While newer or modified applications can take advantage of new features of the processor design to achieve performance improvements. As the full x86 16-bit and 32-bit instruction sets remain implemented in hardware without any intervening emulation, these older executables can run with little or no performance penalty, The compatibility mode defined in the architecture allows 16- and 32-bit user applications to run unmodified, coexisting with 64-bit applications if the 64-bit operating system supports them. (Each register can store one or two double-precision numbers or one to four single-precision numbers, or various integer formats.) In 64-bit mode, instructions are modified to support 64-bit operands and 64-bit addressing mode. Floating-point arithmetic is supported via mandatory SSE2-like instructions, and x87/ MMX style registers are generally not used (but still available even in 64-bit mode) instead, a set of 16 vector registers, 128 bits each, is used. for stack management) to 16 (fully general), and provides numerous other enhancements. x86-64 also expands general-purpose registers to 64-bit, and expands the number of them from 8 (some of which had limited or fixed functionality, e.g. With 64-bit mode and the new paging mode, it supports vastly larger amounts of virtual memory and physical memory than was possible on its 32-bit predecessors, allowing programs to store larger amounts of data in memory. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging mode. X86-64 (also known as 圆4, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999.

The five-volume set of the x86-64 Architecture Programmer's Manual, as published and distributed by AMD in 2002

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