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From: gp@picuxa.UUCP (Greg Pasquariello X1190)
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Accessing data from an interrupt handler
Message-ID: <330@picuxa.UUCP>
Date: Thu, 8-Oct-87 10:11:04 EDT
Article-I.D.: picuxa.330
Posted: Thu Oct  8 10:11:04 1987
Date-Received: Sun, 11-Oct-87 11:50:56 EDT
Organization: AT&T Information Systems, Parsippany NJ
Lines: 31
Xref: mnetor comp.sys.ibm.pc:8851 comp.lang.c:4778

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
set up an interrupt handler for the clock interrupt (0x1C).  The interrupt
handler itself was a small ASM module that simply printed out stars whenever
the interrupt occured.  Fine.  The next thing I did was change the int
handler to increment a word of memory (a variable) rather than print stars, 
and when the interrupt was done being serviced, the C program printed the 
value of the variable.  Problem was, the value was ALWAYS 0.  I then ran
the interrupt handler and the C code (a single, small model program) through
Codeview, and found that the variable *WAS* being incremented.  Now can anyone
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?

The MSC routines are compiled using the small memory model option, and the
ASM routine is set up to be compatible with the C code (_TEXT segment, _DATA
segment, DGROUP, and all that).  All publics are declared  within the ASM
module, and properly begin with an '_'.  The code is linked as one program
with the interrupt handler embedded, and the routine that sets up the 
new interrupt vector does so correctly.

Any help will be appreciated.  Thanks in advance.





BTW: 	Thanks to all who responded to my question about the "hole" in the
	stack.  The answer dawned on me just after I posted the question!


Greg Pasquariello