INES Mapper 038

iNES Mapper 038 represents the board used for Crime Busters. It is another mapper almost identical to GNROM (66) except the bits and writeable port moved around.

Overview

• PRG ROM size: 128 KiB
• PRG ROM bank size: 32 KiB
• PRG RAM: Impossible
• CHR capacity: 32 KiB ROM
• CHR bank size: 8 KiB
• Nametable mirroring: Hardwired vertical mirroring
• Subject to bus conflicts: no

Registers

Bank Select ($7000-$7FFF)

7  bit  0
---- ----
xxxx CCPP
     ||||
     ||++- Select 32 KB PRG ROM bank for CPU $8000-$FFFF
     ++--- Select 8 KB CHR ROM bank for PPU $0000-$1FFF

Hardware

Writes to $F000-$FFFF are very likely to also trigger a bankswitch, but well behaved code has no reason to. The 74138 used by this board uses /ROMSEL and R/W on its enable inputs, but /ROMSEL is 1 while M2 is low, not just while M2 is high and and A15 is low. As a result, before M2 is high, if the 2A03's A14, A13, and A12 lines are 1 and R/W is 0, the 74161's /LOAD line will still load from the data bus.

Emulator oversize support

 FCEUX (2.1.5):
  $7000-$7FFF: [CCCC CCPP]

 Nestopia (1.4.0h):
  $6000-$7FFF: [CCCC CC..]
               [PPPP PPPP]

See also: Oversize