Laserforce IIgs (Britannica, 1989)

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#873 - By H. Aalbers. Someone's playing games with you... It started out as a routine space probe. But suddenly you've been swept into someone else's idea of a good time: watching you fight for your life.

Disk structure
We have a standard 3.5" disk with a bad block, block #$63F or 1599. It seems we are in front of the standard Britannica protection (see Task force for more information).

More information on the bad block
At first, I thought the block was not formatted. That is not correct, the block contains data. But the data can be read if and only if the data address markers are correctly modified. The standard values are D5AAAD for the data field. Here it is ADD5AA.
I browsed the different program files and found no evidence of checking the values of the pseudo bad block. So, if we bypass the check, that may be sufficient.

How to copy
Use ZZCopy or Photonix II to copy your original disk. Then, store it in a dry and safe place.

Boot trace
The disk boots, GS/OS arrives and launches LASER.SYS16 (the intro program) and then quits to PROG.2 (we can understand that because there is a switch between the SHR and text screen, meaning a quit to some other file)

Funny protections
Laserforce contains three protection checks, all different and not used apart from the bad block one:
- Track length: file LASER.SYS16, segment 8 (called by end of segment 1)
- Nibble check: file PROG.2, segment 9 (called by end of segment 1)
- Bad block: file PROG.2, segment A (called at $078F)
Did Britannica include the three to change the one to use overtime? We'll never know. But what we know if that Huibert, its developer, has always worked on a protection-free version, the protection was later added by Britannica.

How to crack
Launch Block.Warden and

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And some code. See the usage of a jump table. A jump table is a kind of OMF segment that contains call addresses within dynamic segments (those not loaded at the time of launch)

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And the Block.Warden views

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Sorry Huibert!