ARMv7 Common Memory System Architecture

Introduction

This article seeks to provide a concise summary of the ARM version 7 Common Memory System Architecture, which is a very convoluted name for the ARM Caching and Branch Prediction architecture. ARM has 3 main "Memory Systems Architectures" which form the components of its memory access scheme:

• CMSA: Common Memory System Architecture: Caches and Branch Prediction. CMSA is common to all profiles, with adjustments as needed for the particular implementation and intended use of the processor.
• VMSA: Virtual Memory System Architecture: TLBs and Virtual Memory as a memory protection and isolation, and linearization scheme. VMSA is generally implemented by ARM profile-A processor implementations.
• PMSA: Protected Memory System Architecture: a simplified memory protection scheme, also known in general CPU design parlance as an MPU (Memory Protection Unit), often held in contrast to an MMU (Memory Management Unit). An MPU is generally preferred to the memory and speed overheads of an MMU, in very resource-constrained, or performance critical microcontrollers. PMSA is generally implemented by ARM profile-R processor implementations.

Any particular ARM implementation may have one or more of these "Memory Systems" (CMSA, VMSA and PMSA).

This article is not current with the ARM version 8 CMSA.

Components of the CMSA

Branch predictors

The ARM architecturally visible Branch Prediction architecture consists of the Branch Target Buffer (BTB), the Pipeline, and the Instruction Prefetch Buffer (IPB). The following sections explain why software may need to care about these components.