FIX: Operator "." and the Operator "*" (27835)

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 "esi" is being used as an index into a structure and "eax" 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 "." operator has a higher precedence than the "*" operator. MASM turns:

   [eax.esi*2]

into

   [(eax+esi)*2]

as a result of the precedence. MASM evaluates "(eax+esi)" as a constant instead of a register. Thus, the result of the constant expression "(eax+esi)" 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 "+" operator which has lower precedence than the "." operator and produces the correct result. Alternately parenthesis can be used to give higher precedence to the "*" operator.

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 "+" instead of "."
  mov eax,[eax.(esi*2)] ; This is also the desired instruction
                        ; Note the use of () around the "*"
  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