x86 Assembly/MASM Syntax

This page will explain x86 Programming using MASM syntax, and will also discuss how to use the macro capabilities of MASM. Other assemblers, such as NASM and FASM, use syntax different from MASM, similar only in that they all use Intel syntax.

Instruction Order

MASM instructions typically have operands reversed from GAS instructions. for instance, instructions are typically written as Instruction Destination, Source.

The mov instruction, written as follows:

mov al, 05h

will move the value 5 into the al register.

Instruction Suffixes

MASM does not use instruction suffixes to differentiate between sizes (byte, word, dword, etc).

Macros

MASM is known as either the "Macro Assembler", or the "Microsoft Assembler", depending on who you talk to. But no matter where your answers are coming from, the fact is that MASM has a powerful macro engine, and a number of built-in macros available immediately.

MASM directives

MASM has a large number of directives that can control certain settings and behaviors. It has more of them compared to NASM or FASM, for example.

.model small
.stack 100h

.data
msg	db	'Hello world!$'

.code
start:
	mov	ah, 09h   ; Display the message
	lea	dx, msg
	int	21h
	mov	ax, 4C00h  ; Terminate the executable
	int	21h

end start

A Simple Template for MASM510 programming

;template for masm510 programming using simplified segment definition
 title YOUR TITLE HERE
 page 60,132 
 ;tell the assembler to create a nice .lst file for the convenience of error pruning
 .model small 
 ;maximum of 64KB for data and code respectively
 .stack 64
 .data
 ;PUT YOUR DATA DEFINITION HERE
 .code
 main proc far 
 ;This is the entry point,you can name your procedures by altering "main" according to some rules
 mov ax,@DATA 
 ;load the data segment address,"@" is the opcode for fetching the offset of "DATA","DATA" could be change according to your previous definition for data
 mov ds,ax 
 ;assign value to ds,"mov" cannot be used for copying data directly to segment registers(cs,ds,ss,es)
 ;PUT YOUR CODE HERE
 mov ah,4ch
 int 21h 
 ;terminate program by a normal way
 main endp 
 ;end the "main" procedure
 end main 
 ;end the entire program centering around the "main" procedure