UxROM

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The generic designation UxROM refers to the Nintendo cartridge boards NES-UNROM, NES-UOROM, their HVC counterparts, and clone boards. The iNES format assigns mapper 2 to UxROM.

Contents

[edit] Overview

  • PRG ROM size: 128 KB for UNROM, 256 KB for UOROM (DIP-28/32 Nintendo pinout)
  • PRG ROM bank size: 16 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

[edit] Banks

  • CPU $8000-$BFFF: 16 KB switchable PRG ROM bank
  • CPU $C000-$FFFF: 16 KB PRG ROM bank, fixed to the last bank

[edit] Solder pad config

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

[edit] Registers

[edit] Bank select ($8000-$FFFF) 

7  bit  0
---- ----
xxxx pPPP
     ||||
     ++++- Select 16 KB PRG ROM bank for CPU $8000-$BFFF
          (UNROM uses bits 2-0; UOROM uses bits 3-0)

[edit] Hardware

The UNROM and UOROM boards contain a 74HC161 binary counter used as a quad D latch (4-bit register) and a 74HC32 quad OR gate to make one bank always visible.

[edit] Variants

The mapper used in Codemasters games published by Camerica extends UxROM with CIC defeat circuitry.

In theory, it would be possible to implement the bank select register with a 74373/74374/74377/74573 octal D latch along with an additional 74LS32, allowing up to 4 megabytes of PRG ROM, but due to mask ROM cost in the NES era, no non-pirate NES cart used this much memory. Leaving out the additional 74LS32 would allow making a multicart.

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