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1992-03-20
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Show-Vectors, Version 1.00, Copyright (c) 1992 SoftCircuits
Redistributed by permission
Title: SHOWVECT.DOC
Overview
========
This package provides a simple program to display the vectors in
the interrupt table on IBM-compatible computers. The assembly
language source code is also included.
Description
===========
IBM-compatible computers use interrupts. An interrupt acts like a
call. It causes the CPU's thread of execution to jump to another
location and then return to where it left off when the interrupt
is finished.
An interrupt can be invoked using the INT instruction. Interrupts
are also triggered by the hardware. In fact, by saving the
current state of the CPU and then restoring it when the interrupt
returns, hardware interrupts are able to occur without the
currently executing program even being aware it was interrupted!
In fact, there are many such interrupts in most PC's including
the system timer interrupt which occurs about 18 times a second.
When an interrupt occurs, the CPU saves the current address and
the flags register, and then jumps to the handler routine for
that interrupt. It determines the address, or vector, of the
handler by a table that starts in low memory (0000:0000). The
vector for interrupt 0 is at this address. Since each vector
requires a segment (2 bytes) and an offset (2 bytes), the vector
for interrupt 1 is at address 0000:0004. So in response to an
interrupt, the CPU calculates the vector address by multiplying
the interrupt number by 4. It then reads the vector from that
address and finally jumps to where the vector pointed.
Show-Vectors displays the vectors for each of the 256 interrupts
on IBM-compatible computers. It also gives a description of what
each interrupt is normally used for. Although it's a very simple
program, it can be useful for troubleshooting, and getting low
level information about your machine.
Distribution Information
========================
This program was created and released by:
SoftCircuits Programming
P.O. Box 16262
Irvine, CA 92713
It is provided "as is" with no warranty of any kind, expressed or
otherwise. SoftCircuits assumes no responsibility as to the
performance or usability of this program.
You may use and distribute this program freely on the condition
that the complete package is distributed without modification and
that no fee is charged for such distribution with the exception
of reasonable shipping and media charges.
This package contains the following files:
SHOWVECT.COM Program executable file
SHOWVECT.ASM Program assembly language source code
SHOWVECT.DOC Program documentation (the file you're
reading)