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	$\textdollar I mm$	$I mm$	Immediate
Register	$r_{a}$	$R [r_{a}]$	Register
Memory	$I mm$	$M [I mm]$	Absolute
Memory	$(r_{a})$	$M [R [r_{a}]]$	Indirect
Memory	$I mm (r_{b})$	$M [I mm + R [r_{b}]]$	Base + displacement
Memory	$(r_{b}, r_{i})$	$M [R [r_{b}] + R [r_{i}]]$	Indexed
Memory	$I mm (r_{b}, r_{i})$	$M [I mm + R [r_{b}] + R [r_{i}]]$	Indexed
Memory	$(, r_{i}, s)$	$M [R [r_{i}] \cdot s]$	Scaled indexed
Memory	$I mm (, r_{i}, s)$	$M [I mm + R [r_{i}] \cdot s]$	Scaled indexed
Memory	$(r_{b}, r_{i}, s)$	$M [R [r_{b}] + R [r_{i}] \cdot s]$	Scaled indexed
Memory	$I mm (r_{b}, r_{i}, s)$	$M [I mm + R [r_{b}] + R [r_{i}] \cdot s]$	Scaled indexed

Bibliography

Bryant, Randal E., and David O’Hallaron. Computer Systems: A Programmer’s Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

My Notebook

Explorer

Explorer

Instructions

Overview

Instruction Classes

Bibliography

Memory Access

Arithmetic Operations

Condition Operations

Logical Operations