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iNES, NES 2.0, and UNIF ROM format reference. Use when reading, writing, parsing, or debugging .nes/.unf ROM files, or handling NES ROM headers.
Codex 또는 Claude로 설치 이 Prompt를 복사해 Codex, Claude 또는 다른 어시스턴트에 붙여 넣으면 Skill 페이지를 검토하고 설치를 진행할 수 있습니다.
SOC 직업 분류 기준
Write, read, debug, and optimize 6502 assembly language code. Use this skill whenever the user asks about 6502 assembly, MOS 6502, 6510, 65C02, or 65C816 programming, or is working on projects for the NES, SNES, Commodore 64, Apple II, Atari 2600/8-bit, VIC-20, BBC Micro, or any other 6502-based platform. Also trigger when the user mentions opcodes, addressing modes, zero page, stack operations, or assembly language concepts in the context of 8-bit/retro computing. Covers instruction set, addressing modes, common algorithms (16-bit math, multiplication, division), memory layout, optimization, and platform-specific patterns. Even if the user just says "assembly" in a retro computing context, use this skill.
Write and run FCEUX Lua scripts for NES ROM automation — screenshots, PPU memory dumps, automated input, code execution tracing, and API discovery. Use when the user wants to automate FCEUX, capture ROM data, dump CHR/nametable/palette, trace code execution, or script NES emulation tasks. Triggers on mentions of FCEUX, Lua scripting for NES, ROM analysis automation, ppu.readbyte, memory.registerexec, gui.savescreenshotas, joypad.set.
Disassemble retro ROMs (NES, CHIP-8) into bit-perfect reassemblable assembly with retrodisasm. Use when the user wants to disassemble a ROM, reverse-engineer NES code, inspect ROM internals, batch-process ROMs, verify reassembly, or convert binaries to assembly source.
| name | nes-format |
| description | iNES, NES 2.0, and UNIF ROM format reference. Use when reading, writing, parsing, or debugging .nes/.unf ROM files, or handling NES ROM headers. |
Three ROM formats exist: iNES, NES 2.0, and UNIF. NES 2.0 is the current standard and recommended for new dumps. UNIF is deprecated.
| Signature in file | Format |
|---|---|
NES + $1A + byte 7 bits 3-2 = 10 | NES 2.0 |
NES + $1A + byte 7 bits 3-2 ≠ 10 | iNES |
UNIF | UNIF |
Created by Marat Fayzullin for the iNES emulator. Uses an 8-bit mapper number (0–255). Bytes 8–15 are loosely defined and vary between iNES revisions. NES 2.0 repurposes them with a formal specification.
[16-byte Header] [optional 512-byte Trainer] [PRG-ROM data] [CHR-ROM data]
Some ROM images also have a 128-byte title at the end of the file.
| Byte | Field |
|---|---|
| 0–3 | $4E $45 $53 $1A — "NES" + MS-DOS EOF |
| 4 | PRG-ROM size in 16KB units |
| 5 | CHR-ROM size in 8KB units (0 = CHR-RAM used) |
| 6 | Flags 6: NNNN FTBM |
| 7 | Flags 7: NNNN xxTT |
| 8 | PRG-RAM size in 8KB units (0 infers 8KB for compatibility) |
| 9 | TV system: bit 0 = 1 if PAL |
| 10 | Unofficial extension: TV system, PRG-RAM presence, bus conflicts |
| 11–15 | Padding (should be zero, but some tools wrote identifying strings here) |
NNNN FTBM)| Bit | Name | Meaning |
|---|---|---|
| 0 | M | Nametable arrangement: 0 = vertical mirroring, 1 = horizontal mirroring |
| 1 | B | Battery-backed PRG-RAM at $6000–$7FFF (or other persistent memory) |
| 2 | T | Trainer present between header and PRG-ROM (512 bytes at $7000–$71FF) |
| 3 | F | Alternative nametable layout (mapper-dependent, often 4-screen) |
| 7–4 | N | Mapper number low nibble (D3–D0) |
Mirroring notes:
%....1..0 = 1-screen, %....1..1 = 4-screen).Trainer: 512 bytes loaded at $7000–$71FF. Holds mapper translation / CHR-RAM caching code for early RAM cartridges and Nesticle. Not present on unmodified dumps of original cartridges.
NNNN xxTT)| Bit | Name | Meaning |
|---|---|---|
| 1–0 | T | Console type: 0 = NES/Famicom, 1 = Vs. System, 2 = PlayChoice-10 |
| 3–2 | x | NES 2.0 identifier: must be 10 (2) for NES 2.0 |
| 7–4 | N | Mapper number high nibble (D7–D4) |
Mapper number = (byte 7 bits 7–4) << 4 | (byte 6 bits 7–4) → 0–255 for iNES.
PlayChoice-10 games have 8KB of Hint Screen data stored after CHR data. Vs. games have a coin slot and different palettes.
See references/mappers.md for the full mapper reference with detailed specifications.
Not part of the original spec; few emulators honor it.
| Bits | Field |
|---|---|
| 1–0 | TV system: 0 = NTSC, 2 = PAL, 1/3 = dual compatible |
| 5–4 | PRG-RAM: 0 = present, 1 = absent |
| 6 | Bus conflicts: 0 = none, 1 = has bus conflicts |
Should be zero. If the last 4 bytes are not all zero and this is not NES 2.0, an emulator should mask off the upper 4 bits of the mapper number or refuse to load. Some tools wrote identifying strings here (e.g., "DiskDude!").
Identified by (byte 7 & $0C) == $08. Backward-compatible with iNES for bytes 0–7. Extends the mapper number to 12 bits (0–4095), adds a submapper for disambiguating board variants, and formally specifies RAM/NVRAM sizing.
[16-byte Header] [optional Trainer] [PRG-ROM] [CHR-ROM] [optional Misc ROMs]
| Bits | Field |
|---|---|
| 3–0 | Mapper number D11–D8 (extends to 12 bits total, 0–4095) |
| 7–4 | Submapper number (0–15, disambiguates board/chip variants without relying on checksums) |
See references/mappers.md for the full mapper reference with detailed specifications.
| Bits | Field |
|---|---|
| 3–0 | PRG-ROM size MSB (together with byte 4 → 12-bit PRG size) |
| 7–4 | CHR-ROM size MSB (together with byte 5 → 12-bit CHR size) |
Simple mode (MSB nibble $0–$E): PRG = value × 16KB, CHR = value × 8KB. Max: ~63 MB PRG, ~31 MB CHR.
Exponent-multiplier mode (MSB nibble = $F): Used when simple notation cannot represent the exact size. Header bits are reinterpreted:
If actual ROM size is odd and unrepresentable, pad to a representable size.
For Vs. Dual System: first half of PRG belongs to unit 1, second half to unit 2. Known splits exist for Vs. Gumshoe and Vs. Raid on Bungeling Bay.
| Bits | Field |
|---|---|
| 4–0 | PRG-RAM (volatile) shift count |
| 7–5 | PRG-NVRAM/EEPROM (non-volatile) shift count |
Size formula: 64 << shift_count bytes. Shift count 0 = absent (compatibility: 0 infers no RAM).
Battery bit rule: If the upper nibble (NVRAM) is non-zero, byte 6 bit 1 (B) must be set. The reverse also applies unless the only battery-backed memory is mapper-internal or PRG-ROM is self-flashable.
Excludes: mapper-internal memory not CPU-mapped, MMC5 EXRAM, self-flashable PRG-ROM, external storage (cassette, Turbo File).
| Bits | Field |
|---|---|
| 4–0 | CHR-RAM (volatile) shift count |
| 7–5 | CHR-NVRAM (non-volatile) shift count |
Same encoding as byte 10. Critical: emulators must not assume 8KB CHR-RAM when CHR-ROM is absent — all CHR-RAM must be explicitly specified. Nametable memory (PPU $2000–$2FFF) is excluded from this count even if 4-screen bit is set.
Two low bits:
| Value | Name | Meaning |
|---|---|---|
| 0 | RP2C02 | NTSC NES |
| 1 | RP2C07 | Licensed PAL NES |
| 2 | Multiple region | Self-adjusting to both NTSC and PAL |
| 3 | UA6538 | Dendy (PAL variant, 50.0063 Hz) |
Convention for assigning the correct value:
Interpretation depends on byte 7 bits 1–0 (Console Type):
When Console Type = 1 (Vs. System):
| Bits | Field |
|---|---|
| 3–0 | Vs. PPU variant (RP2C03-xxxx, RP2C04-0001 through -0004, RC2C05-01 through -04) |
| 7–4 | Vs. Hardware type: 0=Normal, 1=RBI Baseball, 2=TKO Boxing, 3=Super Xevious, 4=Ice Climber, 5=Dual System Normal, 6=Dual System Raid on Bungeling Bay |
RP2C04/RC2C05 PPUs use different palettes for copy protection. RC2C05 swaps $2000/$2001 and returns a signature in $2002.
When Console Type = 3 (Extended Console):
| Value | Console |
|---|---|
| 0 | Regular NES/Famicom/Dendy |
| 1 | Nintendo Vs. System |
| 2 | PlayChoice-10 |
| 3 | Famiclone with Decimal Mode CPU |
| 4 | NES/Famicom with EPSM module |
| 5 | VT01 (red/cyan STN palette) |
| 6 | VT02 |
| 7 | VT03 |
| 8 | VT09 |
| 9 | VT32 |
| A | VT369 |
| B | UMC UM6578 |
| C | Famicom Network System |
Bits 1–0: count of extra ROMs after CHR-ROM (0–3). Defined uses:
7-bit ID for the expected peripheral at $4016/$4017. $00 = unspecified, $01 = standard controllers. This typically denotes the device required to play the game; for optional devices it lists the device if having it connected doesn't preclude normal controllers. Does not cover cartridge-connected devices (those belong to the mapper definition).
See references/device-ids.md for the complete 80-entry device ID table.
UNIF uses chunked TLV (Type–Length–Value) blocks, allowing flexible ordering. Deprecated — its originator abandoned the format in December 2008. NES 2.0 is recommended for new work. A few game rips still only exist as UNIF.
[32-byte Header] [Chunk] [Chunk] ...
Header:
| Offset | Size | Value |
|---|---|---|
| 0 | 4 | UNIF (literal ASCII) |
| 4 | 4 | LE32 — minimum version needed to parse all chunks |
| 8 | 24 | Null bytes |
Each chunk:
| Offset | Size | Value |
|---|---|---|
| 0 | 4 | Type ID (4 ASCII characters) |
| 4 | 4 | LE32 — content length |
| 8 | N | Content (encoding depends on type) |
| Type | Content | Notes |
|---|---|---|
MAPR | Null-terminated UTF-8 | PCB identifier (e.g., "NES-SNROM", "HVC-UNROM"). Not a mapper number, not a free-form description. |
| Type | Content | Min Ver |
|---|---|---|
PRGn | Raw binary, n-th PRG-ROM chip (n in hex, 0-indexed) | 4 |
CHRn | Raw binary, n-th CHR-ROM chip | 4 |
PCKn | LE32 CRC-32 of n-th PRG-ROM | 5 |
CCKn | LE32 CRC-32 of n-th CHR-ROM | 5 |
| Type | Content | Notes |
|---|---|---|
NAME | Null-terminated UTF-8 | Game name |
READ | Null-terminated UTF-8 | Comments / license info |
DINF | 204-byte struct | Dumper name (100 bytes), date (day/month/year), tool name (100 bytes) |
BATR | 1-byte boolean | Battery-backed RAM present (doesn't disambiguate which RAM) |
MIRR | 1 byte | Mirroring: 0=horiz, 1=vert, 2=1scrA, 3=1scrB, 4=4scr, 5=mapper-controlled |
TVCI | 1 byte | TV: 0=NTSC only, 1=PAL only, 2=compatible with both |
CTRL | 1-byte bitmask | Controllers: 1=standard, 2=Zapper, 4=R.O.B., 8=Arkanoid, 16=Power Pad, 32=Four Score |
VROR | 1 byte | Treat CHR-ROM as RAM (deprecated, ignored by most emulators) |