INF: Differences in Functionality Between /ND and /Aw Switches

ID Number: Q66841

6.00 6.00a 6.00ax 7.00 | 6.00 6.00a

MS-DOS | OS/2

Summary:

This article describes the differences in functionality between the

/Aw and /ND switches.

The /Aw switch is used by the compiler to determine if local variables

(typically stack based) can be based on DS, as well as SS. For

example, if a variable needs to be copied from local data to global

data, which would normally include several MOV instructions, if the

compiler can assume DS=SS, it can generate a LEA instruction on the

stack-based variable and set up ES:DI with the destination. DS will

already be set to the correct value. If DS != SS, the compiler will

have to generate more code to correctly access the stack-based

variable.

For the /ND switch, the data segment for the module is not the default

segment. The compiler will do an explicit DS load at function entry

point because of just this fact; that is, there is no way for the

compiler to know that DS is properly set to the nonstandard segment

name (not DGROUP). While this behavior is indeed the same behavior as

the _loadds keyword, it has nothing to do with the main reason for /Aw

(DS!=SS).

To summarize, the /Aw switch doesn't cause the segment load, but it

also doesn't stop it. This is one case where one switch (/ND) can

override one of the effects of another switch (/Aw). However, the main

effect of the /Aw switch is not changed (don't assume DS = SS).