https://www.nesdev.org/wiki/NES_2.0_Mapper_474
I've created a mapper that maps 0x4020 to 0xFFFF to PRG ROM.
I called it Akerasoft Mapper 474. This is NROM-383.
It maps more than NROM-368. But an NROM-368 is also planned.
My prototype uses FLASH memory but is designed to be programmed before soldering and is not reflashable.
https://www.nesdev.org/wiki/NROM-368
My design however is this 64KB of ROM space:
Bank 0:
Memory 0x0000 - 0x3FFF (empty or ignored, all zeros?)
Bank 1:
Memory 0x4000 - 0x041F (empty or ignored, all zeros?)
Memory 0x4020 - 0x7FFF (PRG)
Bank 2:
Memory 0x8000 - 0xBFFF (PRG)
Bank 3:
Memory 0xC000 - 0xFFFF (PRG)
Mapper 474 submapper 0 - 0x4020 - 0xFFFF (NROM-383)
Mapper 474 submapper 1 - 0x4800 - 0xFFFF (NROM-368)
https://www.nesdev.org/wiki/NES_2.0_Mapper_474
I've created a mapper that maps 0x4020 to 0xFFFF to PRG ROM.
I called it Akerasoft Mapper 474. This is NROM-383.
It maps more than NROM-368. But an NROM-368 is also planned.
My prototype uses FLASH memory but is designed to be programmed before soldering and is not reflashable.
https://www.nesdev.org/wiki/NROM-368
My design however is this 64KB of ROM space:
Bank 0:
Memory 0x0000 - 0x3FFF (empty or ignored, all zeros?)
Bank 1:
Memory 0x4000 - 0x041F (empty or ignored, all zeros?)
Memory 0x4020 - 0x7FFF (PRG)
Bank 2:
Memory 0x8000 - 0xBFFF (PRG)
Bank 3:
Memory 0xC000 - 0xFFFF (PRG)
Mapper 474 submapper 0 - 0x4020 - 0xFFFF (NROM-383)
Mapper 474 submapper 1 - 0x4800 - 0xFFFF (NROM-368)