Return to cswitch CVS log

Up to [OS/2 SDKs] / pmsdk / samples / notes

Microsoft OS/2 SDK PM 02-24-1989

When compiling a C program for PM, here are some points to note:
----------------------------------------------------------------

Segment setup
-------------

There are 3 switches which specify segment setup (note that SS never changes):

-Ad	SS==DS.
	SS and DS are loaded with the same value when the program
	starts up and are assumed to be the same later on.
	This is the default.

-Aw	SS!=DS, DS not reloaded on function entry.
	SS is not assumed to equal DS, so if DS must contain the same value
	as SS for some operation, DS is pushed, loaded, used and popped.

-Au	SS!=DS, DS reloaded on function entry, for all functions.
	At the beginning of each function, DS is pushed
	then loaded with the module's data segment (usually DGROUP).
	At the end of each function, DS is popped.  Also, any other
	time DS must be changed, it is pushed, loaded, used and popped.
	The _loadds keyword has the same effect, but on a
	function-by-function basis.

If you have a thread whose stack lies outside the default data segment
(eg. obtained by a call to DosAllocSeg, or if you compile large-model and
use malloc()), you must specify SS!=DS.  Multi-threaded apps nearly always
have SS!=DS.

Any routine which is called by code which uses a different data segment
(different value in DS) must use the _loadds keyword.  Your program will
execute correctly if you use the -Au switch, but this switch generates DS
save, load and restore code for all functions, even ones which use the same
data segment.  Generally, only window procedures or dll entry points need
this code.

Summary:
If your program has multiple threads
	Use the -Aw switch
Else
	Use the -Ad switch

Use the _loadds keyword when declaring window procedures and dll entry points.



Window procedures
-----------------

The preferred method of typing window procedures is to use the
CALLBACK keyword, defined in os2def.h:

#define CALLBACK pascal far _loadds

Note that window procedures need not be exported in PM (unlike Windows).

The -Gw switch should not be used for PM application programs, since it
causes unnecessary code to be emitted.	It will not cause incorrect
execution, but it is wasteful.



C switches used in the samples
------------------------------

-Zi	causes symbolic information used by CodeView to be generated

-Zp	packs structure members

-Gs	turns off stack checking.  Useful if SS!=DS and C runtime libraries
	are used.

-G2	causes 286 instructions to be generated

-Gc	makes the Pascal calling convention the default

-Od	disables optimization.	Useful when running CodeView.

-NT	sets the name of the text (code) segment

-c	compiles without linking

-u	removes definitions of all predefined identifiers

-J	changes the default type for char from signed to unsigned

-W3	sets the compiler output warning level to its most sensitive

This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.