Programming UNROM

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UNROM is the common name for a discrete mapper found on the UNROM board as well as the less common UOROM board. UNROM has 64 KB or 128 KB PRG-ROM (divided into 8 16k banks) and CHR-RAM. The UOROM board works the same way and can take PRG-ROM up to 256 KB (16 banks). It is very easy to use once you know how to load CHR RAM.

iNES header

Here is an NES 2.0 header for mapper 2 on the UNROM and UOROM boards. It should be backward-compatible with emulators supporting only the older iNES header format, but they may emulate extra RAM at $6000-$7FFF, where official boards have open bus.

.segment "HEADER"
  .byte "NES", $1A
  .byte 8         ;UNROM has 8 16k banks; change this to 4 or 16 as needed
  .byte 0         ;No CHR ROM
  .byte $20, $08  ;Mapper 2, horizontal mirroring, NES 2.0
  .byte $00       ;No submapper
  .byte $00       ;PRG ROM not 4 MiB or larger
  .byte $00       ;No PRG RAM
  .byte $07       ;8192 (64 * 2^7) bytes CHR RAM, no battery
  .byte $00       ;NTSC; use $01 for PAL
  .byte $00       ;No special PPU

To use vertical mirroring instead of horizontal mirroring, change the .byte $20, $08 to .byte $21, $08

Bankswitching

UNROM has four or eight banks 16 KB in size; UOROM has 16 banks. The last of these banks is fixed at $C000-$FFFF. The rest (numbered 0-2, 0-6, or 0-14) are switchable at $8000-$BFFF.

Switching banks requires a write to $8000-$FFFF. In UNROM, bits 0-2 of the byte written to $8000-$FFFF will select the bank; UOROM uses bits 0-3. When writing to $8000-$FFFF, the value you are writing must match the value located at the destination address in ROM (see Bus conflict). One way to ensure this is to have a bankswitch lookup table. You can read from this table and then immediately write that value back to the table.

.segment "RODATA"
banktable:              ; Write to this table to switch banks.
  .byte $00, $01, $02, $03, $04, $05, $06
  .byte $07, $08, $09, $0A, $0B, $0C, $0D, $0E
  ; UNROM needs only the first line of this table (0-6)
  ; but UOROM needs both lines (0-14).

.segment "ZEROPAGE":    ; The mapper is read-only; need to track its state separately
current_bank: .res 1

.segment "CODE"
bankswitch_y:
  sty current_bank      ; save the current bank in RAM so the NMI handler can restore it
bankswitch_nosave:
  lda banktable, y      ; read a byte from the banktable
  sta banktable, y      ; and write it back, switching banks
  rts

The lookup table and the bankswitching subroutine are normally located in the fixed bank ($C000-$FFFF).

To save 12 cycles per bankswitch at a cost of 5 bytes of ROM, the bankswitch_y subroutine can be made into a macro.

With the lookup table and bankswitching subroutine in place, switching banks is as easy as this:

  ldy #$02
  jsr bankswitch_y     ;switch to bank 2

If you switch banks in the NMI handler, such as to run a sound engine, do not write to current_bank. Instead, do this at the end of the NMI handler just before pulling registers:

  ldy current_bank
  jsr bankswitch_nosave

See also