A-Profile Architectures

A-Profile Architectures

Arm produces a whole family of processors that share common instruction sets and programmer’s models and have some degree of backward compatibility. The Application (‘A’) profile targets high performance markets such as mobile and enterprise.

Armv8-A Architecture

The Armv8-A architecture is the latest generation Arm architecture targeted at the Applications ('A') profile. 

It introduces the ability to use 64-bit and 32-bit Execution states, known as AArch64 and AArch32 respectively. The AArch64 Execution state supports the A64 instruction set, holds addresses in 64-bit registers and allows instructions in the base instruction set to use 64-bit registers for their processing. The AArch32 Execution state is a 32-bit Execution state that preserves backwards compatibility with the Armv7-A architecture and enhances that profile so that it can support some features included in the AArch64 state. It supports the T32 and A32 instruction sets.

Armv8-A is the only profile that supports AArch64 execution, where the relationship between AArch64 and AArch32 is known as interprocessing. In addition, the Armv8-A architecture allows different levels of AArch64 and AArch32 support, for example:

• AArch64 only designs.
• AArch64 designs that also support AArch32 operating systems/virtual machines.
• AArch64 support with AArch32 at (unprivileged) application level only.

Major Features in Armv8-A

The latest Armv8-A architecture introduces a number of new features, which enable more secure and significantly higher performance processor implementations to be designed.

Arm v7-A Architecture

The Armv7-A architecture introduced the concept of architecture profiles which has been carried onwards into the Armv8 architectures. It implements a traditional Arm architecture with multiple modes, supports a Virtual Memory System Architecture (VMSA) based on a Memory Management Unit (MMU), and supports the Arm (A32) and Thumb (T32) instruction sets.

Architectural extensions

This architecture also supports multiple extensions. These are:

• Security Extensions. These are an optional set of extensions that provide a set of security features that facilitate the development of secure applications.
• Multiprocessing Extensions. These are an optional set of extensions that provide a set of features that enhance multiprocessing functionality.
• Large Physical Address Extension. This is an optional extension that provides an address translations system supporting physical addresses of up to 40 bits at a fine grain of translation. It requires implementation of the Multiprocessing Extensions.
• Virtualization Extensions. These are an optional set of extensions that provide hardware support for a virtual machine monitor, called a Hypervisor, to switch between Guest operating systems. It requires implementation of the Security Extensions and the Large Physical Address Extension.
• Generic Timer Extension. This is an optional extension that provides a system timer and a low-latency register interface to it. It is included as part of the Large Physical Address Extension or the Virtualization Extension, but can also been implemented with earlier versions of the Armv7-A architecture.
• Performance Monitors Extension. This extension defines a recommended performance monitors implementation and reserves register space for performance monitors.

Most of the functionality provided by these extensions is included in the Armv8-A architecture, though the Performance Monitors Extension remains optional.