Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!lll-tis!ames!rutgers!mtune!westmark!dave
From: dave@westmark.UUCP (Dave Levenson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Writing a TSR in C
Message-ID: <116@westmark.UUCP>
Date: 26 Feb 88 02:00:17 GMT
References: <5527@burdvax.PRC.Unisys.COM>
Distribution: na
Organization: Westmark, Inc., Warren, NJ, USA
Lines: 40
Summary: MS-C 5.0 Does it for you

In article <5527@burdvax.PRC.Unisys.COM>, coltoff@PRC.Unisys.COM (Chilly Willy) writes:

> I need to write a TSR in C. Actually I want to write it in C
> since assembler is a royal pain in the posterior end. It is
> actually a very simple task that I need to do but I'm a PC
> novice and don't know about all the little gotchas hiding
> hither and yon.

Joel:
	In Microsoft C Release 5.0 (the latest, as of this date) you
can code a function as:

interrupt void foo()
{
...
}


The code generated for such a function saves the machine state, and
upon reaching the return (or final } of the code) generates an iret
rather than a subroutine return.  This means that you can write your
interrupt service routine in C.

They also supply runtime library calls to several MS-DOS system
functions that are helpful here:  Terminate and stay resident (aka
keep process) and set vector.  If your main() function sets a the
interrupt vector to be a pointer to another function of type
interrupt, and then calls the keep process function, you've
installed your code.

The gotchas begin when your interrupt service function issues dos
system calls.  If dos was executing when the interrupt happened, you
may run into the lack of re-entrancy in the dos kernel.  There are
arcane and poorly-documented ways of handling this, depending on
what your interrupt function has to do.

-- 
Dave Levenson
Westmark, Inc.		A node for news.
Warren, NJ USA
{rutgers | clyde | mtune | ihnp4}!westmark!dave