NSF2

NSF 2 is a proposed extension to the NSF file format. It is intended to be backward compatible with the original NSF, provide IRQ timer functionality, and allow track times and other metadata to be contained.

It is currently an unfinished proposal. Currently Nintendulator partially supports it. See link below for reference.

The goals are:

* IRQ supprt
* "no return" init addresses
* information block


For IRQ support, I figured allowing the use of frame IRQs and DPCM IRQs just like on the NES, and then a 16 bit IRQ timer which would be connected to the CPU in the usual way (via its IRQ input).

This would allow a real NES to play 2.0 NSFs using say a powerpak or similar.

So my proposal for IRQ hardware:

* a 16 bit IRQ timer that sits at 0x4018-0x401a
    * 4018 = lower 8 bits of the 16 bit timer reload value
    * 4019 = upper 8 bits of the 16 bit timer reload value
    * 401a = control register. 
        * bit 0 - 0 = timer off, reset.  1 = timer is running
    * all registers are readable and writable.  on a real NES, these
       addresses are fully readable/writable to the cartridge.

* allowing the use of DPCM and frame IRQs. 
    * these will work like on a real NES. you MUST write to 0x4017
       to enable the external (timer) IRQ, and to reset the frame IRQ,
      just like a regular NES.

* IRQ vector.
    * write the vector to FFFE and FFFF.  These two locations hold the
       IRQ vector like usual, but are writable.  When read by  the CPU,
       these two locations must return the two bytes written there.
    * when written, you DO NOT write to the underlying NSF data bank.
    * in effect, FFFE/FFFF become two bytes of RAM which are separate
      from the rest of NSF space.

And the proposal for no return on the init address:

    * Allow for the init address to never return.

    this basically means:
    * init becomes the reset vector
    * play becomes the NMI vector
    * and IRQ has its vector at FFFE/FFFF





Proposed header changes:


0005    1   BYTE    Version number (currently 01h)

this will be bumped to 02h for version 2.0


007c    4   ----    4 extra bytes for expansion (must be 00h)

these will be used as follows:

007c    1  BYTE   NSF 2.0 feature enables
       bit:
       0    - when set, enables the IRQ features. when clear, disables them
       1    - when set, allows for a non-returning init address.
       2    - when set, allows play calling to be disabled
       3-6 - maintain 0
       7    - an extended info block follows nsf data. (see below)

007d    3  WORD  length of NSF data block, in bytes.  LSB first (little endian)


Extended data block:

   * an extended block of data that is optional to include.
   * it has the following features:
       * stores a unicode? title up to N characters long
       * stores the same for copyright, author, and ripper
       * allow for separate author/copyright/title on each track?
       * lengths of tracks
       * any other possible ancillary data?
   * the reason for placing it at the end, is so that 1.0 players can still
      use these NSFs.  they will append the extra data into NSF space, and
      it should not affect the playback if it doesn't use any of the other
      features (IRQs, non-return init addresses)
    * You MUST still populate the original author/copyright/title strings
       in the original header for backwards compatibility.


So that's basically it.  I think it adds all the features that can be added and still function properly on an NES with a powerpak or similar player cartridge.

I'm open to suggestions or feedback on it.  If people like it, I will formalize it and update the existing NSF document, and modify my FPGA synth to conform to the document for testing.

IRQ timer

The timer would decrement at the CPU clock rate, whether it be NTSC or PAL rate (so 1.79MHz or 1.66MHz or so).

The counter is a modulus N counter and has the following behaviour:

When the counter is off (whenever 401a bit 0 is clear) it is constantly being reloaded with the values in 4018/4019, and the counter IRQ flag is cleared.

When the counter is on (401a bit 0 is set), it will decrement once per CPU cycle. When it hits 0, it is reloaded from 4018/4019, the IRQ flag is set and an IRQ is asserted.

This means an IRQ will be generated every N+1 clock cycles, where N is the value loaded into the counter. (it is N+1 because 0 is counted too).

To clear the IRQ flag, you read 401a. I should probably put the IRQ flag at bit 7 (read only) to allow easy testing of IRQ source.

Code might look like this:

timervalue:  .equ 01fffh       ;desired # of cpu cycles minus 1

starttimer: LDA #000h
               STA 0401ah         ;reset and shut off timer in case it was on

               LDA #<irqvector
               STA 0fffeh           ;store interrupt vector low of our handler
               LDA #>irqvector
               STA 0ffffh            ;store interrupt vector high

               LDA #<timervalue
               STA 04018h         ;low byte of timer value
               LDA #>timervalue
               STA 04019h         ;high byte of timer value

               LDA #0c0h
               STA 04017h         ;turn off frame IRQs
               LDA 04015h         ;ack any pending DPCM IRQ if, it exists
               CLI                     ;enable IRQs

               LDA #01h
               STA 0401ah         ;turn the timer on
               RTS

stoptimer: SEI                    ;turn off IRQs
                LDA #000h
                STA 0401ah        ;turn timer off
                RTS

irqvector:  <perform our interrupt code here>
                ....
                ....
               
                LDA 0401ah        ;reading 401a resets IRQ flag
                RTI                    ;return from interrupt


alternatively, if you wish to determine WHAT caused the IRQ (if you're using more than one source) you'd do something like this...

irqvector:   BIT 0401ah        ;bit 7 indicates we have a timer IRQ waiting
                 BPL +
                 JSR timer           ;if bit 7 was set, call timer subroutine
               + BIT 04015h
                 BPL +
                 PHP                   ;save flags if we have DPCM int.
                 JSR dpcm           ;if bit 7 was set, call DPCM sub
                 PLP
               + BVC +
                 JSR frame          ;bit 6 of 4015 = frame IRQ
               + RTI                    ;exit interrupt

timer:        <do stuff here>
                 RTS

dpcm:        <do stuff here>
                 RTS

frame:        <do stuff here>
                  RTS

There's a lot of ways to skin this cat, but this is one particular method. The idea is to read the status regs to figure out which source caused the interrupt, then run code to service it, then go back and check the other sources just in case one of them also needs servicing.

Like the APU period timers and the MMC3 scanline timer, the NSF2 cycle timer includes both the 0 and the reload value in its sequence of states. When it hits 0, the NEXT clock performs a reload.

For example, when the reload value is $0010, the counter would have a period of 17 CPU cycles:

10, 10, 10, [Enable now] 0F, 0E, 0D, 0C, ..., 02, 01, 00, 10*, 0F, 0E, 0D, 0C, ...

where * denotes the IRQ flag becoming true.

Track information

my approach would be to have records that contain track # and then information fields... something like this:

record:


offset, # of bytes, type, description
----------------------------------------
0 1 BYTE record type
1 2 WORD record length
3 1 BYTE track #
4 N --- record data

Records would be one after another, and a record of 4 00h bytes would signify the end of the data.

the type would be something like:

0 - last record
1 - title
2 - composer
3 - copyright

etc.

Track # 0ffh could be reserved, and used as a wildcard indicator,
allowing for things like this:

composer, track 0ffh, jimbob
title, track 0, my first song
title, track 1, my second song
title, track 2, a song by someone else
composer, track 2, billy
title, track 3, another song

this would populate all the "composer" entries on the tracks which do not have a specific composer (i.e. everything but track 2 in the above example).

This would allow removal of most duplicated material and allow for individual track fields to be populated with different info if required.

Also, it'd be pretty trivial to process this type of data on a real NES or other hardware player.

Possible fields could be (with tentative ID's):

01 Title followed by ASCII or unicode? data. A max length should be specified.
02 Composer (Same as above)
03 Copyright (Same as above)
04 Ripper (Same as above)
05 length (in seconds? milliseconds? NMI counts?)
06 type (maybe? i.e. SFX, BGM, title tune, etc)
07 ancillary data (i.e. compo entry #? "this is a cover of xyz")

Any other fields that would be useful? As usual, all fields are optional, and you only use the ones you need/want.

This would make it pretty easy to read via a real NES, vs. some feel-good text format. As such, it'd probably be a decent idea to only allow ASCII in the fields since an NES cannot read unicode too easily. (Also, a simple converter could be written to "compile" a desired text format into the binary format for stuffing onto the end of the NSF.)

As for a pure text format, I ran into this issue with .SAP files. I had to write a somewhat annoying complicated parser for these files, because they have a human-generated header with the binary data just appended on. Because of this, you have to account for all sorts of weird cases; tabs, spaces, CRLF, LF only, etc.

It's kind of a nightmare to handle in 6502 asm.

The data would be encoded in UTF-8, with a warning if the player cannot display a particular character.

Reference

• NSF 2.0 forum discussion