Introduction

Source code gets compiled to assembly and then assembly gets compiled to machine code. Assembly has a direct one-to-one mapping of its instructions to those in machine language. This makes assembly the only possible way to disambiguously take a look at what a program does. Assembly is essentially a human readable version of machine code.

Intel vs AT&T Syntax

There are two general syntax formats for writing Assembly - Intel and AT&T. I will be using Intel throughout my notes, but here is a list of common differences between the two because you never know which one you might have to read:

Intel

• Instruction format - operation destination, source
• Instruction sufixes - none
• Register & Immediate value prefixes - none
• Dereferencing - done with []

AT&T

• Instruction format - operation source, destination
• Mnemonic sufixes - mnemonics have a suffix depending on the size of their operands - b for byte, w for word, l long

  movb %bl,%al
  movw %bx,%ax
  movl  %ebx,%eax
  movl (%ebx),%eax
  

• Register & Immediate value prefixes - registers are prefixed with % and immediate values with $
• Dereferencing - done with ()