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).
TypeFormOperand ValueName
ImmediateImmediate
RegisterRegister
MemoryAbsolute
MemoryIndirect
MemoryBase + displacement
MemoryIndexed
MemoryIndexed
MemoryScaled indexed
MemoryScaled indexed
MemoryScaled indexed
MemoryScaled indexed

Bibliography

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

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