FIX: MASM 6.0 Does Not Detect Overflow Initializing Structures

The information in this article applies to:

• Microsoft Macro Assembler for MS-DOS, version 6.0
• Microsoft Macro Assembler for OS/2, version 6.0

SYMPTOMS

In certain situations, the Microsoft Macro Assembler (MASM) version 6.0 will not detect an overflow condition when initializing structure elements. This will most likely occur when the MASM 5.1 compatibility option is used (that is, /Zm on the command line, OPTION M510 in the source code, or when using the MASM driver) and the initialized type is of word size. In cases where the error is not generated but should be, the specified initialization value, modulo the maximum size for the type, is used as the value of the variable.

STATUS

Microsoft has confirmed this to be a problem in MASM version 6.0. This problem was corrected in MASM version 6.0a.

MORE INFORMATION

In the sample code below, a structure is declared without initializing the elements in the structure. Later, another structure is declared that contains the first structure and it initializes the structure to a value that will create an overflow condition. This error is not caught by the assembler.

In the second case, a data item is allocated for the first structure and initialized to a value that would be an overflow. Again, the assembler does not generate an error if assembled with MASM 5.1 compatibility.

In the third case, a structure is declared and initialized at the same time. In this instance, MASM will generate the following error if an overflow condition exists.

   error A2071: initializer magnitude too large for specified size

To work around this, either avoid using the MASM 5.1 compatibility options or use hex notation when initializing variables.

Sample Code

; Assemble options needed: /Zm /Fl

.MODEL small

         OPTION M510
Struc1   struc
         StrucWord   dw     ?
Struc1   ends

Struc2   struc
         mystruct    Struc1 {65536}      ; Should generate A2071 Error
Struc2   ends

Struc3   struc
         StrucWord2  dw     65536        ; Does generate A2071 Error
Struc3   ends

test1    Struc1  {65536}                 ; Should generate A2071 Error

test2    Struc2  { }                     ; Should generate A2071 Error

test3    Struc3  { }                     ; Does generate A2071 Error

END