FIX: DWORD Local Variables Use Wrong Offset in MASM (68945)


The information in this article applies to:

  • Microsoft Macro Assembler (MASM) 5.1
This article was previously published under Q68945

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
Modification Type:MajorLast Reviewed:10/17/2003
Keywords:kbfix KB68945
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