DOCUMENT:Q68945 04-MAY-2001 [masm] TITLE :FIX: DWORD Local Variables Use Wrong Offset in MASM PRODUCT :Microsoft Macro Assembler PROD/VER::5.1 OPER/SYS: KEYWORDS: ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Macro Assembler (MASM), version 5.1 ------------------------------------------------------------------------------- SYMPTOMS ======== When using the LOCAL directive in the Microsoft Macro Assembler (MASM) version 5.1 to declare stack space for a DWORD variable, the offset that is generated for the variable is [BP-2]. This may result in the saved value of the BP register to be overwritten when a value is stored in the DWORD local variable. STATUS ====== Microsoft has confirmed this to be a problem in MASM version 5.1. This problem was corrected in version 5.1a. MORE INFORMATION ================ Beginning with MASM 5.10, if the optional language parameter is used with the .MODEL directive, the LOCAL directive may be used to declare local variables for a procedure (PROC). When the LOCAL directive is used in a procedure, stack space is set aside for the number and size of the local variables that were declared. For example, upon executing the first line of the sample assembly routine below, the stack frame appears as follows if assembled with MASM 5.10: ------------ | Return | 2 bytes | address | ------------ | Stored | 2 bytes | BP | ------------ | storage | 2 bytes SP--> | for myvar| ------------ The problem is that DWORD needs four bytes of storage. Because the "saved BP" is at a higher memory location than the storage of myvar, myvar "overflows" into the saved BP area. Using MASM 5.1a will solve the problem by properly allocating 4 bytes of storage for a DWORD. Sample Code ----------- ; Assemble options needed: none .MODEL SMALL, C PUBLIC C myproc .CODE myproc PROC LOCAL myvar:DWORD nop ret myproc ENDP END Additional query words: 5.10 buglist5.10 fixlist5.10a ====================================================================== Keywords : Technology : kbMASMsearch kbAudDeveloper kbMASM510 Version : :5.1 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.