View source for BNROM

The designation BNROM refers to the Nintendo cartridge board NES-BNROM. The [[iNES]] format assigns [[iNES Mapper 034|mapper 34]] to BNROM (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 [[Category:Mappers with CHR RAM]]
* CHR bank size: Not bankswitched
* Nametable [[mirroring]]: Solder pads select vertical or horizontal mirroring
* Subject to [[bus conflict]]s: Yes [[Category:Mappers with bus conflicts]]

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 [[74161|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 [[AxROM|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.

{{octal D|8 megabytes of PRG ROM}}

[[Category:Discrete logic mappers]]