Unlocking and Detecting SGB Functions

Cartridge Header

SGB games are required to have a cartridge header with Nintendo logo and proper checksum just as normal Game Boy games. Also, two special entries must be set in order to unlock SGB functions:

• SGB flag: Must be set to $03 for SGB games
• Old licensee code: Must be set to $33 for SGB games

When these entries aren’t set, the game will still work just like all “monochrome” Game Boy games, but it cannot access any of the special SGB functions.

Detecting SGB hardware

SGB hardware can be detected by examining the initial value of the C register directly after startup: a value of $14 indicates SGB or SGB2 hardware. It is also possible to separate between SGB and SGB2 by examining the initial value of the A register directly after startup. Note that the DMG and MGB share initial A register values with the SGB and SGB2 respectively.

For initial register values on all systems, see the table of all CPU registers after power-up.

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.

Only the SGB2 contains a link port.

SGB hardware has traditionally been detected by sending MLT_REQ commands, but this method is more complicated and slower than checking the value of the A and C registers after startup. The MLT_REQ command enables two (or four) joypads; a normal handheld Game Boy will ignore this command, but an SGB will return incrementing joypad IDs each time when deselecting keypad lines (see MLT_REQ description). The joypad state/IDs can then be read out several times, and if the IDs are changing, then it is an SGB (a normal Game Boy would typically always return $0F as the ID). Finally, when not intending to use more than one joypad, send another MLT_REQ command in order to disable the multi-controller mode. Detection works regardless of how many joypads are physically connected to the SNES. However, unlike the C register method, this detection works only when SGB functions are unlocked from the cartridge header.