DiskCopy 4.2

http://www.nulib.com/library/FTN.e00005.htm

I recently encountered a 32-meg image with the following values:

offset	description	value
+80	disk format	$ff
+81	format byte	$22

checksum algorithm (c)

/*
 * length must be even.
 */
uint32_t checksum(uint8_t *data, size_t length)
{
  size_t i;
  uint32_t rv = 0;

  for (i = 0; i < length; i += 2)
  {
    rv += data[i] << 8;
    rv += data[i+1];
    
    rv = (rv >> 1) + (rv << 31);
  }

  return rv;
}