AMD64 is a set of 64-bit extensions to AMD x86-compatible microprocessors.
AMD64 extends the industry-standard x86 instruction set architecture while maintaining full compatibility with existing x86 applications.
The 64-bit architecture of AMD64 enables applications to utilize up to 256 terabytes of memory, overcoming the 4 gigabytes limitation of 32-bit processors.
AMD64 doubles the number of general purpose registers to 16 and doubles their size to 64-bits.
AMD64 also doubles the number of 128-bit XMM registers to 16.
AMD64 includes support for SSE and SSE2 instructions. Later releases of AMD64 add SSE3.
AMD64 improves system security by adding support for the NX bit. The NX bit enables the operating system to forbid code execution in data space. This helps to prevent attacks such as buffer overflows. Both Windows and Linux support use of the NX bit.
Intel has copied AMD64 in their EM64T architecture.