Instructions

Overview

x86-64 instructions are designed so that commonly used instructions and those with fewer operands are encoded in a smaller number of bytes. Instructions range in length from 1 to 15 bytes.

x86-64 assembly comes in two flavors: ATT and Intel. ATT is most common in Linux systems so I focus on that. The most important distinction between the two is operand ordering: Intel syntax lists multiple operands in reverse order compared to ATT.

Instruction Classes

There are three types of operands:

Immediates. These denote constant values. In ATT assembly, they are written with a $ followed by an integer using standard C notation.
Registers. These denote the contents of a register.
Memory. These denote some memory location according to a computed address (i.e. the effective address).

Type	Form	Operand Value	Name
Immediate	$$ I m m$	$I m m$	Immediate
Register	$r_{a}$	$R [r_{a}]$	Register
Memory	$I m m$	$M [I m m]$	Absolute
Memory	$(r_{a})$	$M [R [r_{a}]]$	Indirect
Memory	$I m m (r_{b})$	$M [I m m + R [r_{b}]]$	Base + displacement
Memory	$(r_{b}, r_{i})$	$M [R [r_{b}] + R [r_{i}]]$	Indexed
Memory	$I m m (r_{b}, r_{i})$	$M [I m m + R [r_{b}] + R [r_{i}]]$	Indexed
Memory	$(, r_{i}, s)$	$M [R [r_{i}] \cdot s]$	Scaled indexed
Memory	$I m m (, r_{i}, s)$	$M [I m m + R [r_{i}] \cdot s]$	Scaled indexed
Memory	$(r_{b}, r_{i}, s)$	$M [R [r_{b}] + R [r_{i}] \cdot s]$	Scaled indexed
Memory	$I m m (r_{b}, r_{i}, s)$	$M [I m m + R [r_{b}] + R [r_{i}] \cdot s]$	Scaled indexed