A composite image of two screenshots, both showing DOS Navigator's hexeditor, with the affected byte-pattern above, and below with the fix.

Notices where this attachment appears

1. Charlie Balogh (chainq@mastodon.social)'s status on Sunday, 17-Nov-2024 11:32:33 UTC Charlie Balogh

   I involuntarily (cheers @root42!) spent my Saturday on tracing down an ancient Turbo C bug. 😅

   The intr() function, used to call DOS/BIOS interrupts will pass the wrong BP register value to the interrupt. BP is heavily used in stack handling, so intr() generates a BP-reload-and-INT-trampoline on the stack, but the BP reload offset it creates is off-by-four.

   Fix: Look for the sequence C6 46 F4 E2 in affected binaries, and change E2 to DE.

   More details in reply. 🧵

   #msdos #retrocomputing