NES 2.0 Mapper 268

NES 2.0 Mapper 268 is used for multicarts and Chinese RPGs using the Coolboy hardware. The 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 (submapper 0, without BMC- or BTL- prefix).

A variant that works identically but puts the outer bank registers in the $5xxx range instead of the $6xxx range uses the UNIF board name MINDKIDS (submapper 1, again, without prefix). As the Mindkids multicarts have battery-backed RAM, this was likely done to prevent writes to the outer bank registers from corrupting saved game data.

Registers

These registers overlay, but are independent from, any PRG-RAM present.

Mask: $E003/$F003 (submapper 0/1)

$6000/$5000 (submapper 0/1)

 7  bit  0
 ---- ----
 ABCC DEEE
 |||| ||||
 |||| |+++-- PRG offset (PRG A19, A18, A17)
 |||| +----- Alternate CHR A17
 ||++------- PRG offset (PRG A24, A23)
 |+--------- PRG mask (PRG A17 from 0: MMC3; 1: offset)
 +---------- CHR mask (CHR A17 from 0: MMC3; 1: alternate)

$6001/$5001 (submapper 0/1)

 7  bit  0
 ---- ----
 GHIJ KKLx
 |||| |||
 |||| ||+--- GNROM mode bank PRG size (1: 32 KiB bank, PRG A14=CPU A14; 0: 16 KiB bank, PRG A14=offset A14)
 |||+-++---- PRG offset (in order: PRG A20, A22, A21)
 ||+-------- PRG mask (PRG A20 from 0: offset; 1: MMC3)
 |+--------- PRG mask (PRG A19 from 0: offset; 1: MMC3)
 +---------- PRG mask (PRG A18 from 0: MMC3; 1: offset)

$6002/$5002 (submapper 0/1)

 7  bit  0
 ---- ----
 xxxx MMMM
      ||||
      ++++-- CHR offset for GNROM mode (CHR A16, A15, A14, A13)

$6003/$5003 (submapper 0/1)

 7  bit  0
 ---- ----
 NPxP QQRx
 || | |||
 || | +++--- PRG offset for GNROM mode (PRG A16, A15, A14)
 || +------- 1: GNROM mode; 0: MMC3 mode
 || |         (1: PRG A16...13 from QQ, L, R, CPU A14, A13 + CHR A16...10 from MMMM, PPU A12...10;
 || |          0: PRG A16...13 from MMC3 + CHR A16...A10 from MMC3 )
 |+-+------- Banking mode
 |+--------- "Weird MMC3 mode"
 +---------- Lockout (prevent further writes to these four registers, only works in MMC3 mode)

Register interpretation

The following things are always true, regardless of mode:

• The CCKK bits are always connected to PRG A24..A21.
• The PRG and CHR mask controls (I, H, G, B, & A) always control whether PRG A20..A17 and CHR A17 are connected to the internal MMC3 registers, or the literal values in other controls (J, EEE, & D).

Banking mode = $00

“Normal” oversize MMC3 mode.

Supports MMC3 anywhere between 128 KiB and 2 MiB, otherwise identical to ordinary MMC3. Goofy mask values will produce goofy memory layouts.

The combined register of CCKKJEEE could be thought of as a way to select one of 256 different 128 KiB PRG MMC3 games, or CCKK selecting one of 16 different 2 MiB MMC3 games.

Lockout works in this mode.

Banking mode = $10

“Normal” GNROM mode.

PRG A16 and A15 come from the QQ controls.

When in 16 KiB PRG mode (L control), PRG A14 comes from the R control. Otherwise, it comes from CPU A14.

CHR A16, A15, A14, and A13 come from MMMM controls. CHR A12, A11, and A10 come from PPU A12, A11, and A10.

The combined register of CCKKJEEEQQR could be thought of as a way to select one of 2048 different 16 KiB NROM games.

Note: The CHR and PRG mask registers STILL AFFECT THIS MODE, allowing weird splicing of MMC3 banking (with 128 KiB granularity) overlaying GNROM-style banking.

Lockout DOES NOT work in this mode.

Banking mode = $40

“Weird” MMC3 mode.

If MMC3 PRG ROM bank mode is "normal" (8+8+16F), then the PRG banks at 0xC000 and 0xE000 are fixed to 8 KiB bank 0, instead of 0xFE and 0xFF. Supposedly this effect is completely ignored when MMC3 PRG ROM bank mode is inverted (8F+8+8+8F).

The second half of each of the 2 KiB CHR banks is replaced with bank 0, and the LSB of MMC3 registers 0 and 1 work (instead of being replaced by PPU A10).

Otherwise, behaves as “normal” MMC3 mode above.

Banking mode = $50

“Weird” GNROM-ish mode.

Combine the rules mentioned above for “Normal” GNROM mode with “Weird” MMC3 mode. (These descriptions should not be contradictory.)

Lockout DOES NOT work in this mode.

WRAM usage

MMC3-mode operation with WRAM on the Coolboy board (submapper 0) involves setting up the extra registers with the MMC3's WRAM disabled, setting the Lockout bit, then enabling WRAM again:

LDA #$00
STA $A001   ; Disable WRAM
; initialize PRG/CHR offsets and masks
; ...
LDA #$80
STA $6003   ; Enable Lockout
STA $A001   ; Enable WRAM

Because Lockout does not work in GNROM mode, the Coolboy board cannot have a GNROM game use WRAM, with any WRAM write inadvertently changing the outer bank registers. The one known Chinese RPG (楚留香新传) running in GNROM mode with WRAM uses the Mindkids board (submapper 1) instead.

Notes

• No cart has been discovered so far that makes use of the "weird" mode.
• Some new cartridges from MINDKIDS have /WE and /OE pins connected to mapper, which allows you to rewrite flash memory without soldering. This also allows console to write data to the cartridge. No cart has been discovered so far that makes use of this feature, but this can be used for homebrew.
• Several games from Hengedianzi with 1 MiB PRG-ROM that are commonly set to INES Mapper 004 contain the necessary Coolboy writes to enable oversize MMC3 mode. They thus should be set to Mapper 268 instead of 4.