NROM

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NROM
Company Nintendo, others
Boards NROM, HROM*, RROM, RTROM, SROM, STROM
PRG ROM capacity 16K or 32K
PRG ROM window n/a
PRG RAM capacity 2K or 4K in Family Basic only
PRG RAM window n/a
CHR capacity 8K
CHR window n/a
Nametable mirroring Fixed H or V, controlled by solder pads (*V only)
Bus conflicts Yes
IRQ No
Audio No
iNES mappers 000


The generic designation NROM refers to the Nintendo cartridge boards NES-NROM-128, NES-NROM-256, their HVC counterparts, and clone boards. The iNES format assigns mapper 0 to NROM.

Overview

  • PRG ROM size: 16 KiB for NROM-128, 32 KiB for NROM-256 (DIP-28 standard pinout)
  • PRG ROM bank size: Not bankswitched
  • PRG RAM: 2 or 4 KiB, not bankswitched, only in Family Basic (but most emulators provide 8)
  • CHR capacity: 8 KiB ROM (DIP-28 standard pinout) but most emulators support RAM
  • CHR bank size: Not bankswitched, see CNROM
  • Nametable mirroring: Solder pads select vertical or horizontal mirroring
  • Subject to bus conflicts: Yes, but irrelevant

Banks

All Banks are fixed,

  • CPU $6000-$7FFF: Family Basic only: PRG RAM, mirrored as necessary to fill entire 8 KiB window, write protectable with an external switch
  • CPU $8000-$BFFF: First 16 KB of ROM.
  • CPU $C000-$FFFF: Last 16 KB of ROM (NROM-256) or mirror of $8000-$BFFF (NROM-128).

Solder pad config

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

Registers

None. This has normally no mapping capability whatsoever! Nevertheless, tile animation can be done by swapping between pattern tables $0000 and $1000, using PPUCTRL bits 4-3 as a "poor man's CNROM".

Variants

RROM and SROM are NROM with different CHR ROM pinouts. RTROM and STROM split PRG into two 8KiB ROMs. HROM is an early NROM variant without the solder pads to select mirroring, only vertical mirroring available (as if the 'H' pad were selected).

Family Basic, released by Nintendo, Sharp, and Hudson, contains 2 KiB WRAM (or 4 KiB WRAM for v3) accessible at $6000-$7FFF and decoded with a 74HC20, that is backed with 2 AA batteries. The cartridge is equipped with a back up switch that, when enabled, disables all access to WRAM in order to prevent possible data corruption when the Famicom is turned on or off.

Galaxian uses a smaller 8KiB PRG ROM chip. iNES images are all therefore overdumps.

Modern homebrew programs sometimes use mapper 0 with 8 KB of CHR RAM, which would not be supported on official NROM PCBs without rewiring. In theory, BNROM might be a more appropriate choice. But most emulators will do the right thing with a 16K or 32K PRG ROM with mapper 0.

NROM-368 is a recent invention that allows addressing more memory without bank switching.

See also