PRB: No Error for Same Register Listed Twice in USES Clause

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SYMPTOMS

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. The assembler will generate code to push and pop the register twice.

CAUSE

This is correct behavior. It is the programmer's responsibility to catch this type of error.

MORE INFORMATION

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.

Sample Code

; Assemble Options Needed: /Fl /Sg /c

.MODEL small, c .STACK 4096 .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