NES 2.0 Mapper 268

NES 2.0 Mapper 268 is used for multicarts using the Coolboy hardware. It is functionally similar to the FK23C hardware, although the bank mask and base bits seem to be deliberately mixed-up. These multicarts have rather large amounts of PRG-ROM (usually 32 MiB) and 128 KiB or 256 KiB of CHR-RAM that is filled when a particular game has been selected. It uses an MMC3 clone with four outer bank registers. Its UNIF board name is just COOLBOY.

Registers

Outer Bank Registers ($6000-$6003)

$6000  7654 3210
       ---------
       cpPP CPPP
       |||| |+++- PRG Base bits 0-3 (0x07)
       |||| +---- CHR Bank bit 7 (0x80) 128 KiB
       ||++------ PRG Base bits 6-7 (0xC0)
       |+-------- PRG Mask bit 4 (0x20), inverted
       +--------- CHR Mask bit 7, inverted

$6001  7654 3210
       ---------
       pppP PPb.
       |||| ||+-- PRG Bank size in GNROM mode (1=32 KiB, 0=16 KiB)
       |||| ++--- PRG Base bits 4-5 (0x30)
       |||+------ PRG Base bit 3 (0x08)
       ||+------- PRG Mask bit 7 (0x80)
       |+-------- PRG Mask bit 6 (0x40)
       +--------- PRG Mask bit 5 (0x20), inverted

$6002  7654 3210
       ---------
       .... CCCC
            ++++- Inner 8 KiB CHR bank in GNROM mode

$6003  7654 3210
       ---------
       LW.M PPP.
       || | ||+-- Inner 16 KiB PRG bank in GNROM mode if $6001 bit 1=0
       || | ++--- Inner 32 KiB PRG bank in GNROM mode
       || +------ Mode: 0=MMC3, 1=GNROM
       |+-------- 1=Weird mode
       +--------- 1=Lock (only in MMC3 mode)

Banking modes

• In MMC3 mode ($6003 bit 4 clear), the MMC3's PRG/CHR bank is masked off with the PRG/CHR Mask bits, which are then replaced by the respective bits of the PRG/CHR Base SHL 4.
• In GNROM mode, the lower 4 bits of the MMC3's PRG bank are replaced with $6003 bits 1-3 (16 KiB mode, $6001 bit 1 clear)/bits 2-3 (32 KiB mode, $6001 bit 1 set) to form two 16 KiB/one 32 KiB inner PRG bank. The lower 7 bits of the MMC3's CHR bank are replaced with $6002 bits 0-3 to form one 8 KiB inner CHR bank.
• Register $6003 bit 6 enables what FCEUX's source code calls a "Weird mode - Don't even try do understand" that seems to reverse the bank order in some way.