GMA87
Description
On Track 38 look for signature of raw bytes after a sync: 69 XX XX XX A9
Should occur only once per track.
Logic
Starting from next sync 10 sync lengths are determined by counting on X register.
The next sync length is not evaluated.
The second sync length is used as reference length.
The following 8 sync lengths are compared to the second sync length,
if a sync length is longer than the second length a 1 is shifted in a byte
if a sync length is shorter than the second length a 0 is shifted in a byte.
GMA uses a single byte as EOR decode code that is determined on the basis of sync length on track 38.
Example from Exolon:
We know that the GMA byte decode code is 4F -> 01001111
On Track 38 we see in G64Conv text format of the track:
First the signature bytes:
sync 42 ; Following raw bytes: 69 57 57 a9 a9 55 55 55 55 55 55 55 55 55 55 aa ff
Then the following syncs appear (raw bytes after syncs omitted):
sync 39 sync 87 <- This is the reference sync length sync 40 <- This is shorter -> 0 sync 136 <- This is longer -> 1 sync 40 <- This is shorter -> 0 sync 40 <- This is shorter -> 0 sync 136 <- This is longer -> 1 sync 136 <- This is longer -> 1 sync 136 <- This is longer -> 1 sync 136 <- This is longer -> 1 result of 8 bits after shift: 01001111 -> 4F
Pages that refer to this protection
- Descriptions of old disk copy protections (← links)
- Starfox (← links)
- Jagd auf Roter Oktober (← links)
- Enlightenment: Druid II (← links)
- Cholo (← links)
- Delta (← links)
- Beau Jolly Big Box 2 (← links)
- Kat Trap: Planet of the Cat-Men (← links)
- Triaxos (← links)
- Hit Pak: Trio (← links)
- Bride of Frankenstein (← links)
- Bubble Bobble (← links)
- Flying Shark (← links)
- James Bond 007 in The Living Daylights: The Computer Game (← links)
- Sidewize (← links)
- James Bond 007 in Der Hauch des Todes: Das Computerspiel (← links)
- Twin Tornado (← links)
- Evening Star / Southern Belle (← links)
- Revs+ (← links)
- Zynaps (← links)
- Mystery of the Nile (← links)