FIX: MASM 5.1, 6.x, and the LOCK and REP Prefixes (76727)

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

MASM versions 5.1 and 5.1a will assemble without error, however, code that uses the LOCK prefix with an incorrect instruction, but the code generated will run without errors only on some machines.

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

An invalid opcode exception results from using the LOCK prefix before any other instruction, or with these instructions that do not modify memory (that is, if the destination is a register).

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.

MORE INFORMATION

Sample Code

; Assemble options needed: /c
.MODEL small
.186
.DATA
astr    DB    "Hello, world.", 13, 10, 0
msg     DB    16 DUP(?)

.CODE
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[ASCII 146]s Manual Programmer[ASCII 146]s Reference.

Intel 386 DX Microprocessor Programmer[ASCII 146]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