette 2 (0-511) 7-8 System Palette number for SGB Color Palette 3 (0-511) 9 Attribute File Bit 0-5 - Attribute File Number (00h-2Ch) (Used only if Bit7=1) Bit 6 - Cancel Mask (0=No change, 1=Yes) Bit 7 - Use Attribute File (0=No, 1=Apply above ATF Number) A-F Not used (zero)
Before using this function, System Palette data should be initialized by PAL_TRN command, and (when used) Attribute File data should be initialized by ATTR_TRN.
SGB Command 0Bh - PAL_TRN
Used to initialize SGB system color palettes in SNES RAM. System color palette memory contains 512 pre-defined palettes, these palettes do not directly affect the display, however, the PAL_SET command may be later used to transfer four of these 'logical' palettes to actual visible 'physical' SGB palettes. Also, the OBJ_TRN function will use groups of 4 System Color Palettes (4*4 colors) for SNES OBJ palettes (16 colors).
Byte Content 0 Command*8+Length (fixed length=1) 1-F Not used (zero)
The palette data is sent by VRAM-Transfer (4 KBytes).
000-FFF Data for System Color Palette 0-511
Each Palette consists of four 16bit-color definitions (8 bytes). Note: The data is stored at 3000h-3FFFh in SNES memory.
SGB Color Attribute Commands
SGB Command 04h - ATTR_BLK
Used to specify color attributes for the inside or outside of one or more rectangular screen regions.
Byte Content 0 Command*8+Length (length=1..7) 1 Number of Data Sets (01h..12h) 2-7 Data Set #1 Byte 0 - Control Code (0-7) Bit 0 - Change Colors inside of surrounded area (1=Yes) Bit 1 - Change Colors of surrounding character line (1=Yes) Bit 2 - Change Colors outside of surrounded area (1=Yes) Bit 3-7 - Not used (zero) Exception: When changing only the Inside or Outside, then the Surrounding line becomes automatically changed to same color. Byte 1 - Color Palette Designation Bit 0-1 - Palette Number for inside of surrounded area Bit 2-3 - Palette Number for surrounding character line Bit 4-5 - Palette Number for outside of surrounded area Bit 6-7 - Not used (zero) Data Set Byte 2 - Coordinate X1 (left) Data Set Byte 3 - Coordinate Y1 (upper) Data Set Byte 4 - Coordinate X2 (right) Data Set Byte 5 - Coordinate Y2 (lower) Specifies the coordinates of the surrounding rectangle. 8-D Data Set #2 (if any) E-F Data Set #3 (continued at 0-3 in next packet) (if any)
When sending three or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. The format of the separate Data Sets is described below.
SGB Command 05h - ATTR_LIN
Used to specify color attributes of one or more horizontal or vertical character lines.
Byte Content 0 Command*8+Length (length=1..7) 1 Number of Data Sets (01h..6Eh) (one byte each) 2 Data Set #1 Bit 0-4 - Line Number (X- or Y-coordinate, depending on bit 7) Bit 5-6 - Palette Number (0-3) Bit 7 - H/V Mode Bit (0=Vertical line, 1=Horizontal Line) 3 Data Set #2 (if any) 4 Data Set #3 (if any) etc.
When sending 15 or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. The format of the separate Data Sets (one byte each) is described below. The length of each line reaches from one end of the screen to the other end. In case that some lines overlap each other, then lines from lastmost data sets will overwrite lines from previous data sets.
SGB Command 06h - ATTR_DIV
Used to split the screen into two halfes, and to assign separate color attributes to each half, and to the division line between them.
Byte Content 0 Command*8+Length (fixed length=1) 1 Color Palette Numbers and H/V Mode Bit Bit 0-1 Palette Number below/right of division line Bit 2-3 Palette Number above/left of division line Bit 4-5 Palette Number for division line Bit 6 H/V Mode Bit (0=split left/right, 1=split above/below) 2 X- or Y-Coordinate (depending on H/V bit) 3-F Not used (zero)
SGB Command 07h - ATTR_CHR
Used to specify color attributes for separate characters.
Byte Content 0 Command*8+Length (length=1..6) 1 Beginning X-Coordinate 2 Beginning Y-Coordinate 3-4 Number of Data Sets (1-360) 5 Writing Style (0=Left to Right, 1=Top to Bottom) 6 Data Sets 1-4 (Set 1 in MSBs, Set 4 in LSBs) 7 Data Sets 5-8 (if any) 8 Data Sets 9-12 (if any) etc.
When sending 41 or more data sets, data is continued in further packet(s). Unused bytes at the end of the last packet should be set to zero. Each data set consists of two bits, indicating the palette number for one character. Depending on the writing style, data sets are written from left to right, or from top to bottom. In either case the function wraps to the next row/column when reaching the end of the screen.
SGB Command 15h - ATTR_TRN
Used to initialize Attribute Files (ATFs) in SNES RAM. Each ATF consists of 20x18 color attributes for the gameboy screen. This function does not directly affect display attributes. Instead, one of the defined ATFs may be copied to actual display memory at a later time by using ATTR_SET or PAL_SET functions.
Byte Content 0 Command*8+Length (fixed length=1) 1-F Not used (zero)
The ATF data is sent by VRAM-Transfer (4 KBytes).
000-FD1 Data for ATF0 through ATF44 (4050 bytes) FD2-FFF Not used
Each ATF consists of 90 bytes, that are 5 bytes (20x2bits) for each of the 18 character lines of the gameboy window. The two most significant bits of the first byte define the color attribute (0-3) for the first character of the first line, the next two bits the next character, and so on.
SGB Command 16h - ATTR_SET
Used to transfer attributes from Attribute File (ATF) to gameboy window.
Byte Content 0 Command*8+Length (fixed length=1) 1 Attribute File Number (00-2Ch), Bit 6=Cancel Mask 2-F Not used (zero)
When above Bit 6 is set, the gameboy screen becomes re-enabled after the transfer (in case it has been disabled/frozen by MASK_EN command). Note: The same functions may be (optionally) also included in PAL_SET commands, as described in the chapter about Color Palette Commands.
SGB Sound Functions
SGB Command 08h - SOUND
Used to start/stop internal sound effect, start/stop sound using internal tone data.
Byte Content 0 Command*8+Length (fixed length=1) 1 Sound Effect A (Port 1) Decrescendo 8bit Sound Code 2 Sound Effect B (Port 2) Sustain 8bit Sound Code 3 Sound Effect Attributes Bit 0-1 - Sound Effect A Pitch (0..3=Low..High) Bit 2-3 - Sound Effect A Volume (0..2=High..Low, 3=Mute on) Bit 4-5 - Sound Effect B Pitch (0..3=Low..High) Bit 6-7 - Sound Effect B Volume (0..2=High..Low, 3=Not used) 4 Music Score Code (must be zero if not used) 5-F Not used (zero)
See Sound Effect Tables below for a list of available pre-defined effects. "Notes" 1) Mute is only active when both bits D2 and D3 are 1. 2) When the volume is set for either Sound Effect A or Sound Effect B, mute is turned off. 3) When Mute on/off has been executed, the sound fades out/fades in. 4) Mute on/off operates on the (BGM) which is reproduced by Sound Effect A, Sound Effect B, and the Super NES APU. A "mute off" flag does not exist by itself. When mute flag is set, volume and pitch of Sound Effect A (port 1) and Sound Effect B (port 2) must be set.
SGB Command 09h - SOU_TRN
Used to transfer sound code or data to SNES Audio Processing Unit memory (APU-RAM).
Byte Content 0 Command*8+Length (fixed length=1) 1-F Not used (zero)
The sound code/data is sent by VRAM-Transfer (4 KBytes).
000 One (or two ???) 16bit expression(s ???) indicating the transfer destination address and transfer length. ...-... Transfer Data ...-FFF Remaining bytes not used
Possible destinations in APU-RAM are:
0400h-2AFFh APU-RAM Program Area (9.75KBytes) 2B00h-4AFFh APU-RAM Sound Score Area (8Kbytes) 4DB0h-EEFFh APU-RAM Sampling Data Area (40.25 Kbytes)
This function may be used to take control of the SNES sound chip, and/or to access the SNES MIDI engine. In either case it requires deeper knowledge of SNES sound programming.
SGB Sound Effect A/B Tables
Below lists the digital sound effects that are pre-defined in the SGB/SNES BIOS, and which can be used with the SGB "SOUND" Command. Effect A and B may be simultaneously reproduced. The P-column indicates the recommended Pitch value, the V-column indicates the numbers of Voices used. Sound Effect A uses voices 6,7. Sound Effect B uses voices 0,1,4,5. Effects that use less voices will use only the upper voices (eg. 4,5 for Effect B with only two voices).
Sound Effect A Flag Table
Code Description P V Code Description P V 00 Dummy flag, re-trigger - 2 18 Fast Jump 3 1 80 Effect A, stop/silent - 2 19 Jet (rocket) takeoff 0 1 01 Nintendo 3 1 1A Jet (rocket) landing 0 1 02 Game Over 3 2 1B Cup breaking 2 2 03 Drop 3 1 1C Glass breaking 1 2 04 OK ... A 3 2 1D Level UP 2 2 05 OK ... B 3 2 1E Insert air 1 1 06 Select...A 3 2 1F Sword swing 1 1 07 Select...B 3 1 20 Water falling 2 1 08 Select...C 2 2 21 Fire 1 1 09 Mistake...Buzzer 2 1 22 Wall collapsing 1 2 0A Catch Item 2 2 23 Cancel 1 2 0B Gate squeaks 1 time 2 2 24 Walking 1 2 0C Explosion...small 1 2 25 Blocking strike 1 2 0D Explosion...medium 1 2 26 Picture floats on & off 3 2 0E Explosion...large 1 2 27 Fade in 0 2 0F Attacked...A 3 1 28 Fade out 0 2 10 Attacked...B 3 2 29 Window being opened 1 2 11 Hit (punch)...A 0 2 2A Window being closed 0 2 12 Hit (punch)...B 0 2 2B Big Laser 3 2 13 Breath in air 3 2 2C Stone gate closes/opens 0 2 14 Rocket Projectile...A 3 2 2D Teleportation 3 1 15 Rocket Projectile...B 3 2 2E Lightning 0 2 16 Escaping Bubble 2 1 2F Earthquake 0 2 17 Jump 3 1 30 Small Laser 2 2
Sound effect A is used for formanto sounds (percussion sounds).
Sound Effect B Flag Table
Code Description P V Code Description P V 00 Dummy flag, re-trigger - 4 0D Waterfall 2 2 80 Effect B, stop/silent - 4 0E Small character running 3 1 01 Applause...small group 2 1 0F Horse running 3 1 02 Applause...medium group 2 2 10 Warning sound 1 1 03 Applause...large group 2 4 11 Approaching car 0 1 04 Wind 1 2 12 Jet flying 1 1 05 Rain 1 1 13 UFO flying 2 1 06 Storm 1 3 14 Electromagnetic waves 0 1 07 Storm with wind/thunder 2 4 15 Score UP 3 1 08 Lightning 0 2 16 Fire 2 1 09 Earthquake 0 2 17 Camera shutter, formanto 3 4 0A Avalanche 0 2 18 Write, formanto 0 1 0B Wave 0 1 19 Show up title, formanto 0 1 0C River 3 2
Sound effect B is mainly used for looping sounds (sustained sounds).
SGB System Control Commands
SGB Command 17h - MASK_EN
Used to mask the gameboy window, among others this can be used to freeze the gameboy screen before transferring data through VRAM (the SNES then keeps displaying the gameboy screen, even though VRAM doesn't contain meaningful display information during the transfer).
Byte Content 0 Command*8+Length (fixed length=1) 1 Gameboy Screen Mask (0-3) 0 Cancel Mask (Display activated) 1 Freeze Screen (Keep displaying current picture) 2 Blank Screen (Black) 3 Blank Screen (Color 0) 2-F Not used (zero)
Freezing works only if the SNES has stored a picture, ie. if necessary wait one or two frames before freezing (rather than freezing directly after having displayed the picture). The Cancel Mask function may be also invoked (optionally) by completion of PAL_SET and ATTR_SET commands.
SGB Command 0Ch - ATRC_EN
Used to enable/disable Attraction mode. It is totally unclear what an attraction mode is ???, but it is enabled by default.
Byte Content 0 Command*8+Length (fixed length=1) 1 Attraction Disable (0=Enable, 1=Disable) 2-F Not used (zero)
SGB Command 0Dh - TEST_EN
Used to enable/disable test mode for "SGB-CPU variable clock speed function". This function is disabled by default.
Byte Content 0 Command*8+Length (fixed length=1) 1 Test Mode Enable (0=Disable, 1=Enable) 2-F Not used (zero)
Maybe intended to determine whether SNES operates at 50Hz or 60Hz display refresh rate ??? Possibly result can be read-out from joypad register ???
SGB Command 0Eh - ICON_EN
Used to enable/disable ICON function. Possibly meant to enable/disable SGB/SNES popup menues which might otherwise activated during gameboy game play. By default all functions are enabled (0).
Byte Content 0 Command*8+Length (fixed length=1) 1 Disable Bits Bit 0 - Use of SGB-Built-in Color Palettes (1=Disable) Bit 1 - Controller Set-up Screen (0=Enable, 1=Disable) Bit 2 - SGB Register File Transfer (0=Receive, 1=Disable) Bit 3-6 - Not used (zero) 2-F Not used (zero)
Above Bit 2 will suppress all further packets/commands when set, this might be useful when starting a monochrome game from inside of the SGB-menu of a multi-gamepak which contains a collection of different games.
SGB Command 0Fh - DATA_SND
Used to write one or more bytes directly into SNES Work RAM.
Byte Content 0 Command*8+Length (fixed length=1) 1 SNES Destination Address, low 2 SNES Destination Address, high 3 SNES Destination Address, bank number 4 Number of bytes to write (01h-0Bh) 5 Data Byte #1 6 Data Byte #2 (if any) 7 Data Byte #3 (if any) etc.
Unused bytes at the end of the packet should be set to zero, this function is restricted to a single packet, so that not more than 11 bytes can be defined at once. Free Addresses in SNES memory are Bank 0 1800h-1FFFh, Bank 7Fh 0000h-FFFFh.
SGB Command 10h - DATA_TRN
Used to transfer binary code or data directly into SNES RAM.
Byte Content 0 Command*8+Length (fixed length=1) 1 SNES Destination Address, low 2 SNES Destination Address, high 3 SNES Destination Address, bank number 4-F Not used (zero)
The data is sent by VRAM-Transfer (4 KBytes).
Free Addresses in SNES memory are Bank 0 1800h-1FFFh, Bank 7Fh 0000h-FFFFh. The transfer length is fixed at 4KBytes ???, so that directly writing to the free 2KBytes at 0:1800h would be a not so good idea ???
SGB Command 12h - JUMP
Used to set the SNES program counter to a specified address. Optionally, it may be used to set a new address for the SNES NMI handler, the NMI handler remains unchanged if all bytes 4-6 are zero.
Byte Content 0 Command*8+Length (fixed length=1) 1 SNES Program Counter, low 2 SNES Program Counter, high 3 SNES Program Counter, bank number 4 SNES NMI Handler, low 5 SNES NMI Handler, high 6 SNES NMI Handler, bank number 7-F Not used, zero
Note: The game "Space Invaders 94" uses this function when selecting "Arcade mode" to execute SNES program code which has been previously transferred from the SGB to the SNES. The type of the CPU which is used in the SNES is unknown ???
SGB Multiplayer Command
SGB Command 11h - MLT_REQ
Used to request multiplayer mode (ie. input from more than one joypad). Because this function provides feedback from the SGB/SNES to the gameboy program, it is also used to detect SGB hardware.
Byte Content 0 Command*8+Length (fixed length=1) 1 Multiplayer Control (0-3) (Bit0=Enable, Bit1=Two/Four Players) 0 = One player 1 = Two players 3 = Four players 2-F Not used (zero)
In one player mode, the second joypad (if any) is used for the SGB system program. In two player mode, both joypads are used for the game. Because SNES have only two joypad sockets, four player mode requires an external "Multiplayer 5" adapter.
Reading Multiple Controllers (Joypads)
When having enabled multiple controllers by MLT_REQ, data for each joypad can be read out through JOYPAD register (FF00) as follows: First set P14 and P15 both HIGH (deselect both Buttons and Cursor keys), you can now read the lower 4bits of FF00 which indicate the joypad ID for the following joypad input:
0Fh Joypad 1 0Eh Joypad 2 0Dh Joypad 3 0Ch Joypad 4
Next, set P14 and P15 low (one after each other) to select Buttons and Cursor lines, and read-out joypad state as normally. When completed, set P14 and P15 back HIGH, this automatically increments the joypad number (or restarts counting once reached the lastmost joypad). Repeat the procedure until you have read-out states for all two (or four) joypads.
SGB Border and OBJ Commands
SGB Command 13h - CHR_TRN
Used to transfer tile data (characters) to SNES Tile memory in VRAM. This normally used to define BG tiles for the SGB Border (see PCT_TRN), but might be also used to define moveable SNES foreground sprites (see OBJ_TRN).
Byte Content 0 Command*8+Length (fixed length=1) 1 Tile Transfer Destination Bit 0 - Tile Numbers (0=Tiles 00h-7Fh, 1=Tiles 80h-FFh) Bit 1 - Tile Type (0=BG Tiles, 1=OBJ Tiles) Bit 2-7 - Not used (zero) 2-F Not used (zero)
The tile data is sent by VRAM-Transfer (4 KBytes).
000-FFF Bitmap data for 128 Tiles
Each tile occupies 16bytes (8x8 pixels, 16 colors each). When intending to transfer more than 128 tiles, call this function twice (once for tiles 00h-7Fh, and once for tiles 80h-FFh). Note: The BG/OBJ Bit seems to have no effect and writes to the same VRAM addresses for both BG and OBJ ???
SGB Command 14h - PCT_TRN
Used to transfer tile map data and palette data to SNES BG Map memory in VRAM to be used for the SGB border. The actual tiles must be separately transferred by using the CHR_TRN function.
Byte Content 0 Command*8+Length (fixed length=1) 1-F Not used (zero)
The map data is sent by VRAM-Transfer (4 KBytes).
000-7FF BG Map 32x32 Entries of 16bit each (2048 bytes) 800-87F BG Palette Data (Palettes 4-7, each 16 colors of 16bits each) 880-FFF Not used, don't care
Each BG Map Entry consists of a 16bit value as such:
Bit 0-9 - Character Number (use only 00h-FFh, upper 2 bits zero) Bit 10-12 - Palette Number (use only 4-7, officially use only 4-6) Bit 13 - BG Priority (use only 0) Bit 14 - X-Flip (0=Normal, 1=Mirror horizontally) Bit 15 - Y-Flip (0=Normal, 1=Mirror vertically)
Even though 32x32 map entries are transferred, only upper 32x28 are actually used (256x224 pixels, SNES screen size). The 20x18 entries in the center of the 32x28 area should be set to 0000h as transparent space for the gameboy window to be displayed inside. Reportedly, non-transparent border data will cover the gameboy window.
SGB Command 18h - OBJ_TRN
Used to transfer OBJ attributes to SNES OAM memory. Unlike all other functions with the ending _TRN, this function does not use the usual one-shot 4KBytes VRAM transfer method. Instead, when enabled (below execute bit set), data is permanently (each frame) read out from the lower character line of the gameboy screen. To suppress garbage on the display, the lower line is masked, and only the upper 20x17 characters of the gameboy window are used - the masking method is unknwon - frozen, black, or recommended to be covered by the SGB border, or else ??? Also, when the function is enabled, "system attract mode is not performed" - whatever that means ???
Byte Content 0 Command*8+Length (fixed length=1) 1 Control Bits Bit 0 - SNES OBJ Mode enable (0=Cancel, 1=Enable) Bit 1 - Change OBJ Color (0=No, 1=Use definitions below) Bit 2-7 - Not used (zero) 2-3 System Color Palette Number for OBJ Palette 4 (0-511) 4-5 System Color Palette Number for OBJ Palette 5 (0-511) 6-7 System Color Palette Number for OBJ Palette 6 (0-511) 8-9 System Color Palette Number for OBJ Palette 7 (0-511) These color entries are ignored if above Control Bit 1 is zero. Because each OBJ palette consists of 16 colors, four system palette entries (of 4 colors each) are transferred into each OBJ palette. The system palette numbers are not required to be aligned to a multiple of four, and will wrap to palette number 0 when exceeding 511. For example, a value of 511 would copy system palettes 511, 0, 1, 2 to the SNES OBJ palette. A-F Not used (zero)
The recommended method is to "display" gameboy BG tiles F9h..FFh from left to right as first 7 characters of the bottom-most character line of the gameboy screen. As for normal 4KByte VRAM transfers, this area should not be scrolled, should not be overlapped by gameboy OBJs, and the gameboy BGP palette register should be set up properly. By following that method, SNES OAM data can be defined in the 70h bytes of the gameboy BG tile memory at following addresses:
8F90-8FEF SNES OAM, 24 Entries of 4 bytes each (96 bytes) 8FF0-8FF5 SNES OAM MSBs, 24 Entries of 2 bits each (6 bytes) 8FF6-8FFF Not used, don't care (10 bytes)
The format of SNES OAM Entries is:
Byte 0 OBJ X-Position (0-511, MSB is separately stored, see below) Byte 1 OBJ Y-Position (0-255) Byte 2-3 Attributes (16bit) Bit 0-8 Tile Number (use only 00h-FFh, upper bit zero) Bit 9-11 Palette Number (use only 4-7) Bit 12-13 OBJ Priority (use only 3) Bit 14 X-Flip (0=Normal, 1=Mirror horizontally) Bit 15 Y-Flip (0=Normal, 1=Mirror vertically)
The format of SNES OAM MSB Entries is:
Actually, the format is unknown ??? However, 2 bits are used per entry: One bit is the most significant bit of the OBJ X-Position. The other bit specifies the OBJ size (8x8 or 16x16 pixels).