INES
iNES was an early NES emulator developed by Marat Fayzullin. Its most lasting contribution to the NES scene was its ROM file format andd mapper numbering system.
iNES emulator
The iNES emulator is distributed as freeware on several UNIX platforms and as highly crippled shareware on MS-DOS and Microsoft Windows.
.NES format
The iNES format (file name suffix .nes) is the de facto standard for distribution of NES binary programs. It consists of the following sections, in order:
- Header (16 bytes)
- Trainer, if present (0 or 512 bytes)
- PRG ROM (16384 * x bytes)
- CHR ROM, if present (8192 * y bytes)
- PlayChoice hint screen, if present (0 or 8192 bytes)
The format of the header is as follows:
- 0-3: Constant $4E $45 $53 $1A ("NES" followed by MS-DOS end-of-file)
- 4: Size of PRG ROM in 16 KB units
- 5: Size of CHR ROM in 8 KB units (Value 0 means the board uses CHR RAM)
- 6: Flags 6
- 7: Flags 7
- 8: Size of PRG RAM in 8 KB units (Value 0 infers 8 KB for compatibility; see PRG RAM circuit)
- 9: Flags 9
- 10: Flags 10 (unofficial)
- 11-15: Zero filled
Flags 6
76543210 |||||||| ||||+||+- 0xx0: horizontal mirroring; 0xx1: vertical mirroring; 1xxx: four-screen mirroring |||| |+-- 1: SRAM in CPU $6000-$7FFF, if present, is battery backed |||| +--- 1: 512-byte trainer at $7000-$71FF (stored before PRG data) ++++----- Lower nybble of mapper number
Famicom copiers did not implement all the Famicom mappers. Instead, games for the copiers were patched to use the mappers that the copier supported, and some of these patches relied on extra code called a "trainer" that the copier loaded into PRG RAM before starting the game.
Flags 7
76543210 |||| || |||| |+- VS Unisystem |||| +-- PlayChoice-10 (8KB of Hint Screen data stored after CHR data) ++++----- Upper nybble of mapper number
The PlayChoice-10 bit is not part of the official specification, and most emulators simply ignore the extra 8KB of data. PlayChoice games are designed to look good with the RGB PPU, which handles color emphasis differently from a standard NES PPU.
Vs. games have a coin slot and different palettes. The detection of which palette a particular game uses is left unspecified.
Flags 9
76543210 |||||||| |||||||+- TV system (0: NTSC; 1: PAL) +++++++-- Reserved, set to zero
Though in the official specification, very few emulators honor this bit as virtually no ROM images in circulation make use of it.
Flags 10
76543210 || || || ++- TV system (0: NTSC; 2: PAL; 1/3: dual compatible) |+----- SRAM in CPU $6000-$7FFF is 0: present; 1: not present +------ 0: Board has no bus conflicts; 1: Board has bus conflicts
This byte is not part of the official specification, and relatively few emulators honor it.
The PRG RAM Size value (stored in byte 8) was recently added to the official specification; as such, virtually no ROM images in circulation make use of it.
Older versions of the iNES emulator ignored bytes 7-15, and several ROM management tools wrote messages in there. Commonly, these will be filled with "DiskDude!", which results in 64 being added to the mapper number.
A general rule of thumb: if the last 4 bytes are not all zero, an emulator should either mask off the upper 4 bits of the mapper number or simply refuse to load the ROM.
Mapper numbers
In the iNES file format, cartridge boards were divided into classes called "mappers" based on similar board hardware and behavior, and each mapper was given a number from 0 to 255.
A short list of common mappers can be found below - a more complete list can be found here.
| Number | Boards | Primary mapper hardware |
|---|---|---|
| 0 | NROM | None |
| 1 | SxROM | MMC1 |
| 2 | UxROM | 74LS161 |
| 3 | CNROM | 74LS161 |
| 4 | TxROM, HKROM | MMC3, MMC6 |
| 5 | ExROM | MMC5 |
| 7 | AxROM | 74LS161 |
| 9 | PxROM | MMC2 |
| 10 | FxROM | MMC4 |
| 11 | Color Dreams | |
| 13 | CPROM | 74LS161 |
| 34 | BxROM, NINA-001 | 74LS161 |
| 66 | GNROM | |
| 71 | Camerica | |
| 118 | TLSROM | MMC3 |
| 119 | TQROM | MMC3 |
