DOCUMENT:Q76727 06-MAY-2001 [masm] TITLE :FIX: MASM 5.1, 6.x, and the LOCK and REP Prefixes PRODUCT :Microsoft Macro Assembler PROD/VER:MS-DOS:5.10,5.10a,6.0,6.0a,6.0b,6.1,6.10a,6.11 OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- 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 ------------------------------------------------------------------------------- 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 ====================================================================== 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 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2001.