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From: CMH117@PSUVM.BITNET (Charles Hannum)
Newsgroups: alt.msdos.programmer
Subject: Re: Int 3
Message-ID: <90017.100521CMH117@PSUVM.BITNET>
Date: 17 Jan 90 15:05:21 GMT
References: <10442.740.forumexp@mts.rpi.edu>
Organization: Penn State University
Lines: 29


In article <10442.740.forumexp@mts.rpi.edu>, Greg_d._Moore@mts.rpi.edu
(Commander Krugannal) says:
>
>     Looking over some old code fragments today, I cam across a call
>   to int 3. Neither I nor any of the other programmers in the office
>   could recall what it did. Does anyone know?

Ah, how I long for easy questions like this!

Int 3 is the "debugging" or "single step" interrupt.  Debuggers (including the
infamous DOS DEBUG program) insert Int 3's at "breakpoints".  A breakpoint
refers to either the end of the next instruction (as with single-stepping) or
a user-defined breakpoint, as with the G(oto) command in DEBUG.  When the CPU
executes the Int 3, the debugger regains control.

Of course, I haven't mentioned anything so far that couldn't be done with, say,
Int 0xFF (Yeah, I like C.).  What makes Int 3 special is that it can be coded
in only ONE BYTE, whereas all other interrupts take two bytes.


Personally, I find the NMI interrupt (Int 2) much more interesting ...


Virtually,
- Charles Martin Hannum II       "Klein bottle for sale ... inquire within."
    (That's Charles to you!)     "To life immortal!"
  cmh117@psuvm.{bitnet,psu.edu}  "No noozzzz izzz netzzzsnoozzzzz..."
  c9h@psuecl.{bitnet,psu.edu}    "Mem'ry, all alone in the moonlight ..."