Q76727: FIX: MASM 5.1, 6.x, and the LOCK and REP Prefixes
Article: Q76727
Product(s): Microsoft Macro Assembler
Version(s): MS-DOS:5.10,5.10a,6.0,6.0a,6.0b,6.1,6.10a,6.11
Operating System(s):
Keyword(s):
Last Modified: 06-MAY-2001
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The information in this article applies to:
- Microsoft Macro Assembler (MASM), versions 5.10, 5.10a, 6.0, 6.0a, 6.0b, 6.1, 6.10a, 6.11
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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.
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(?)
.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
Additional query words: 5.10 5.10a 6.00 6.00a 6.00b 6.10 6.10a buglist5.10 buglist5.10a fixlist6.00
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Keywords :
Technology : kbMASMsearch kbAudDeveloper kbMASM600 kbMASM610 kbMASM611 kbMASM600a kbMASM600b
Version : MS-DOS:5.10,5.10a,6.0,6.0a,6.0b,6.1,6.10a,6.11
Solution Type : kbfix
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