NES 2.0 Mapper 355
NES 2.0 Mapper 355 is used for the original 1989 release of 3D Block (copyright Hwang Shinwei rather than RCM Group) and the original single-cartridge version of Block Force, which runs on the same PCB that has "3D-Block" written on it. Its UNIF board name is 3D-BLOCK without prefix.
It is basically NROM-256 with 8 KiB of CHR-RAM rather than CHR-ROM, but with an unusual protection mechanism: a PIC16C54 microcontroller with an embedded internal program ROM watches A4..A14 of the 6502 CPU address bus and raises IRQs based on what it sees.
To emulate this mapper, the embedded ROM's data, included as a Misc. ROM in the NES 2.0 specification, must be executed by a PIC16C54 emulator (running at one fourth of the 6502 clock/M2 switching rate), with the emulated microcontrollers' input/output ports being connected to the cartridge's M2, IRQ and CPU A4..A14 signals.
Note that even though 3D Block seems to write specific values to addresses in the $4xxx range, since the CPU D0..D7 signals are not connected to the microcontroller, their values cannot affect IRQ generation, though their addresses could.
