Programming NROM: Difference between revisions
m (copypasta fix) |
(omit now-redundant link to redirect; replace iNES sample headers with NES2.0 sample. ... this page is really stub shaped.) |
||
| Line 1: | Line 1: | ||
[[NROM]] and the other boards that make up | [[NROM]] and the other boards that make up '''mapper 0''' are the simplest of all NES cartridge boards. | ||
All address decoding and chip enable handling are handled by the NES hardware; the only integrated circuits on the board are the PRG ROM, CHR ROM, and (in 72-pin carts) CIC. | All address decoding and chip enable handling are handled by the NES hardware; the only integrated circuits on the board are the PRG ROM, CHR ROM, and (in 72-pin carts) CIC. | ||
| Line 11: | Line 11: | ||
There are probably a few games that rely on the mirroring, but experiments with [[Forbidden Four|a multicart engine]] show that most can run with garbage in $8000-$BFFF. | There are probably a few games that rely on the mirroring, but experiments with [[Forbidden Four|a multicart engine]] show that most can run with garbage in $8000-$BFFF. | ||
== | == [[NES 2.0]] header == | ||
<source lang="6502"> | |||
.segment "HEADER" | |||
.byte "NES", $1A | |||
.byte 2 ; 1 or 2 for NROM-128 or NROM-256 respectively | |||
.byte 1 ; 8 KiB CHR ROM | |||
.byte $00 ; Mapper 0; $00 or $01 for horizontal or vertical mirroring respectively | |||
.byte $08 ; Mapper 0; NES 2.0 | |||
.byte $00 ; No submapper | |||
.byte $00 ; PRG ROM not 4 MiB or larger | |||
.byte $00 ; No PRG RAM | |||
.byte $00 ; No CHR RAM | |||
.byte $00 ; 0 or 1 for NTSC or PAL respectively | |||
.byte $00 ; No special PPU | |||
</source> | |||
Revision as of 03:32, 7 May 2014
NROM and the other boards that make up mapper 0 are the simplest of all NES cartridge boards. All address decoding and chip enable handling are handled by the NES hardware; the only integrated circuits on the board are the PRG ROM, CHR ROM, and (in 72-pin carts) CIC.
NROM has two configurations:
- NROM-256 with 32 KiB PRG ROM and 8 KiB CHR ROM
- NROM-128 with 16 KiB PRG ROM and 8 KiB CHR ROM
Your program is mapped into $8000-$FFFF (NROM-256) or both $8000-$BFFF and $C000-$FFFF (NROM-128). Most NROM-128 games actually run in $C000-$FFFF rather than $8000-$BFFF because it makes the program easier to assemble and link. Some kinds of data used by the NES CPU, such as the vectors and sampled sound, have to be in $C000-$FFFF, and it simplifies the linker script if everything is in the same memory region. There are probably a few games that rely on the mirroring, but experiments with a multicart engine show that most can run with garbage in $8000-$BFFF.
NES 2.0 header
<source lang="6502"> .segment "HEADER"
.byte "NES", $1A .byte 2 ; 1 or 2 for NROM-128 or NROM-256 respectively .byte 1 ; 8 KiB CHR ROM .byte $00 ; Mapper 0; $00 or $01 for horizontal or vertical mirroring respectively .byte $08 ; Mapper 0; NES 2.0 .byte $00 ; No submapper .byte $00 ; PRG ROM not 4 MiB or larger .byte $00 ; No PRG RAM .byte $00 ; No CHR RAM .byte $00 ; 0 or 1 for NTSC or PAL respectively .byte $00 ; No special PPU
</source>
