NES 2.0 Mapper 398

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NES 2.0 Mapper 398 denotes the 晶太 YY840820C circuit board, used for a single multicart:

  • 1995 Super HiK 5-in-1 - 新系列米奇老鼠組合卡 (JY-048)

It adds an outer bank register to a VRC4 clone (A0/A1, VRC4f) together with ridiculously complex logic to implement a GNROM-like mode. The first 128 KiB of PRG-ROM and 512 KiB (first chip) of CHR-ROM contain World Hero, the second 128 KiB of PRG-ROM and 128 KiB (second chip) of CHR-ROM contain four CNROM-256 games.

Outer Bank Register ($8000-$FFFF)

Mask: $8000
 
A~FEDC BA98 7654 3210
  -------------------
  1... .... BA.. ....
            ++-------- PRG A17..A16
            |+-------- CHR A16
            +--------- CHR chip select (CHR A19 in NES 2.0 ROM image)
            |           0: first (512 KiB)
            |           1: second (128 KiB)
            +--------- Mode Select
                        0: VRC4, first CHR chip
                        1: GNROM-like, second CHR chip
Power-on value: $C0

The Outer Bank Register overlaps the VRC4 registers in the same address range.

GNROM-like mode operation

In the GNROM-like mode (Outer Bank Register 'B' bit =1), the address bits come from the following sources:

PRG-ROM A13: CPU A13
PRG-ROM A14: CPU A14
PRG-ROM A15: VRC4 CHR A12
PRG-ROM A16: Outer Bank Register 'A' bit
PRG-ROM A17: Outer Bank Register 'B' bit, i.e. always 1
CHR-ROM A10: PPU A10
CHR-ROM A11: PPU A11
CHR-ROM A12: PPU A12
CHR-ROM A13: VRC4 CHR A10
CHR-ROM A14: VRC4 CHR A11
CHR-ROM A15: VRC4 CHR A12
CHR-ROM A16: Outer Bank Register 'A' bit

As with SOROM, SUROM and SXROM, if the several CHR bank registers' CHR A12 bit does not specify the same outer bank, then PRG-ROM will would be bankswitched as the PPU renders.