FIX: MASM 5.1, 6.x, and the LOCK and REP Prefixes

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, versions 5.1, 5.1a, 6.0, 6.0a, 6.0b, 6.1, 6.1a, and 6.11

SYMPTOMS

The Microsoft Macro Assembler (MASM) version 6.x will not assemble a statement with both a LOCK and a REP prefix. MASM 6.x generates the following error message:

   error A2008: syntax error : rep

CAUSE

The validity of using LOCK with REP depends upon the processor on which the program is to be executed. A LOCK REP instruction is allowed on 186 processors and is even required in certain cases. However, on 386, 486, and the Pentium processors, the LOCK REP instruction will generate an invalid opcode exception.

For the 80186 processor, LOCK REP MOVS and LOCK REP OUTS are valid instructions and are required in some cases, especially to work with the peripheral control block (PCB) registers.

For the 80286 and higher processors, the LOCK prefix is valid only with the following instructions and only when they modify memory:

   BTS, BTR, BTC   -> bit test and change instructions
   XCHG, XADD, CMPXCHG -> exchange instructions
   INC, DEC, NOT, NEG -> single operand instructions
   ADD, ADC, SUB, SBB, AND, OR, XOR -> two operand arithmetic and logic

Because of the danger of allowing improper use of the LOCK instruction, MASM 6.x does not allow it to be used with the REP prefix even though the Intel 80186 manual specifically calls for such use in certain circumstances. Programmers developing embedded applications using the 80186 series may need this capability, however, and the sample code shows a technique for doing this. In these cases, the programmer will need to be certain that the code will never be run on a CPU where it could cause errors.

STATUS

Microsoft has confirmed that allowing LOCK to be assembled for invalid instructions is a problem in MASM versions 5.1 and 5.1a. This problem was corrected in MASM version 6.0, and by design is not allowed to be used with the REP prefixes.

RESOLUTION

When programming for the 80186 processor using MASM 6.x, a byte can be inserted into the program that contains 0F0h, the prefix for LOCK. The sample code below demonstrates this solution. Macros can be used to make this an easier workaround.

MORE INFORMATION

Sample Code

; Assemble options needed: /c .MODEL small .186 .DATA

astr    DB    "Hello, world.", 13, 10, 0
msg     DB    16 DUP(?)

proc1   PROC
        mov     cx, 10h                 ; Load the SI, DI, and CX
        mov     si, OFFSET astr         ;  registers and clear the
        mov     di, OFFSET msg          ;  direction flag for the
        cld                             ;  string transfer.

;       DB      0F0h                    ; Uncomment this line and
                                        ;  remove the LOCK prefix on
                                        ;  the following instruction
                                        ;  to work around the problem
        lock rep movsb                  ; This line does not assemble
                                        ; in MASM 6.x.
ENDP    proc1
END

REFERENCES

Intel iAPX 86/88, 186/188 User’s Manual Programmer’s Reference.

Intel 386 DX Microprocessor Programmer’s Reference manual.

Also see the Intel manuals for the 486, Pentium, and newer 186 CPUs. Manuals are available directly from Intel Corporation at the following address:

   Intel Literature Sales
   P.O. Box 7641
   Mt. Prospect, IL  60056-7641
   phone: 800-548-4725