An ARM Glossary

As someone who works on compilers, I am often faced with sorting through ARM’s endless collection of architectures, instruction sets, ABIs, and the like. This page is my attempt to bring some order to the madness.

Architectures

There are several ARM architecture currently found in the wild. Each defines a set of architectural state and one (or more) instruction sets. We will overlook the ancient history that is everything prior to ARMv6.

ARMv6

The 6th version of the ARM architecture. This is a 32-bit-only architecture and is today only found in Cortex M0, M0+, and M1 embedded devices.

ARMv7

The 7th version of the ARM architecture. Like ARMv6, this is a 32-bit-only architecture and is generally what is found in most consumer devices made before 2016. Being a 32-bit architecture, these devices generally support up to 4GB of physical memory.¹ Note that floating-point hardware (the ARM Vector Floating Point, VFP, unit) is optional for ARMv7 devices.

ARMv8 (wikichip).

The 8th version of the ARM architecture. This architecture defines two modes of execution: AArch64 (a 64-bit mode using the new A64 instruction set) and AArch32 (a 32-bit mode admitting the T32 and A32 instruction sets). Implementations may support AArch32, AArch64, or both (most implementations support exclusively AArch64 or both execution modes).

In addition, ARMv8 defined a set of profiles:

• ARMv8-A (Application)
• ARMv8-R (Real-time)
• ARMv8-M (Microcontroller)

In typical consumer and server environments one most often finds ARMv8-A devices. Unlike ARMv7, the ARMv8-A profile unconditionally provides floating-point hardware (the ARMv8-R and ARMv8-M profiles makes floating-point optional).

Instruction sets

The ARM architectures each can support several instruction sets.

ARM (abbreviated A32)

This is the “native” fixed-width (32-bit opcode length) instruction set used by ARMv7 and earlier.

Thumb

This is a compressed subset of the typical A32 instruction set introduced in the ARMv7 era. The encoding is still fixed-width, but 16-bits per opcode.

Thumb-2 (abbreviated T32)

This extends the Thumb instruction set with additional 32-bit instructions.

A64

This is the 64-bit instruction set used by AArch64 (wikichip). It has a fixed 32-bit encoding.

Typical ARM triples

When working with ARM hardware you see a dizzying array of triples. This is an attempt to make sense of them.

See Linaro’s documentation.

The ABI used on ARMv7 is defined by the ARM EABI specification