Other MBCs

Multicart MBCs

MBC1M uses the MBC1 IC, but the board does not connect the MBC1’sA18 address output to the ROM. This allows including multiple 2 Mbit (16bank) games, with SRAM bank select ($4000) to select which of up tofour games is switched in. In theory, a MBC1M board could be made for 1Mbit or 512 kbit games by additionally not connecting A17 and A16outputs, but this appears not to have been done in licensed games.

Bung andEMS MBCs are reported to exist.

EMS

Aheader matching any of the following is detected as EMS mapper:

• Header name is “EMSMENU”, NUL-padded
• Header name is “GB16M”, NUL-padded
• Cartridge type ($0147) = $1B and region ($014A) = $E1

Registers:

• $2000 write: Normal behavior, plus save written value in $2000 latch
• $1000 write: $A5 enables configure mode, $98 disables it, and other values have no known effect
• $7000 write while configure mode is on: Copy $2000 latch to OR mask

After the OR mask has been set, all reads from ROM will OR A21-A14 (thebank number) with the OR mask. This chooses which game is visible to theCPU. If the OR mask is not aligned to the game size, the results may benonsensical.

The mapper does not support an outer bank for battery SRAM.

To start a game, perform the following steps with code running from RAM:

1. Write $A5 to $1000
2. Write game’s first bank number to $2000
3. Write any value to $7000
4. Write $98 to $1000
5. Write $01 to $2000 (so that 32K games work)
6. Jump to $0100

Wisdom Tree

The Wisdom Tree mapper is a simple, cost-optimized one-chip designconsisting of a 74LS377 octal latch in addition to the ROM chip. Becausethe mapper consists of a single standard 74 series logic chip, it hastwo unusual properties:

First, unlike a usual MBC, it switches the whole 32 KiB ROM area insteadof just the $4000-$7FFF area. Therefore, if you want to usethe interrupt vectorswith this cart, you should duplicate them across all banks.Additionally, since the 74LS377’s contents can’t be guaranteed when powering on,the ROM header and some code for switching to a known bank should alsobe included in every bank. This also means that the Wisdom Tree mappercould be used as a multicart mapper for 32 KiB ROMs, assuming there isenough ROM space in each bank for some small initialization code, andnone of the ROMs wrote to the $0000-$7FFF area. For example, if thelast 5 bytes of all banks are unused, games can be patched as follows:

; At $0100 in all banks but the first    nop    jp $7FFB

; At $7FFB in all banks    ld hl, $0100    ld [hl], a    jp hl

Second, because the 74LS377 latches data on thepositive write pulse edge,and the value on the Game Boy data bus is no longer valid when thepositive edge arrives, the designer of this mapper chose to use theA7-A0 address lines for selecting a bank instead of the data lines.Thus, the value you write is ignored, and the lower 8 bits of theaddress is used. For example, to select bank $XX, you would write anyvalue to address $YYXX, where $YY is in the range $00-$7F.

Magic values for detection of multicarts in emulators

Sometimes it may be useful to allow a ROM to be detected as a multicartin emulator, for example for development of a menu for physicalmulticart hardware.

Emulator authors who are interested in supporting the other multicartmappers are also encouraged to support detection of the following values.

• Detect as Wisdom Tree mapper
  • ROM title is “WISDOM TREE” (the space may be a$00 NUL character instead), $0147 = $00, $0148 = $00, size > 32k.This method works for the games released by Wisdom Tree, Inc.
  • $0147 = $C0, $014A = $D1.
• Detect as EMS multicart
  • $0147 = $1b, $014a = $e1
• Detect as Bung multicart
  • $0147 = $be