x86-64 includes several enhancements to the basic x86 assembler. Chief amoung these:
  • Register Extension: x86-64 support 8 new general purpose registers & 8 new XMM registers (used with SIMD). Sign extends all previous registers to 64 bits.
  • Long mode: Supports 64 bit and IP relative addressing.

x86-64 is completely backwards compatible with IA-32. It is generally believed that AMD's implementation will provide better 32 bit performance that a comparible Itanium processor, but that Intel's Itanium will hold the edge in 64 bit computing. This is still speculation, because as of 11/6/2002 there is no implementation of x86-64 to compare with. The first chips (currently named Opteron) are scheduled to being shipping in 2003.