Q12401: PRB: Single Stepping IN to 8259A Controller Gives Wrong Result

Article: Q12401
Product(s): Microsoft Programming Utilities
Version(s): 2.2,3.0,3.05,3.06,3.07,3.11,3.14,4.0,4.01,4.05,4.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 26-JUN-2001

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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, 4.1 
- Microsoft CodeView for Windows, versions 3.0, 3.05, 3.06, 3.07, 4.0, 4.01, 4.1 
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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.

Additional query words: 2.20 3.00 4.00 4.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbCodeView kbZNotKeyword3 kbCodeView220DOS kbCodeView300DOS kbCodeView311DOS kbCodeView314DOS kbCodeView400DOS kbCodeView401DOS kbCodeView405DOS kbCodeView410DOS kbCodeView300 kbCodeView305 kbCodeView306 kbCodeView307 kbCodeView400 kbCodeView401 kbCodeView410
Version           : :2.2,3.0,3.05,3.06,3.07,3.11,3.14,4.0,4.01,4.05,4.1

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