MAIN HEADER FIELDS

Offset Size Description
====== ==== ==========================
  00     3  Identifier string ("GBS")
  03     1  Version (1)
  04     1  Number of songs (1-255)
  05     1  First song (usually 1)
  06     2  Load address ($400-$7fff)
  08     2  Init address ($400-$7fff)
  0a     2  Play address ($400-$7fff)
  0c     2  Stack pointer
  0e     1  Timer modulo  (see TIMING)
  0f     1  Timer control (see TIMING)
  10    32  Game Title string
  30    32  Author string
  50    30  Copyright string
  6e     2  Length of Code/Data section (1 = 16 Bytes)
  70     n  Code and Data (see RST VECTORS)
n+70     x  Extended Header (see EXTENDED HEADER)

If the word at offset 0x6e is not zero, the extended header is considered
valid if both the magic is present and the extended header crc is ok.
If the extended header is not valid, the copyright string is assumed to be
32 characters.



EXTENDED HEADER

The extended header was designed with the following in mind:
- Backwards compatible extension
- Reliable to detect
- Easy read support even on the native hardware
- Selfsufficient
- Can be a separate file

Offset Size Description
====== ==== ==========================
  00     4  Magic 'GBSX'
  04     4  Size of Extended Header - 8
  08     4  Extended Header CRC
  0c     4  Original File Size
  10     4  Original File CRC
  14     2  Offset to Extended Title string
  16     2  Offset to Extended Author string
  18     2  Offset to Extended Copyright string
  1a     2  Offset to (multiline) Comment string
  1c     1  Number of Entries in Subsong Info Table
  1d     3  Reserved (0)
  20     a  Subsong Info Table (see SUBSONG INFO TABLE)
a+20     b  String area

Reserved bits shall be zero.
The Offsets refer to the beginning of the String area.
If the Offset is -1 (FFFF) the String is not set.
The Extended String Offsets shall be -1 except when the String
does not fit into the main header.  Strings in the String area
are zero terminated.
The file CRC is calculated over the (original) length of the gbs
without the extended header and padding.
The extended header CRC is calculated after the file CRC has been set,
with the extended header CRC field set to zero.

If the extended header is stored in a separate file, an IFF header
with the ID 'GBSI' (GBS Info) is added and the Title, Author and
Copyright strings are included unconditionally to meet the selfsufficient
requirement.  FORM-IFFs may include a 'FILE' chunk containing the filename
of the GBS, CAT- and LIST-IFFs have to include it.



SUBSONG INFO TABLE

Offset Size Description
====== ==== ==========================
  00     4  Length of Subsong (1024 == 1 second, 0 == unlimited)
  04     2  Offset to Subsong Title
  06     2  Reserved (0)



FILE CHUNK

Offset Size Description
====== ==== ==========================
  00     4  'FILE'
  04     4  Length of filename
  08     x  Filename (zeroterminated)