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Q11817: How Interrupts Are Handled in CodeView

Article: Q11817
Product(s): See article
Version(s): 1.00 1.10 2.00 2.10 2.20 2.30
Operating System(s): MS-DOS
Keyword(s): ENDUSER | TAR55548 | mspl13_basic
Last Modified: 9-AUG-1989

Question:

How does CodeView handle interrupt vectors? Does it mask any
interrupts when it runs?

Response:

CodeView saves and restores about 60 interrupt vectors as a safety
feature. However, it only redirects the following nine vectors for its
own use:

   0       Divide By 0
   1       Single Step
   2       NMI
   3       Breakpoint
   9       Keyboard
   21H     DOS functions
   22H     DOS terminate
   23H     ^C
   24H     Critical Error

The remaining estimated 51 vectors are never altered by CodeView
unless something else (e.g. the program being debugged) alters them
after CodeView has started up. In this case, CodeView restores them to
their original value as it exits.

When you enter G(o) and let your program run, CodeView relinquishes
control and lets all interrupts (except the nine listed above) flow
into your application for processing. However, when you hit a
breakpoint, the 8259 Programmable Interrupt Controller is masked. This
prevents interrupts from coming into your application while your
program is suspended. If you enter G(o) again, CodeView reenables
interrupts to your application.

To avoid a bug in the 8086 family, CodeView masks the interrupt
controller during T(race) commands and some P(rogram step) commands.
CodeView Versions 2.00 and above solve this restriction by emulating
the interrupts for your application.

You normally do not single-step or trace real-time code, so this
should not be a problem for developers writing interrupt-driven code.
However, it is something you should be aware of if you are depending
on interrupts to get to your application. For example, it will not
work if you enter "T 1000" and expect your program to catch and
process interrupts during the trace. You will need to set a breakpoint
at the instruction with which you are concerned, then enter G(o).

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