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From: jru@etn-rad.UUCP (John Unekis)
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Re: Accessing data from an interrupt handler
Message-ID: <291@etn-rad.UUCP>
Date: Fri, 9-Oct-87 13:45:43 EDT
Article-I.D.: etn-rad.291
Posted: Fri Oct  9 13:45:43 1987
Date-Received: Sun, 11-Oct-87 17:41:28 EDT
References: <330@picuxa.UUCP>
Reply-To: jru@etn-rad.UUCP (John Unekis)
Organization: Eaton Inc. IMSD, Westlake Village, CA
Lines: 35
Xref: mnetor comp.sys.ibm.pc:8886 comp.lang.c:4808

In article <330@picuxa.UUCP> gp@picuxa.UUCP (Greg Pasquariello X1190) writes:
>I ran into a small problem today, and I was wondering if anyone could
>shed some light on it.  I wrote a test program in MSC 4.0 that will
...
>tell me why the variable is incremented when the routine is used as a called
>function, but not when the routine is used as an interrupt routine?

Remember that addresses on the intel processor are composed of two parts,
a segment, which comes from one of the segment registers, and an offset
which may be a register or part of the instruction. When you use the small
model compiler it is assumed that all of your data fits in one segment, and 
the data segment(DS) register is loaded once at the start of your program
and never changed. When you call your routine as a subroutine, the data
segment register is properly set up, and your routine accesses the increment
variable location as an offset from the DS register. When you come into 
your routine off an interrupt, the data segment is set up to whatever DOS
left it at before jumping to your routine. When you try to increment your 
variable as an offset from the data segment you are actually incrementing
some location in the DOS data space(bad idea). In order to find the variable
that you really want to increment, you will have to have a separate entry 
point to your routine that will save the user mode data segment register 
in a memory loacation in the code portion of your interrupt service routine.
Then when you catch an interrupt, you should push the DS on the stack, 
retreive the user mode DS from the location in your code segment, increment
your variable at the correct location, then pop the DOS mode DS back off the
stack.

BTW: This is why Motorola 680X0 processors are so popular for real - time
applications that require interrupt handling and data sharing, NO SEGMENTS.
An address is an address is an address. The propagation of this archaic
segmented architecture into Intel's state-of-the-art 80X86 chips in order 
to preserve IBM PC compatibility reminds me of an old joke:

Intel to IBM : Mommy, Mommy, why do I keep walking in circles?
IBM to Intel : Shut up or I'll nail your other foot to the floor.