The Snapper (Silicon Valley Systems, 1982)

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#1006 - That is an arcade game that 4am proposed me to crack. That was fun but as with other one-pass loading programs, once you win the boot trace, you are done or nearly done...

Disk structure
This is a non standard disk. It cannot be copied with Locksmith Fast Disk Backup. It loads fast, it seems it is using 4*4 encoded nibbles (where two bytes on disk make one byte in RAM)

Protection type
The program is heavily protected against boot tracing. Moreover, it uses:
- 4*4 encoded nibbles,
- nibbles on disk tell where to execute code,
- it uses false opcodes to lose the pirate (07 06),
- uses the text page to put code.

Boot trace
I invite you in reading the next message. It contains all my notes to get it into memory, ie. crack it.

How to normalize
I used a WOZ image (see applesaucefdc.com) provided by 4am. I booted it under OpenEmulator. So, once I had the program in RAM, I saved the emupack of it and then reconstructed a disk image:
- T0: the standard RWTS from DOS 3.3
- T1/S0-T7/S7: the game (from $800..$6FFF, a waste of two tracks for no image)
- T7/S8-T7/SF: the original $0000..$07FF area that is relocated by a code at offset $80 on T0/S0

Information
Note that the keys (stored at $4AFC) are ? ] ^ ^ > < / M N . , and ESC
ESC leaves the game, it does NOT pause it!!

The disk image is available at http://www.brutaldeluxe.fr/crack/

Reboot and... enjoy,
LoGo
7/2019

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My cracking notes:

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The original boot 3 code at $400..$7FF: