02_syntax_program_structure.md

Syntax and Program Structure

An Assembly line often has this shape:

label:  instruction  operand1, operand2  ; comment

Parts:

• label: a name for the current address.
• instruction: a CPU instruction or assembler directive.
• operand: register, memory, immediate value, or label.
• comment: text after ;, ignored by the assembler.

NASM Linux x86-64 example:

bits 64

section .text
global _start

_start:
    mov rax, 60     ; syscall exit
    mov rdi, 0      ; exit code 0
    syscall

Common sections:

• .text: machine code.
• .data: initialized data.
• .bss: uninitialized data.
• .rodata: read-only data, when supported by the linker/script.

Labels:

loop_start:
    dec rcx
    jnz loop_start

A label does not generate instruction bytes. It only names an address.

Intel syntax:

mov rax, rbx    ; rax = rbx
add rax, 5      ; rax = rax + 5

Destination is on the left, source is on the right.