NES 2.0 Mapper 270 denotes OneBus console multicarts that use the consoles' universal input/output (UIO) register $412C to bankswitch higher-order PRG address lines or several PRG chips, and select CHR-RAM via $4242.
Responds to all bank bits of submappers 1-3 at the same time. This is permissible, since the only bit that is used by several submappers has the same function in each of them.
D~7654 3210
---------
.... ..A.
+-- PRG/CHR A24
D~7654 3210
---------
.... ..AB
|+- PRG/CHR A25
+-- PRG/CHR A24
D~7654 3210
---------
.... B...
+---- 0: Jumper cartridge not inserted
1: Jumper cartridge inserted
D~7654 3210
---------
.... ...C
+- 0: Use normal OneBus CHR banking
1: Use 8 KiB of unbanked CHR-RAM
D~7654 3210
---------
.... .A..
+--- PRG/CHR A24
Categories: NES 2.0 mappers with submappers, Mappers with scanline IRQs, MMC3-like mappers, Multicart mappers