x86 64 Machine Level Programming Randal E Bryant David R O Hallaron September 8 2008 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 Copyright c 2005 2008 R E Bryant D R O Hallaron All rights reserved 1 1 History and Motivation for x86 64 Over the more than twenty years since introduction of the i386 the capabilities of microprocessors have changed dramatically In 1985 a fully configured high end desktop computer had around 1 megabyte of random access memory RAM and 50 megabytes of disk storage Microprocessor based workstation systems were just becoming the machines of choice for computing and engineering professionals A typical microprocessor had a 5 megahertz clock and ran around one million instructions per second Nowadays a typical high end system has 2 gigabyte of RAM 40X increase 1 terabyte of disk storage 20 000X increase and a 4 gigahertz clock running around 5 billion instructions per second 5000X increase Microprocessor based systems have become pervasive Even today s supercomputers are based on harnessing the power of many microprocessors computing in parallel Given these large quantitative improvements it is remarkable that the world s computing base mostly runs code that is binary compatible with machines that existed over 20 years ago The 32 bit word size of the IA32 has become a major limitation in growing the capacity of microprocessors Most significantly the word size of a machine defines the range of virtual addresses that programs can use giving a 4 gigabyte virtual address space in the case of 32 bits It is now feasible to buy more than this amount of RAM for a machine but the system cannot make effective use of it For applications that involve manipulating large data sets such as scientific computing databases and data mining the 32 bit word size makes life difficult for programmers They must write code using out of core algorithms1 where the data reside on disk and are explicitly read into memory for processing Further progress in computing technology requires a shift to a larger word size Following the tradition of growing word sizes by doubling the next logical step is 64 bits In fact 64 bit machines have been available for some time Digital Equipment Corporation introduced its Alpha processor in 1992 and it became a popular choice for high end computing Sun Microsystems introduced a 64 bit version of its SPARC architecture in 1995 At the time however Intel was not a serious contender for high end computers and so the company was under less pressure to switch to 64 bits Intel s first foray into 64 bit computers were the Itanium processors based on the IA64 instruction set Unlike Intel s historic strategy of maintaining backward compatibility as it introduced each new generation of microprocessor IA64 is based on a radically new approach jointly developed with Hewlett Packard Its Very Large Instruction Word VLIW format packs multiple instructions into bundles allowing higher degrees of parallel execution Implementing IA64 proved to be very difficult and so the first Itanium chips did not appear until 2001 and these did not achieve the expected level of performance on real applications Although the performance of Itanium based systems has improved they have not captured a significant share of the computer market Itanium machines can execute IA32 code in a compatibility mode but not with very good performance Most users have preferred to make do with less expensive and often faster IA32 based systems Meanwhile Intel s archrival Advanced Micro Devices AMD saw an opportunity to exploit Intel s misstep with IA64 For years AMD had lagged just behind Intel in technology and so they were relegated to competing with Intel on the basis of price Typically Intel would introduce a new microprocessor at a price premium AMD would come along 6 to 12 months later and have to undercut Intel significantly to 1 The physical memory of a machine is often referred to as core memory dating to an era when each bit of a random access memory was implemented with a magnetized ferrite core 2 get any sales a strategy that worked but yielded very low profits In 2002 AMD introduced a 64 bit microprocessor based on its x86 64 instruction
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