GCM Format.txt

.gcm files are Gamecube DVD image files.

GCM format:
0,440: Header (boot.bin).
440,2000: BI2 data.
2440,(size varies): Apploader.

Header (boot.bin) format:
000,6: Game ID.
006,1: Disc number?
007,1: Revision?
020,60: Internal game name.
420,4: Offset to main.dol.
424,4: Offset to file system table.
428,4: File system table size.

BI2 data format:
018,4: Region.
  0 - Japan
  1 - USA
  2 - PAL

Apploader format:
00,20: Header.
00,10: Apploader date.
14,4: Apploader size.
18,4: Trailer size.
(Total size of apploader.img: 0x20 for header + apploader size + trailer size.)

File system table format:
Entry length 0xC.
00,4: Bitfield:
  FF000000 - 00 if this entry is a file, or 01 if it's a directory.
  00FFFFFF - Offset to this entry's name within the name table.
04,4: File offset within the partition. Or for directories, FST index of the parent directory.
08,4: File size. Or for directories, FST index of the next entry not in this directory.

File named "opening.bnr": Has the banner. Size: 0x1960.
Format:
00,4: "BNR1" (or "BNR2" for a slightly different format for some games).
04,1C: Padding, all 00s.
20,1800: Banner image data.
  The banner is a 96x32 image. Each pixel is a short, which is the RGB5A3 color for that pixel.
Banner information:
1820,20: Short game name.
1840,20: Short game maker name.
1860,40: Long game name.
18A0,40: Long game maker name.
18E0,80: Long game maker name.
For BNR2, there are 6 banner informations instead of one (for 6 different languages).