Missing PAM write protection corrupts the VGA BIOS during Windows 98 shutdown

[jlxip]

After installing VMDisp9x in Windows 98 SE, shutting down leaves a black screen with the CPU busy. Pressing a key makes CPU usage drop, but forcing a restart then runs ScanDisk because Windows did not shut down correctly.

During shutdown, the VGA BIOS clears B8000–BFFFF. This triggers Windows VDD's page-fault handling, which ends up overwriting C0000–C7FFF. PAM1=0x11 makes that shadow RAM read-only, but the code saves this value without protecting the memory. The VGA BIOS gets overwritten with 0x0720 text cells and shutdown never completes.

Honoring the PAM write-enable bits fixes it. The slop86 fix.

[copy]

The slop86 fix.

Quite sloppy indeed. I think a better fix would be move this range into in_mapped_range.