AMD64 Instruction Set Overview

The AMD64 architecture [AMD64] is an extension of x86 instruction set to enable 64-bit computing while remaining compatible with existing x86 software. The CPU can operate in 64-bit mode, where semantic of several x86 instructions has been changed. Most notably:

Unlike earlier 64-bit architectures GCC has been ported to, some AMD64 features are unique, such as CISC instruction set, generally usable IP relative addressing, partial support for 64-bit immediate operands and more.

Jan Hubicka 2003-05-04