INES Mapper 242
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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 (PCB code ET-113) 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 A4-A0 are replaced with the values of five solder pads, which when the menu code reads particular ROM locations effectively selects one of up to 32 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.
See also
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.
