Intel x86 Assembly Language & Microarchitecture Assemblers Microsoft Assembler - MASM


Example

Given that the 8086/8088 was used in the IBM PC, and the Operating System on that was most often from Microsoft, Microsoft's assembler MASM was the de facto standard for many years. It followed Intel's syntax closely, but permitted some convenient but "loose" syntax that (in hindsight) only caused confusion and errors in code.

A perfect example is as follows:

MaxSize      EQU     16          ; Define a constant
Symbol       DW      0x1234      ; Define a 16-bit WORD called Symbol to hold 0x1234

             MOV     AX, 10      ; AX now holds 10
             MOV     BX, MaxSize ; BX now holds 16
             MOV     CX, Symbol  ; ????

Does the last MOV instruction put the contents of Symbol into CX, or the address of Symbol into CX? Does CX end up with 0x1234 or 0x0102 (or whatever)? It turns out that CX ends up with 0x1234 - if you want the address, you need to use the OFFSET specifier

             MOV     AX, [Symbol]      ; Contents of Symbol
             MOV     CX, OFFSET Symbol ; Address of Symbol