BNF descript of .IF statement

[StillLearningMasm]

I have read the BNF Grammar on the .IF statement and according to it I should be able to do the follow fragment of code:

Code Select Expand


.486
.model small

.data

myvalue db 5
example dw 7

.code
.if ebx == (size example eq 2)
inc eax
.else
dec eax
.endif

end



The code for the .IF is correct.

Code Select Expand


00000003  83 FB FF    *     cmp    ebx, size example eq 002h
00000006  75 13    *     jne    @C0001


But If I change the .IF statement from this:
.if ebx == (size example eq 2)
to this:
.if ebx == (size example eq 4)

The code that is created for the .IF statement is this:

Code Select Expand


.if ebx == (size example eq 4)
00000003  0B DB    *     or ebx, ebx
00000005  75 13    *     jne    @C0001


Is this type of expression not supported, even though masm says it is?

notice there is no cmp for the ebx register

[sinsi]

(size example eq 2) evaluates to TRUE (0FFFFFFFFh)
(size example eq 4) evaluates to FALSE (0)

MASM will try to optimise, so 83 FB FF uses the signed byte version of CMP (think of 0FFFFFFFFh as -1)
and uses OR EBX,EBX which is shorter than CMP EBX,0

Code Select Expand


00000000  81 FB FFFFFFFF     cmp ebx,0ffffffffh
00000006  83 FB FF     cmp ebx,-1


[StillLearningMasm]

Thank you

[hutch--]

This is incorrect.

> .model small

It should be,

      .486                                      ; create 32 bit code
      .model flat, stdcall                      ; 32 bit memory model
      option casemap :none                      ; case sensitive


The small memory model belongs to MS-DOS, not win32.