VGM replay.

Song format

The VGM data used by the following replay routine is a stripped down version of the VGM v1.61 format.

First the 512 bytes header is removed.  Only the following commands are kepts.

As there are only 10 PSG registers ($0800-$0809), the HuC6280 command byte is omitted, and the register index and data are directly output.

So if we read a value less to 10, we know that this is a register index and the next byte is the data to be written to this register.  Otherwise it is either a “end of frame” or “end of data” special command.  The format is then.

Translated into pseudo-code, we will have :

do
{
    A = vgm_read_byte
    if (A < 10)
    {   // PSG register index.
        data = vgm_read_byte
        psgport[A] = data
    }
} while (A < 10);

if (A == 0xf0)
{   // Nothing else to do for current frame.
}
else if (A == 0xff)
{   // The song is finished.
}

Tool

A small command line utility to strip down a standard VGM file is available in the tools directory (tools/vgmrip.c).

vgmrip -b bb -o hhll input.vgm out

The command line arguments are,

vgmrip generates an assembly file containing the include directive and the start bank and base address of the VGM data, as long as the bank and address for song looping.  The song data is split into files which size is at most 8192 bytes.

Example

Let’s suppose that the song data was generated using the vgmrip tool with song as the output basename.  The output assembly file will contain the following values,

First the following ZP variables must be inialized,

Once these variables has been setup, the song is played by calling the vgm_update routine at each VSYNC.

   lda    #low(song_base_address)
   sta    <vgm_base
   sta    <vgm_ptr

   lda    #high(song_base_address)
   sta    <vgm_base+1
   sta    <vgm_ptr+1

   lda    #song_bank
   sta    <vgm_bank

   lda    <vgm_base+1
   clc
   adc    #$20
   sta    <vgm_end

   lda    #song_loop_bank
   sta    <vgm_loop_bank
   stw    #song_loop, <vgm_loop_ptr

; [...]

vsync_hook:
   jsr    vgm_update
   ; [...]
   rts