NSF
The basic idea is one rips the music/sound code from an NES game and prepends a small header to the data.
A program of some form (6502/sound emulator) then takes the data and loads it into the proper place into the 6502's address space, then inits and plays the tune.
Header Overview
offset # of bytes Function ---------------------------- 0000 5 STRING "NESM",01Ah ; denotes an NES sound format file 0005 1 BYTE Version number (currently 01h) 0006 1 BYTE Total songs (1=1 song, 2=2 songs, etc) 0007 1 BYTE Starting song (1= 1st song, 2=2nd song, etc) 0008 2 WORD (lo/hi) load address of data (8000-FFFF) 000a 2 WORD (lo/hi) init address of data (8000-FFFF) 000c 2 WORD (lo/hi) play address of data (8000-FFFF) 000e 32 STRING The name of the song, null terminated 002e 32 STRING The artist, if known, null terminated 004e 32 STRING The Copyright holder, null terminated 006e 2 WORD (lo/hi) speed, in 1/1000000th sec ticks, NTSC (see text) 0070 8 BYTE Bankswitch Init Values (see text, and FDS section) 0078 2 WORD (lo/hi) speed, in 1/1000000th sec ticks, PAL (see text) 007a 1 BYTE PAL/NTSC bits, like NES 2.0 byte 12 bit 0: if clear, this is an NTSC tune bit 0: if set, this is a PAL tune bit 1: if set, this is a dual PAL/NTSC tune bits 2-7: not used. they *must* be 0 007b 1 BYTE Extra Sound Chip Support bit 0: if set, this song uses VRC6 audio bit 1: if set, this song uses VRC7 audio bit 2: if set, this song uses FDS audio bit 3: if set, this song uses MMC5 audio bit 4: if set, this song uses Namco 163 audio bit 5: if set, this song uses Sunsoft 5B audio bits 6,7: future expansion: they *must* be 0 007c 4 ---- 4 extra bytes for expansion (must be 00h) 0080 nnn ---- The music program/data follows
NSF is loosely based on the PSID file format for C64 music/sound.
Loading a tune into RAM
If offsets 0070h to 0077h have 00h in them, then bankswitching is *not* used. If one or more bytes are something other than 00h then bankswitching is used. If bankswitching is used then the load address is still used, but you now use (ADDRESS AND 0FFFh) to determine where on the first bank to load the data.
Each bank is 4K in size, and that means there are 8 of them for the entire 08000h-0ffffh range in the 6502's address space. You determine where in memory the data goes by setting bytes 070h through 077h in the file. These determine the initial bank values that will be used, and hence where the data will be loaded into the address space.
Here's an example:
METROID.NSF will be used for the following explanation.
The file is set up like so: (starting at 070h in the file) 0070: 05 05 05 05 05 05 05 05 - 00 00 00 00 00 00 00 00 0080: ... music data goes here...
Since 0070h-0077h are something other than 00h, then we know that this tune uses bankswitching. The load address for the data is specified as 08000h. We take this AND 0fffh and get 0000h, so we will load data in at byte 0 of bank 0, since data is loaded into the banks sequentially starting from bank 0 up until the music data is fully loaded.
Metroid has 6 4K banks in it, numbered 0 through 5. The 6502's address space has 8 4K bankswitchable blocks on it, starting at 08000h-08fffh, 09000h-09fffh, 0a000h-0afffh ... 0f000h-0ffffh. Each one of these is 4K in size, and the current bank is controlled by writes to 05ff8h through 05fffh, one byte per bank. So, 05ff8h controls the 08000h-08fffh range, 05ff9h controls the 09000h-09fffh range, etc. up to 05fffh which controls the 0f000h-0ffffh range. When the song is loaded into RAM, it is loaded into the banks and not the 6502's address space. Once this is done, then the bank control registers are written to set up the inital bank values. To do this, the value at 0070h in the file is written to 05ff8h, 0071h is written to 05ff9h, etc. all the way to 0077h is written to 05fffh. This is should be done before every call to the init routine.
If the tune was not bankswitched, then it is simply loaded in at the specified load address, until EOF
- If the tune is bankswitched
- Load the data into the CPU address space starting at load_address;
- If the tune is not bankswitched
- Load the data starting at (start_address AND 0FFFh) and stop.
Initializing a tune
This is pretty simple. Load the desired song # into the accumulator, minus 1 and set the X register to specify PAL (X=1) or NTSC (X=0). If this is a single standard tune (i.e. PAL *or* NTSC but not both) then the X register contents should not matter. Once the song # and optional PAL/NTSC standard are loaded, simply call the INIT address. Once init is done, it should perform an RTS.
- Clear all RAM at $0000-$07FF and $6000-$7FFF.
- Init the sound registers by writing $00 to $4000-$4013 and $0F to $4015. (The spec originally recommended writing $10 to $4010, but it was since discovered that the APU ignores bits 4 and 5 of this value.)
- If the tune is bankswitched, load the bank values from the header into $5FF8-$5FFF.
- Set the accumulator and X registers for the desired song.
- Call the music init routine.
Playing a tune
Once the tune has been initialized, it can now be played. To do this, simply call the play address several times a second. How many times per second is determined by offsets $006e and $006f in the file. These bytes denote how many microseconds to wait between calls. These playback rates are common:
- 60.002 Hz (recommended by the original NSF specification, close to APU timer IRQ rate): 16666 ($411A)
- 60.099 Hz (actual NTSC NES frame rate): 16640 ($4100)
- 50.007 Hz (PAL NES frame rate): 19997 ($4E1D)
To generate a differing playback rate, use this formula:
1000000
period= ---------
speed
Where period is the value you stick into $006e-$006f in the file, and speed is the desired speed in hertz.
Some NSFs use a much faster playback rate than the typical value of one call per frame. These can be played on an emulator or on an NES by treating the playback routine as if it were called by a mapper IRQ handler.
The player should call the play address of the music at periodic intervals determined by the speed words. Which word to use is determined by which mode you are in- PAL or NTSC.
Sound Chip Support
Byte 007bh of the file stores the sound chip flags. If a particular flag is set, those sound registers should be enabled. If the flag is clear, then those registers should be disabled. All I/O registers within 8000-FFFF are write only and must not disrupt music code that happens to be stored there.
APU
- Uses registers 4000-4013, 4015 and 4017. See APU for more information.
- 4015 is set to 0F on reset by most players. It is better if the NSF does not assume this and initializes this register itself, but there are several existing NSF files that require it (Battletoads, Castlevania and Gremlins 2 are examples).
- The interrupts that can be enabled via 4015 and 4017 are not supported by the NSF format.
VRCVI
- Uses registers 9000-9002, A000-A002, and B000-B002, write only. See VRC6 Audio for more information.
- Note: The A0 and A1 lines are flipped on a few games! If you rip the music and it sounds all funny, flip around the xxx1 and xxx2 register pairs. (i.e. 9001 and 9002) 9000 and 9003 can be left untouched. I decided to do this since it would make things easier all around, and this means you only will have to change the music code in a very few places (6). Esper2 and Madara will need this change, while Castlevania 3j will not for instance.
VRCVII
- Uses registers 9010 and 9030, write only. See VRC7 Audio for more information.
FDS Sound
- Uses registers from 4040 through 4092. See FDS Audio for more information.
Notes:
- 6000-DFFF is assumed to be RAM, since 6000-DFFF is RAM on the FDS. E000-FFFF is usually not included in FDS games because it is the BIOS ROM. However, it can be used on FDS rips to help the ripper (for modified play/init addresses).
- Bankswitching operates slightly different on FDS tunes. 5FF6 and 5FF7 control the banks 6000-6FFF and 7000-7FFF respectively. NSF header offsets 76h and 77h correspond to *both* 6000-7FFF *AND* E000-FFFF. Keep this in mind!
MMC5 Sound
- Uses registers 5000-5015, write only as well as 5205 and 5206, and 5C00-5FF5. see MMC5 Audio for more information.
Notes:
- 5205 and 5206 are a hardware 8*8 multiplier. The idea being you write your two bytes to be multiplied into 5205 and 5206 and after doing so, you read the result back out.
- 5C00-5FF5 should be RAM to emulate EXRAM while in MMC5 mode.
Namco 106 Sound
- Uses registers 4800 and F800. See Namco 106 Audio for more information.
Sunsoft 5B Sound
- Audio in the Sunsoft 5B mapper, a variant of the FME-7, uses registers C000 and E000. See Sunsoft audio.
Caveats
- The starting song number and maximum song numbers start counting at 1, while the init address of the tune starts counting at 0. To "fix", simply pass the desired song number minus 1 to the init routine.
- The NTSC speed word is used *only* for NTSC tunes, or dual PAL/NTSC tunes. The PAL speed word is used *only* for PAL tunes, or dual PAL/NTSC tunes.
- The length of the text in the name, artist, and copyright fields must be 31 characters or less! There has to be at least a single NULL byte (00h) after the text, between fields.
- If a field is not known (name, artist, copyright) then the field must contain the string "<?>" (without quotes).
- There should be 8K of RAM present at 6000-7FFFh. MMC5 tunes need RAM at 5C00-5FF7 to emulate its EXRAM. 8000-FFFF should be read-only (not writable) after a tune has loaded. The only time this area should be writable is if an FDS tune is being played.
- Do not assume the state of *anything* on entry to the init routine except A and X. Y can be anything, as can the flags.
- Do not assume the state of *anything* on entry to the play routine either. Flags, X, A, and Y could be at any state. I've fixed about 10 tunes because of this problem and the problem, above.
- The stack sits at 1FFh and grows down. Make sure the tune does not attempt to use 1F0h-1FFh for variables. (Armed Dragon Villigust did and I had to relocate its RAM usage to 2xx)
- Variables should sit in the 0000h-07FFh area *only*. If the tune writes outside this range, say 1400h this is bad and should be relocated. (Terminator 3 did this and I relocated it to 04xx).
