BNROM

The designation BxROM refers to the Nintendo cartridge board NES-BNROM and its HVC counterpart. The iNES format assigns mapper 34 to BxROM (as well as NINA-001).

Overview

• PRG ROM size: 128 KB (DIP-28 Nintendo pinout)
• PRG ROM bank size: 32 KB
• PRG RAM: None
• CHR capacity: 8 KB RAM
• CHR bank size: Not bankswitched
• Nametable mirroring: Solder pads select vertical or horizontal mirroring
• Subject to bus conflicts: Yes

The bank number at power on is not defined. The 6502's vectors must be present in all banks, along with the NMI, reset, and IRQ handlers.

Banks

• CPU $8000-$FFFF: 32 KB switchable PRG ROM bank

Solder pad config

• Horizontal mirroring : 'H' disconnected, 'V' connected.
• Vertical mirroring : 'H' connected, 'V' disconnected.

Registers

Bank select ($8000-$FFFF)

7  bit  0
---- ----
xxxx xxPP
       ||
       ++- Select 32 KB PRG ROM bank for CPU $8000-$FFFF

Hardware

The BxROM board contains a 74HC161 binary counter used as a quad D latch (4-bit register) to select the current PRG bank.

Variants

The iNES mapper used to implement this mapper also includes three additional registers at $7FFD-$7FFF for emulation of the NINA-001 board (using CHR ROM instead of RAM, as well as supporting 8KB of PRG RAM), which has caused many headaches for NES emulator authors. Emulator developers may consider switching between NINA-001 emulation and BxROM emulation based on the presence of CHR ROM.

The only difference between AMROM and BNROM visible to the program is the mirroring configuration.

The upper 2 bank select bits on the 74HC161 were left unconnected. A hypothetical "oversize" variant of BNROM could have supported up to 512 KB of PRG ROM, but by the time larger ROMs became affordable, more flexible ASIC mappers like MMC1 had become common.

In theory, it would be possible to implement the bank select register with a 74HC377 octal D latch, allowing up to 8 megabytes of PRG ROM, but due to mask ROM cost in the NES era, no non-pirate NES cart used this much memory.