x86-64 Machine-Level Programming

Intel’s IA32 instruction set architecture (ISA), colloquially known as “x86”, is the dominant instruction format for the world’s computers. IA32 is the platform of choice for most Windows, Linux, and, since 2006, even Macintosh computers. The ISA we use today was defined in 1985 with the introduction of the i386 microprocessor, extending the 16-bit instruction set defined by the original 8086 to 32 bits. Even though subsequent processor generations have introduced new instruction types and formats, many compilers, including GCC, have avoided using these features in the interest of maintaining backward compatibility. A shift is underway to a 64-bit version of the Intel instruction set. Originally developed by Advanced Micro Devices (AMD) and named x86-64, it is now supported by high end processors from AMD (who now call it AMD64) and by Intel, who refer to it as Intel64. Most people still refer to it as “x86-64,” and we follow this convention. (Some vendors have shortened this to simply “x64”). Newer versions of Linux and GCC support this extension. In making this switch, the developers of GCC saw an opportunity to also make use of some of the instruction-set features that had been added in more recent generations of IA32 processors. This combination of new hardware and revised compiler makes x86-64 code substantially different in form and in performance than IA32 code. In creating the 64-bit extension, the AMD engineers also adopted some of the features found in reduced-instruction set computers (RISC) [7] that made them the favored targets for optimizing compilers. For example, there are now 16 general-purpose registers, rather than the performancelimiting eight of the original 8086. The developers of GCC were able to exploit these features, as well as those of more recent generations of the IA32 architecture, to obtain substantial performance improvements. For example, procedure parameters are now passed via registers rather than on the stack, greatly reducing the number of memory read and write operations. This document serves as a supplement to Chapter 3 of Computer Systems: A Programmer’s Perspective (CS:APP), describing some of the differences. We start with a brief history of how AMD and Intel arrived at x86-64, followed by a summary of the main features that distinguish x86-64 code from IA32 code, and then work our way through the individual features.