How to parse D64 files, part 2: file chains

Last time, we saw how to crack open a D64 file with Perl and get at the first directory block. You may recall, two of the bytes in a directory entry point to the first block of the file proper, by way of a Track number and Sector number.

Now let's see how to read files in general.

A file in a Commodore disk image is stored in blocks. Those blocks may be scattered across the disk; in fact, due to mechanical considerations, files were almost never written in a contiguous set of blocks. Instead, a block was written, then the next block was written a few sectors away, then nthe next a few more sectors away, and so on.

When a block is read, the next block's location is found in the first two bytes. The remaining 254 bytes are file data proper.

So then, suppose a directory entry indicates that a file begins at track $t, sector $s. Using Perl, the file could be reconstructed in a manner similar to this:

my $fileData = readFile( $startTrack, $startSector );

readFile( $buffer, $t, $s )
{
return $buffer unless $t;
my $byteOffset = 256 * ($sectorOffset[ $t ] + $s);
($t, $s) = unpack "CC", substring( $diskImage, $byteOffset, 2 );
$buffer .= unpack "C*", substring( $diskImage, $byteOffset + 2, 254 );
return readFile( $buffer, $t, $s );
}


@sectorOffset will require some more explanation.