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Microsoft KB Archive/100426

From BetaArchive Wiki

FIX: MASM Local Variables Unavailable in CodeView 4.0x


The information in this article applies to:

  • Microsoft CodeView for MS-DOS


An attempt to display the local variables of an assembly language module created in Microsoft Macro Assembler (MASM) version 6.1 may fail. MASM displays one of the following messages when you attempt to display a local variable either in the command window or in the locals window:

CAN0030: Error: expression cannot be evaluated


CXX0030: Error: expression cannot be evaluated

The actual error message depends on the expression evaluator (C or C++) CodeView is using at the time.


Apparently, this problem is caused by CodeView incorrectly initializing memory. It occurs only after certain programs run, particularly those that use extended memory. Because MASM 6.1 is a DOS-extended application, the error occurs when CodeView runs after the assembler.


At present, there are two methods to work around this problem.

  • Reset memory before running CodeView. On a machine that runs only MS-DOS, you must reboot your computer. On a machine that runs an MS-DOS session in Microsoft Windows, close the MS-DOS window and open another to reset memory.
  • Open a memory window to display the storage location of the local variable. There are three methods available to determine the address of the variable, as follows:

    • Calculate an offset from the base pointer (BP+<offset>) as shown in the Locals window.
    • Add a watch on the address of the variable (&<variable>).
    • Type the command ? &<variable> in the Command window to display the address.
    • Enter the address in the address field of a Memory window to display the desired location.


Microsoft has confirmed this to be a problem in CodeView versions 4.0 and 4.01 for MS-DOS. This problem was corrected in CodeView version 4.1 distributed with Microsoft Visual C++ version 1.0 for Windows.


If you have Visual C++, use LINK version 5.5 and CVPACK version 4.1 from the Visual C++ package with CodeView version 4.1 to debug applications developed in MASM version 6.1.

You can use the Watch window to view global variables without any difficulties. The following code example illustrates the problem with local variables and allows you to experiment with the Memory window to watch the local variable. If you use CodeView to debug this application immediately after you start your computer or immediately after opening an MS-DOS window in Windows, the error does not occur. If you run CodeView after running MASM 6.1, it will not evaluate the local variable.

Sample Code

; Assembly options needed: /Zi
.model small
a WORD ?
mainproc PROC NEAR
        mov ax,30h
        mov a, ax
        mov ax, 0
        call myproc
        mov ax, a
mainproc ENDP
myproc PROC NEAR
        mov ax, 60h
        mov loc, ax
        mov dx, loc
        mov a, dx
myproc ENDP

Additional query words: 4.00

Keywords : kb16bitonly
Issue type :
Technology : kbAudDeveloper kbCodeView kbZNotKeyword3

Last Reviewed: June 26, 2001
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