AMD64 technology

AMD64 technology, originally called x86-64, is a set of extensions that allows the CPU to run as a 64-bit microprocessor. The AMD64 technology includes the following extensions:

• Micro-architecture improvements: wider general-purpose registers, 8 additional 64-bit registers, and 8 additional SSE (XMM) registers.
• New 64-bit instructions
• New Long mode that includes two sub-modes:
  • 64-bit mode: in this mode the CPU can execute only 64-bit applications.
  • Compatibility mode: In this mode the CPU can execute existing 32-bit and 16-bit applications, while running under control of 64-bit operating system.

  To operate in the Long mode a 64-bit Operating System is required.

• Better performance due to wider register width, bigger number of general and SSE registers, and cleaner architecture.
• Larger size of addressable memory. 32-bit microprocessors cannot address more than 4 GB of physical memory. K8-based microprocessors (Athlon 64, Opteron, etc) in 64-bit mode can access up to 1 TB of memory, and K10-based processors (Phenom, Third Generation Opteron) can access up to 256 TB of memory.

Intel released their own set of 64-bit extensions which was called Extended Memory 64 technology (EM64T). In 2006 the EM64T was renamed to Intel 64. Intel's EM64T and "Intel 64" are mostly compatible with the AMD64 technology.