Editing arm/a64

{{arm title|A64}}{{arm isa main}}
'''A64''' is a 32-bit [[fixed-length instructions|fixed-length]] [[instruction set]] that machines in {{\\|AArch64}} state execute. This instruction set was introduced with the {{\\|ARMv8}} [[ISA]].

== Overview ==
The A64 instruction set was introduced with the {{\\|ARMv8}} ISA in order to introduce {{arch|64}} capabilities, primarily targeting the LP64 and LPP64 [[64-bit data models]]. It is [[fixed-length instructions|fixed-length]] with instructions being 32 bits in size and follow almost identical semantics as in {{\\|AArch32}}. This was done in order to maximize the hardware reuse. With the exception of [[branch instructions|branches]] and [[comparison instruction|comparisons]], [[predication]] was also eliminted. Compared to the {{arm|a32|32-bit instruction set}} which offered 14 [[general-purpose registers]], the A64 IS has 31 GPRs. Most instructions are capable of having a 32-bit or 64-bit [[operand]] and the address is assumed to be 64-bit in size. 

== Registers ==
A64 has 31 [[general-purpose registers]], all of which are 64-bit wide. Depending on the instruction, the 32nd register is used to obtain a zero or the stack pointer. The almost doubling of the GPR count was done in order to improve both the performance and power of the implementations (designed to reduce cache accesses). Banking was eliminated altogether. Additionally, the stack pointer is no longer regarded as a GPR and the [[program counter]] is no longer treated as a normal directly accessible register.

=== Packing ===
Compared to {{\\|A32}}, smaller registers no longer packed into the larger ones but are instead mapped one-to-one to the low-order bits of the bigger register.

:[[File:arm a64 registers.svg|400px]]

== Instructions ==
Most instructions are capable of supporting both 32-bit or 64-bit [[operands]]. A64 primarily targeted the LP64 and LPP64 [[64-bit data models|data models]], meaning [[long integer|long]], [[long long integers]], and [[pointers]] are 64-bit wide while [[integers]] are 32 bit. A64 has a limited set of instructions supporting [[predication]] and arbitrary length [[load instruction|load]]/[[store instruction|store]] are no longer supported.

== Advanced SIMD ==
A64 Advanced SIMD ({{\\|NEON}}) shares the same register file as the {{\\|A32}} floating-point [[register file]] but is extended to 128-bit. A64 has 32 128-bit wide vector registers (this is double the amount from {{\\|A32}}). A64 advanced SIMD added support for [[double-precision floating point]] as well as [[IEE 754]] compliance. Additional instructions were also added to support the 2008 version of the standard (e.g., conversion, min/max).

:[[File:arm a64 media registers.svg|550px]]

== Bibliography ==
* Richard Grisenthwaite. (October 26, 2011). ''"Technology Preview: The ARM Architecture - a View of the Future "''.
* ARM Architecture Group. "Armv8 instruction set overview." vol. PRD03-GENC-010197 (2011).