Macros And Defining Values With EQU In Assembly

Macros

A macro is a single instruction that expands into a predefined set of instructions to perform a particular task.

Why Use Macros?

Imagine writing the same sequence of instructions for different memory locations or registers. Macros eliminate this redundancy, making your code cleaner and easier to maintain. Here are some key advantages of macros:

• Reduced Code Size: By using a single macro for repeated code blocks, you shrink your assembly file size.
• Improved Readability: Macros enhance code readability by encapsulating complex operations under meaningful names.
• Error Reduction: Defining a macro ensures consistency and reduces the risk of typos in repeated code sections.
• Flexibility: Macros allow for parameterized instructions, adapting the code based on arguments passed during invocation.

Defining Macros

1. Single Line Macros

Defined using the %define directive, these macros expand to a single instruction or expression when used.

%define SQUARE(x)  ((x) * (x))
mov eax, SQUARE(5)  ; Expands to mov eax, 25

2. Multi-Line Macros

Defined with %macro and %endmacro directives, these macros encompass a block of instructions that execute when called.

%macro exit 0
	mov rax, SYS_EXIT
	mov rdi, 0
	syscall
%endmacro

structure:

%macro <name> <argc>
    ...
    <macro body>
    ...
%endmacro

Macro Parameters:

Both types of macros can accept parameters, allowing you to customize the code during invocation. Parameters are represented by dollar signs ($)

Defining Values With EQU

The EQU directive essentially assigns a symbolic name (label) to a constant value. This value can be a number, a string, or even an expression. Unlike data directives like DB (define byte) or DW (define word), EQU doesn't reserve memory space. Instead, it acts as a placeholder,

Structure

<name> equ <value>

Demo:

SYS_READ equ 0
SYS_WRITE equ 1
SYS_EXIT equ 60

Simple program using macros and EQU

STDIN equ 0
STDOUT equ 1
STDERR equ 2

SYS_READ equ 0
SYS_WRITE equ 1
SYS_EXIT equ 60

section .data
	digit db 0,10

section .text
	global _start

%macro exit 0
	mov rax, SYS_EXIT
	mov rdi, 0
	syscall
%endmacro


%macro printdigit 1
	mov rax, %1
	call _printraxdigit
%endmacro

_start:
	printdigit 49
	printdigit 50
	exit

_printraxdigit:
	;mov rax, 50
	mov [digit], al
	mov rax, SYS_WRITE
	mov rdi, STDOUT
	mov rsi, digit
	mov rdx, 2
	syscall
	ret