Charlie Balogh on Nostr: I involuntarily (cheers nprofile1q…k5zd4!) spent my Saturday on tracing down an ...

I involuntarily (cheers nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpq9hcsfvx2d7nu5d4qz5fmusjrne3n0f0vzg0cet7t4x9ucszasapsgk5zd4 (nprofile…5zd4)!) 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.

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#msdos #retrocomputing