NES 2.0 Mapper 308
NES 2.0 Mapper 308 is used for a bootleg version of the Sunsoft game Batman. It uses a VRC2 clone (VRC2b, CPU A0=chip A0, CPU A1=chip A1, i.e. similar to Mapper 23 Submapper 3) with custom IRQ functionality. Its UNIF board name is UNL-TH2131-1.
VRC2-compatible registers ($8000-$FFFF)
See description at VRC2.
IRQ Acknowledge and Reset ($F000)
Acknowledge IRQ and disable IRQ counting; reset low counter to zero.
IRQ Counter Enable ($F001)
Enable IRQ counting.
IRQ High Counter Value ($F003)
Mask: $F003 D~7654 3210 --------- VVVV .... ++++----- Value for high counter
If IRQ counting is enabled, the 12 bit low counter is increased on every M2 cycle. Upon reaching the value 2048, the high counter is decreased. If the high counter is zero and the low counter is below 2048, an IRQ is asserted.