ID Number: Q72727
6.00 | 6.00
MS-DOS | OS/2
Summary:
The USES clause was added to the PROC directive with the release of
the Microsoft Macro Assembler (MASM) version 6.0. With USES, it is
possible to specify the registers that your procedure will use. The
assembler will then save the registers on the stack at function entry
and restore them upon exit, saving the programmer from having to do
so.
If you mistakenly list the same register twice in the USES clause,
MASM will not generate a warning. This is correct behavior and the
assembler will just generate code to push and pop the register twice.
The sample code below illustrates this situation. In this example, the
ES, DI, DS, SI, and CX registers were meant to be saved. However, DI
is inadvertently saved twice, and SI is not saved at all. It is the
programmer's responsibility to catch this type of error.
Sample Code
-----------
; Assemble Options Needed: /Fl /Sg /c
.model small, c
.stack
.data
.code
StrCmp PROC USES es di ds di cx,
string1:FAR PTR, string2:FAR PTR, len:WORD
cld
mov cx, len
les di, string1
lds si, string2
repe cmpsb
mov ax, cx
StrCmp ENDP
END