64 bit processor architectures

The original IA-32 instruction set has been evolved over time with the addition of the multimedia instruction updates. Many new architectures, including 64-bit ones, have since succeeded IA-32.
However, the ultimate evolution of IA-32 was when it was extended again to 64-bits, but of course at that point it cannot be called IA-32 anymore; the 64-bit extension is called x86-64. It could not be called IA-64 as Intel had already used this label for the Itanium design (a design which is not really an evolution of the IA-32 architecture). AMD's AMD64 was the first x86-64 instruction set designed. Later, Intel followed by imitating AMD's design with what they call Intel 64 (formerly IA-32e and EM64T).

IA-64

Main article: IA64

Intel's IA-64 architecture is not directly compatible with the IA-32 instruction set, despite having a similar name. It completely discards all IA-32 instructions, and starts from scratch with a completely different instruction set as well as using a VLIW design instead of out-of-order execution. IA-64 is the architecture used by the Itanium line of processors. Itanium initially had hardware-support for IA-32, but it was very slow. Intel shifted to the use of a software emulator instead. The nomenclature "IA-64" means "Intel Architecture, 64-bit", but the connection with IA-32 is only in the name.
Further improvements are:

• Sixteen times the number of general purpose registers (now 128)
• Sixteen times the number of floating point registers (now 128)
• Register rotation mechanism to keep values in registers over function calls

AMD64

Main article: x86-64

AMD's AMD64 instruction set, initially called x86-64, is largely built on top of IA-32, and thus maintains the x86 family heritage, despite having a different name. While extending the instruction set, AMD took the opportunity to clean up some of the odd behavior of this instruction set that has existed since its earliest 16-bit days, while the processor is operating in 64-bit mode.
Further improvements are:

• Two times the number of general purpose registers (now 16)
• Two times the number of SSE registers (now 16)
• The general purpose registers are now truly general-purpose registers and are no longer restricted.
• Most of the functionality of the segment registers has been
  deprecated, since their usage has steadily declined even during the
  IA-32 days.

Intel 64

Main article: x86-64#Intel_64

By February 2004, Intel announced the Intel 64 instruction set, formerly known as Yamhill. It was derived from AMD's AMD64.
Intel 64 is generally compatible with code written for the AMD64, though it lacks some AMD64 features. Intel started using the set starting with the Xeon Nocona core in late 2004, introducing it to the desktop market with the Pentium 4 E0 revision in early 2005.