MMC3 + NROM multicart

Overview

• PRG: 512kB ROM1 + 32kB ROM2
• CHR: 512kB ROM
• Mirroring: H or V (selected by software)
• Bus conflicts: no

All MMC3's PRG & CHR registers are available at $8000-$ffff for the whole time. There is no WRAM at $6000-$7fff but additional register (which can be protected from writes using MMC3's $a001 just like it protects WRAM).

After power up & soft/hard reset, this is cleared and ROM2 is selected (for $8000-$ffff).

[011. ...R .wMG GPPP] REG $6000-$7fff (address bits are latched)
         |  ||| ||||
         |  ||| |+++- selects inner 16KB PRG when in NROM mode
         |  ||+-+---- selects outer 128KB PRG & 128KB CHR (in any mode)
         |  |+------- 0: NROM mode, 1: MMC3 mode
         |  +-------- 1: protect whole $6000-$7fff register from further changes
         +----------- 0: selects ROM2, 1: selects ROM1 and protects this single bit from further changes

PRG banking

condition         | $8000-$9fff | $a000-$bfff | $c000-$dfff | $e000-$ffff | notes
------------------+-------------+-------------+-------------+-------------+--------
R=0               |  one 32 KB PRG from ROM2 (mapped to first 32 KB)      | NROM32 
------------------+-------------+-------------+-------------+-------------+--------
R=1 & M=1         |   four CPU banks just as in MMC3 (from outer bank GG) | MMC3
------------------+-------------+-------------+-------------+-------------+--------
R=1 & M=0 & GG=0  |        GGPPPP             |         GGPPPP            | NROM16
------------------+-------------+-------------+-------------+-------------+--------
R=1 & M=0 & GG!=0 |         GGPPP0            |         GGPPP1            | NROM32
------------------+-------------+-------------+-------------+-------------+--------

CHR banking

CHR banking works in the same manner like in MMC3.

Mirroring

Mirroring works in the same manner like in MMC3.

Notes

More info about this multicart can be found here: http://forums.nesdev.org/viewtopic.php?t=7926&start=15