X86-64

x86-64 (also known as AMD64, x86_64, x64 and, rarely, Intel 64) is a 64-bit extension of the x86 instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new operating modes: 64-bit mode and compatibility mode, along with a new four-level paging mechanism. In 64-bit mode, x86-64 supports significantly larger amounts of virtual and physical memory compared to its 32-bit predecessors, allowing computer programs to utilize more memory. The architecture increases the number of general-purpose registers from 8 to 16, and extends each of the register's width (capacity) to 64 bits. Floating-point arithmetic is supported through mandatory SSE2 instructions in 64-bit mode. While registers for older floating-point implementations—the x87 and MMX—are still available, they are generally superseded by 128-bit vector registers for the SSE instruction set. These vector registers are capable of storing two double-precision, or up to four single-precision floating-point numbers, along with various integer formats. In 64-bit mode, instructions are modified to support 64-bit operands and 64-bit addressing mode. The x86-64 architecture defines a compatibility mode that allows 16-bit and 32-bit user applications to run unmodified alongside 64-bit applications, provided the 64-bit operating system supports them. Since the full x86-32 instruction sets remain implemented in hardware without the need for emulation, these older executables can run with little or no performance penalty, while newer or modified applications can take advantage of new features of the processor design to achieve performance improvements. Also, processors supporting x86-64 still power on in real mode to maintain backward compatibility with the original 8086 processor, as has been the case with x86 processors since the introduction of protected mode with the 80286. The original specification, created by AMD and released in 2000, has been implemented by AMD, Intel, and VIA. The AMD K8...