Intel EM64T Technology Article@ 2005/03/23
"The main additions to the IA-32 architecture lie with the need for additional 64-bit registers. Intel has added eight new general purpose registers, along with eight 128-bit Streaming SIMD Extension registers. The pre-existing general purpose registers have all been widened to 64-bits as well. With these new registers comes a new opcode prefix, REX. The defaults for 64-bit mode allow for a 64-bit address size, and a 32-bit operand size."