Microsoft KB Archive/27835

= FIX: Operator &quot;.&quot; and the Operator &quot;*&quot; =

Article ID: 27835

Article Last Modified on 10/17/2003

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APPLIES TO


 * Microsoft Macro Assembler 5.0
 * Microsoft Macro Assembler 5.1 Standard Edition
 * Microsoft Macro Assembler 5.1a

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This article was previously published under Q27835



SYMPTOMS
When using scaled indexed addressing in the following Microsoft Macro Assembler (MASM) 5.0, 5.1, or 5.1a instruction, the opcode generated is incorrect. Note that the variable &quot;esi&quot; is being used as an index into a structure and &quot;eax&quot; is the base. mov eax,[eax.esi*2] The opcodes generated by MASM indicate that MASM is not using scaled indexed addressing. The code generated by MASM is effectively the same as: mov eax,[eax+esi]



CAUSE
The problem occurs because the &quot;.&quot; operator has a higher precedence than the &quot;*&quot; operator. MASM turns: [eax.esi*2]

into

[(eax+esi)*2] as a result of the precedence. MASM evaluates &quot;(eax+esi)&quot; as a constant instead of a register. Thus, the result of the constant expression &quot;(eax+esi)&quot; is multiplied by two and the addressing mode is lost. This results in MASM interpretting the operand as [eax+esi]



RESOLUTION
One workaround is to use the &quot;+&quot; operator which has lower precedence than the &quot;.&quot; operator and produces the correct result. Alternately parenthesis can be used to give higher precedence to the &quot;*&quot; operator.



STATUS
Microsoft has confirmed this to be a problem in MASM version 5.0, 5.1, and 5.1a. This problem was corrected in MASM version 6.0.



MORE INFORMATION
Under Microsoft MASM 6.0 the following error will be generated.

error A2026: constant expected

Sample Code
; Assemble options needed: none

.386 _TEXT SEGMENT PARA PUBLIC USE16 'CODE' ASSUME cs:_TEXT start: mov eax,[eax+esi*2]  ; This is the desired instruction ; Note the use of &quot;+&quot; instead of &quot;.&quot; mov eax,[eax.(esi*2)] ; This is also the desired instruction ; Note the use of around the &quot;*&quot; mov eax,[eax.esi*2]  ; This is the format that is misinterpreted ; Under MASM 6.0 this generates an A2026 mov eax,[eax+esi]    ; This is what is incorrectly generated ; by MASM 5.0, 5.1, and 5.1a mov ax, 4C00h int 21h _TEXT ENDS

END start

Additional query words: 5.00 5.10 5.10a buglist5.00 buglist5.10 buglist5.10a fixlist6.00

Keywords: kbfix KB27835

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