INES Mapper 242

iNES Mapper 242 denotes an address-latch-based multicart circuit board mounting 512 KiB of PRG-ROM plus 8 KiB of unbanked CHR-RAM. A variant used by Chinese RPGs from Waixing adds 8 KiB battery-backed WRAM and does not implement the UNROM-like bankswitching modes. Its UNIF MAPR is UNL-43272.

A variant of this PCB adds provisions for a second PRG-ROM chip that is always 128 KiB in size and that contains a replacement menu plus one or two replacement games for the first 512 KiB PRG-ROM chip that contained a menu of its own, resulting in a non-power-of-two 640 KiB of PRG-ROM. The chip select logic is implemented such that the replacement chip is banked-in on power-on/reset.

Address Latch ($8000-$FFFF, write)

[A~1... .CLm OQQP PpMS]
         ||| |||| |||+-0: PRG A14=p
         ||| |||| |||  1: PRG A14=CPU A14
         ||| |||| ||+- 0: Vertical mirroring
         ||| |||| ||   1: Horizontal mirroring
         ||| |||+-++-- PRG A16..A14 (inner bank)
         ||| |++------ PRG A18..A17 (outer bank)
         ||| +-------- 0: When CPU A14=1: PRG A16..14=LLL
         |||           1: When CPU A14=1: PRG A16..14=PPp
         ||+---------- 0: PRG A4..A0=CPU A4..A0
         ||            1: PRG A4..A0=Solder pad 4-0
         |+----------- Value for PRG A16..14 when CPU A14=1 and O=0
         +------------ 0: Select 128 KiB PRG-ROM chip (appearing second in .NES file)
                       1: Select 512 KiB PRG-ROM chip (appearing first in .NES file)                         
Power-on value: 0

Effective meaning:

Bit 9   Bit 7   Bit 0   Meaning
$200s   $080s   $001s
 (L)     (O)     (S)
  0       0       0     Switchable inner 16 KiB bank PPp at CPU $8000-$BFFF, fixed inner bank #0 at CPU $C000-$FFFF (UNROM-like with fixed bank 0)
  0       0       1     Switchable inner 16 KIB bank PP0 at CPU $8000-$BFFF, fixed inner bank #0 at CPU $C000-$FFFF (UNROM-like with only even banks reachable, pointless)
  1       0       0     Switchable inner 16 KiB bank PPp at CPU $8000-$BFFF, fixed inner bank #7 at CPU $C000-$FFFF (UNROM)
  1       0       1     Switchable inner 16 KIB bank PP0 at CPU $8000-$BFFF, fixed inner bank #7 at CPU $C000-$FFFF (UNROM with only even banks reachable, pointless)
  ?       1       0     Switchable 16 KiB inner bank PPp at CPU $8000-$BFFF, mirrored at CPU $C000-$FFFF (NROM-128)
  ?       1       1     Switchable 32 KiB inner bank PP at CPU $8000-$FFFF (NROM-256)

• On multicart PCBs (no PRG-RAM/battery), CHR-RAM is write-protected when O=1 and write-enabled when O=0, i.e. write-protected in the NROM-128 and NROM-256 modes.
• When the m bit is set, PRG A3-A0 are replaced with the values of four solder pads, which when the menu code reads particular ROM locations effectively selects one of up to sixteen menus with different game counts.
• Because all bits are cleared on reset, both CPU $8000-$BFFF and $C000-$FFFF are set to 16 KiB bank #0 on reset.

Similar mappers

• INES Mapper 227 is a variant with 1 MIB PRG-ROM that moves the m bit to a different bit location.
• NES 2.0 Mapper 380 is an incompatible variant with similar functionality.
• NES 2.0 Mapper 449 is an incompatible variant that adds 32 KiB CHR-RAM bankswitching functionality.