NASM Assembly basics

For x86 architecture

Prerequisites

• nasm
• ld

Build and execute

nasm -f elf file.asm -o file.o
ld -m elf_i386 -s -o bin/file file.o
./bin/file

or, alternatively

./run.sh file.asm

Notes

Registers

Processor registers

• General-purpose registers Data registers to store data and perform arithmetic operations

  AX -> AL (low) + AH (high) -> 8 bit + 8 bit (accumulator)
  BX -> BL + BH -> 8 bit + 8 bit (base)
  CX -> CL + CH -> 8 bit + 8 bit (count)
  DX -> DL + DH -> 8 bit + 8 bit (data)
  
  (extended)
  EDX -> 32 bit
  ECX -> 32 bit
  EBX -> 32 bit
  EAX -> 32 bit
  

• Pointer registers To point to the instruction, stack, and base

  IP -> 16 bit (instruction)
  SP -> 16 bit (stack)
  BP -> 16 bit (base)
  
  (extended)
  EIP -> 32 bit
  ESP -> 32 bit
  EBP -> 32 bit
  

• Control registers Pointer registers + Flags register

  Flag bits (32 bit) to indicate the status of the processor

  OF (overflow)
  DF (direction)
  IF (interrupt)
  TF (trap)
  SF (sign)
  ZF (zero)
  AF (auxiliary)
  PF (parity)
  CF (carry)
  

• Segment registers: 16 bit To contain data,code, and stack segments

  CS (code segment)
  DS (data segment)
  ES (extra segment)
  SS (stack segment)
  FS (extra segment 1)
  GS (extra segment 2)
  

• Index registers: 32 bit Used for string operations

  ESI (source index)
  EDI (destination index)
  

Statements

mov destination, source ; copy source to destination
add AH, 1 ; add 1 to AH (register)
add AH, BH ; add BH to AH (register)
add VAR, 1 ; add 1 to VAR
mov AL, 10 ; move 10 to AL (register)
inc VAR ; increment VAR by 1

Syscalls

mov ebx,1 ; (stdout)

mov eax,6 ; (sys_close)
mov eax,5 ; (sys_open)
mov eax,4 ; (sys_write)
mov eax,3 ; (sys_read)
mov eax,2 ; (sys_fork)
mov eax,1 ; (sys_exit)

int 0x80 ; call kernel

Variable Directives

db (define 1 bit)
dw (define 16 bit)
dd (define 32 bit)
dq (define 64 bit)
resb 1 (reserve byte)
resw 1 (reserve word)
resd 1 (reserve double word)
resq 1 (reserve quad word)
rest 1 (reserve ten bytes )
times 2 (initialize 2 times)

choice db 'Y'       ;ASCII of y = 79H
number1 dw 12345    ;12345D = 3039H
number2 dd 12345679 ;123456789D = 75BCD15H
stars times 9 dw '*'

Constants

equ  (define constant) 
%assign (define numeric constant)
%define (define string literal constant)

%assign STDOUT 1
%define MSG 'Hello world'

NUM1 equ 40
NUM2 equ 30
SUM equ NUM1 + NUM2

section .data
    msg db MSG, 0xA
    len equ $-msg

Arithmetic

inc ebx (increments 32-bit register)
inc dl (increments 8-bit register)
inc [var] (increments var)
dec dl (decrements 8-bit register by 1)
dec [var] (decrements var by 1)
add dest, source (add)
sub dest, source (subtract)

mul dl (multiply)
imul dl (signed multiply)
div dl (division)
idiv dl (signed division)