PRB: Single Stepping IN to 8259A Controller Gives Wrong Result

MS-DOS                             | WINDOWS

The information in this article applies to:

• Microsoft CodeView for MS-DOS, versions 2.2, 3.0, 3.11 3.14, 4.0, 4.01, 4.05, and 4.1
• Microsoft CodeView for Windows, versions 3.0, 3.05, 3.06, 3.07, 4.0, 4.01, and 4.1

SYMPTOMS

CodeView disables all interrupts at the interrupt controller when single-stepping an instruction. If you single-step an IN instruction that accesses an 8259A interrupt controller, the value returned will be 0xFF and not the true value you would expect.

CAUSE

This is a restriction imposed by the 8086 chip, which forces CodeView to disable interrupts when single-stepping.

RESOLUTION

The following are two of the simplest ways to work around this restriction:

1. Issue G IP+1 or G IP+2, depending on whether the instruction is a one- or two-byte IN instruction. This does not single-step the instruction; it sets a temporary breakpoint at the next instruction and executes up to the breakpoint, thus appearing as if you just single-stepped.

   -or-

2. If you accidentally single-step the IN instruction, you can get the real value of the interrupt controller by issuing I 0x21. This returns the value of the port directly, so you can change the value in the AX register by issuing RAX=value and continue single- stepping.