A64

A64 is the 64-bit execution state of the ARMv8-A architecture, commonly referred to as AArch64. It defines a 64-bit instruction set and a 64-bit register file designed to provide higher performance and a larger address space for mobile, embedded, and server-class devices. A64 runs alongside a 32-bit execution state, AArch32, which allows legacy software to execute in compatibility mode while new software is written for 64-bit code.

In A64, software uses 31 general-purpose 64-bit registers named x0 to x30, plus a dedicated stack pointer (sp). The program counter is implicit in instruction flow. The architecture also includes 32 128-bit NEON/Advanced SIMD registers (v0–v31) for vector processing. It supports a large virtual address space and enables modern security and virtualization features, including multiple privilege levels (EL0–EL3). Later revisions added security improvements such as pointer authentication codes (PAC) and memory tagging extension (MTE).

History and usage: ARM announced ARMv8-A in 2011, with first commercial implementations appearing in 2012–2013. The A64 state forms the foundation of most modern ARM-based CPUs, including Apple’s A7 and subsequent chips, as well as many Cortex-A series cores used in smartphones and tablets. ARMv8-A and its A64 execution state have enabled broad 64-bit software ecosystems, while maintaining compatibility with older 32-bit software through AArch32. The architecture continues to influence subsequent ARM designs, with ongoing refinements and expansions in newer generations.