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IA-64 (Intel Itanium architecture) is the instruction set architecture (ISA) of the discontinued Itanium family of 64-bit Intel microprocessors. The basic ISA specification originated at Hewlett-Packard (HP), and was subsequently implemented by Intel in collaboration with HP. The first Itanium processor, codenamed Merced, was released in 2001.
Under the influence of Microsoft, Intel responded by implementing AMD's x86-64 instruction set architecture instead of IA-64 in its Xeon microprocessors in 2004, resulting in a new industry-wide de facto standard. [60] In 2003 Intel released a new Itanium 2 family member, codenamed Madison, initially with up to 1.5 GHz frequency and 6 MB of L3 ...
Intel releases the instruction set for the IA-64 architecture. AMD publicly discloses its set of 64-bit extensions to IA-32, called x86-64 (later branded AMD64). 2000 IBM ships its first 64-bit z/Architecture mainframe, the zSeries z900. z/Architecture is a 64-bit version of the 32-bit ESA/390 architecture, a descendant of the 32-bit System/360 ...
For example, the IA-32 instruction set architecture has 8 general purpose registers, x86-64 has 16, many RISCs have 32, and IA-64 has 128. The advantages of a larger register file are offset by the need to use more bits to encode the register number.
The term IA-64 refers to the Itanium processor, and should not be confused with x86-64, as it is a completely different instruction set. Many operating systems and products, especially those that introduced x86-64 support prior to Intel's entry into the market, use the term "AMD64" or "amd64" to refer to both AMD64 and Intel 64.
The REX prefix provides additional space for encoding 64-bit addressing modes and additional registers present in the x86-64 architecture. Bit-field W changes the operand size to 64 bits, R expands reg to 4 bits, B expands r/m (or opreg in the few opcodes that encode the register in the 3 lowest opcode bits, such as "POP reg"), and X and B expand index and base in the SIB byte.
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There are exceptions, such as the VAX architecture, which includes optional support of the PDP-11 instruction set; the IA-64 architecture, which includes optional support of the IA-32 instruction set; and the PowerPC 615 microprocessor, which can natively process both PowerPC and x86 instruction sets.