INES
iNES was an early NES emulator developed by Marat Fayzullin. Its most lasting contribution to the NES scene was its ROM file format and mapper numbering system.
A second version of this standard, NES 2.0, is currently under proposal.
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 data (16384 * x bytes)
- CHR ROM data, if present (8192 * y bytes)
- PlayChoice INST-ROM, if present (0 or 8192 bytes)
- PlayChoice PROM, if present (16 bytes Data, 16 bytes CounterOut) (this is often missing, see PC10 ROM-Images for details)
Some ROM-Images additionally contain a 128-byte (or sometimes 127-byte) title at the end of the file.
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: vertical arrangement/horizontal mirroring (CIRAM A10 = PPU A11) |||| || 0xx1: horizontal arrangement/vertical mirroring (CIRAM A10 = PPU A10) |||| || 1xxx: four-screen VRAM |||| |+-- 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
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.
Some mappers, such as MMC1, MMC3, and AxROM, can control nametable mirroring. They ignore bit 0. On the other hand, if bit 3 is true, the cart has 4 KiB of RAM at PPU $2000-$2FFF, and it ignores the mapper's CIRAM A10 output (as in Rad Racer 2). One exception is #218, which uses the four-screen bit to switch between one-screen and V/H mirroring.
Unlike the PowerPak, Famicom copiers did not implement even the most popular 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. One of these copiers was made by Front Fareast Industrial; hence the nickname "FFE" for these mapper hacks. It is probably not worth the effort to support FFE hacks in future emulators or copiers because substantially all games available with an FFE hack also have a good dump without a trainer.
Flags 7
76543210 |||||||| |||||||+- VS Unisystem ||||||+-- PlayChoice-10 (8KB of Hint Screen data stored after CHR data) ||||++--- If equal to 2, flags 8-15 are in NES 2.0 format ++++----- 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.
NES 2.0 is a more recent extension to the iNES format that allows more flexibility in ROM and RAM size, among other things.
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, and the header is not marked for NES 2.0 format, an emulator should either mask off the upper 4 bits of the mapper number or simply refuse to load the ROM.
