INES Mapper 234

iNES Mapper 234 represents the Maxi 15 multicart, which allows the combination of slightly-modified 32k/32k CNROM and 64k/64k NINA-03 games. The only instance of this board was released with 512k/512k, but seems to be laid out in a way that would support a total of 1M/1M in 4 512k ROMs.

Overview

• PRG ROM size: 512 KiB or 1MiB
• PRG ROM bank size: 32 KiB inner / 32 or 64 KiB outer
• PRG RAM: No
• CHR capacity: 512 KiB or 1MiB ROM
• CHR bank size: 8 KiB inner / 32 or 64 KiB outer
• Nametable mirroring: Controlled by mapper.
• Subject to bus conflicts: Yes

Registers

Typical for the Atari 2600, but strange for the NES: Registers lie at $FF80-$FFF7 but bankswitching happens on the value that was read. Bus conflicts are thus avoided by storing the library of desired bankswitch values in ROM. Writes to the registers will produce bus conflicts.

All registers are cleared on reset.

Outer Bank Control ($FF80-$FF9F)

7  bit  0
MOQq BBBb  
|||| ||||
|||| ++++-- Block selection
|||+------- ROMs 3+4 Disable (0=normal, 1=disable ROMs 3+4 even if selected by Q)
||+-------- ROM switch (0=select ROMs 1+2, 1=select ROMs 3+4)
|+--------- Mode (0=CNROM, 1=NINA-03)
+---------- Mirroring (0=Vertical, 1=Horizontal)

Since the cartridge was distributed with only ROMs 1+2 populated, the Q and q bits are irrelevant.

The q bit seems to have been intended to have been an extra address line for ROMs 3+4, enabling a total of 1.5M/1.5M in the cartridge, but instead of just being routed to the A19 line for ROMs 3+4, it instead also is ORed with the Output Enable signal and so disables them.

Once any of the Q, q, BBB, or b bits are set, neither this register nor the Lockout defeat register can be updated. However, the Inner Bank Control Register can be set even when updates to the other registers are disabled.

Lockout defeat control ($FFC0-$FFDF)

7  bit  0
.... ..LL
       ||
       ++-- Lockout defeat (charge pump drive)

Emulators do not need to implement the lockout defeat control register.

Inner Bank Control ($FFE8-$FFF7)

7  bit  0
.cCC ...P
 |||    |
 |||    +-- PRG page
 +++------- CHR page

Both CNROM and NINA-03 modes select a 32kB bank of PRG ROM at $8000 and an 8kB bank of CHR ROM at $0000.

In CNROM mode, the 32kB PRG ROM bank is BBBb and the 8kB CHR ROM bank is BBBbCC.

In NINA-03 mode, the 32kB PRG ROM bank is BBBP and the 8kB CHR ROM bank is BBBcCC.