An Instruction Set Architecture (ISA) is part of the abstract model of a computer. It defines how software controls the CPU.
The Arm ISA family allows developers to write software and firmware that conforms to the Arm specifications, secure in the knowledge that any Arm-based processor will execute it in the same way. This is the foundation of the Arm portability and compatibility promise, underlying the Arm ecosystem.
The Arm architecture supports three instruction sets: A64, A32 and T32.
T32 was introduced as a supplementary set of 16-bit instructions that supported better code density for user code. Over time, T32 evolved into a 32- and 16-bit mixed length instruction set, providing an advantage for compilers, with the opportunity to balance performance and code size trade-offs, in a single instruction set.
A64 instruction set
The A64 instruction set, introduced in Armv8-A to support the 64-bit architecture.
A32 instruction set
The A32 instruction set, referred to as ‘ARM’ in pre-Armv8 architectures (e.g. Armv7).
T32 instruction set
The T32 instruction set, referred to as ‘Thumb’ in pre-Armv8 architectures.