Q46948: MSJ 9/88: "Using MSC for TSRs" Omission/TSRs in C Information

Article: Q46948
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# G890710-22224 | mspl13_c
Last Modified: 26-JUL-1989

The September 1988 article in Microsoft Systems Journal (MSJ) titled
"Using Microsoft C Version 5.1 to Write Terminate-and-Stay-Resident
Programs" omits and misstates some important information about writing
TSRs. In addition, the example calls BIOS from within a TSR, which is
not guaranteed to be safe. Although this article is an excellent
introduction to writing TSRs, especially in C, it does not contain all
of the needed information. The article on TSRs in the "MS-DOS
Encyclopedia" is a much more definitive reference.

There are two problems in the article: first, the program calls BIOS
functions from within the TSR. Since BIOS is not re-entrant, you can't
always call BIOS from within a TSR. Second, it is not good advice to
say that it's OK to compile with -AS rather than -Asnw. Unless you're
going to switch to your own stack in the TSR, you should always use
-Asnw.

The article is correct when it says that, in general, you may NOT call
DOS functions from within a TSR without taking special precautions.
DOS is not re-entrant.

Although the MSJ article does prepare you for the concepts you'll need
to write TSRs, the best reference is the "MS-DOS Encyclopedia," which
has a 40-page chapter on the topic. Among the topics are descriptions
of how to safely call BIOS and DOS from within a TSR. The "MS-DOS
Encyclopedia" is available in hardcover and paperback from Microsoft
Press at (800) 888-3303 and is an exceptionally useful reference book.

Programming TSRs in C is a very complicated task. Microsoft is unable
to offer assistance beyond the "MS-DOS Encyclopedia" to help you with
TSR programming. There are a number of small problems: many library
functions call DOS, do stack checks, or make memory allocations (or
all three); so they can't be called. However, you can often work
around this problem by buying the library source from Microsoft [call
(800) 426-9400] and modifying offending routines.

A bigger problem is that there is no routine supplied to change the
stack from the user's stack to your own. It's not wise to rely on the
user's stack being big enough for your needs. To change to your own
stack, you'll need an assembly-language subroutine, which may be a
part of some function packages for writing TSRs in C.

It's a bit easier to program TSRs in assembly language. With assembly
language, you know exactly what's going on all the time. This is a big
help in debugging. For a TSR that's going to be reliable, you'll need
to know exactly what's going on.

There are third-party packages that make programming TSRs in C
easier and safer. You may want to contact one of the PC programmers'
software houses about this.

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