FIX: LES Instruction Assembles Incorrectly (29735)

SYMPTOMS

The program example below demonstrates a problem with the LES instructions in Microsoft Macro Assembler (MASM) versions 5.1 and 5.1a. The first LES instruction (Parm1) assembles correctly, but the second instruction (Parm2) generates the error message:

error A2056: Immediate mode illegal

The only difference between the two instructions is the order of the arguments. In earlier versions of MASM, they produced identical code.

CAUSE

The problem is with the type operator and the way it handles registers inside brackets ([]). The type operator makes things a constant on the left of the expression.

Parm1 works because it handles registers inside brackets differently. "TYPE x" turns bp into a constant; +10 then turns it back into an addressing mode.

However, in Parm2, bp+10 has been turned into an addressing mode, which is then turned into a constant by "TYPE x". This difference is demonstrated in the sample code below.

STATUS

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

MORE INFORMATION

Sample Code

; Assemble options needed: none

.MODEL   small

x STRUC
y DD ?
x ENDS

Parm1 EQU DWORD PTR [bp+TYPE x+10]
Parm2 EQU DWORD PTR [bp+10+TYPE x]

.CODE
begin:
    les di,Parm1
    les di,Parm2

END begin