'V-Max secondary checks'
Author:Lord Crass (guest: search)
Date: Tue, Mar 29th, 2011 @ 23:55 ( . )

Checked out the secondary protection check on Bop'n Rumble (Mindscape V-Max version).

Similar to the Defender of the Crown secondary check, but it's in a smaller package and loads differently. Instead of the code being uploaded to the $700 buffer from the C64, it's contained in a regular V-Max sector.

Every V-Max sector is executed as code once loaded into drive memory. So, normally the sector starts with a short routine which begins with a JSR to send the data payload to the C64, then sets the pointers to the next track and sector in the file chain and continues along.

When Bop'n Rumble loads track 8 sector 15 into drive memory at $200, it's the full protection check code there instead of a sector to send back to the computer.

It searches for the sector 0 header, then verifies that the sector gap bytes match a certain signature leading into the sync mark. It then does the strict sync length check on 38 consecutive sync marks. This is where it fails on emulators due to the odd length it's expecting.

When it fails, it corrupts the current track pointer so the drive thinks it's on track 12 instead of track 8. When the next track seek happens, it winds up on track 20 instead of 24, fails loading, and retries constantly.

Commented protection code attached. Put a Vice monitor breakpoint on 8:$218 before loading the game if you want it to break at the protection routine and take a look.

Attachments:
1301456953_bopnrumble-vmax.txt


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