Microsoft KB Archive/62586

IN AL,21 Instruction Returns 0xFF in SYMDEB Single-Step Mode

PSS ID Number: Q62586 Article last modified on 11-05-1993

3.00 MS-DOS

= SUMMARY =

Single-stepping an IN AL,21 instruction in SYMDEB version 3.00 causes the return value to be 0xFF, regardless of what value is really at port 0x21. This is because SYMDEB executes the IN instruction after setting port 0x21 to 0xFF to disable interrupts. Microsoft is researching this problem and will post new information here as it becomes available. In the meantime, a workaround to the problem is to set a breakpoint on the instruction after the IN instruction, then use the GO command to execute over it.

Additional reference words: 3.00 KBCategory: KBSubcategory: TlsSymMisc Copyright Microsoft Corporation 1993.