The ARM Architecture

ARM is a a 32-bit RISC processor architecture currently being developed by the ARM corporation. The business model behind ARM is based on licensing the ARM architecture to companies that want to manufacture ARM-based CPU’s or system-on-a-chip products. The two main types of licenses are the Implementation license and the Architecture license. The Implementation license provides complete information required to design and manufacture integrated circuits containing an ARM processor core. ARM licenses two types of cores: soft cores and hard cores. A hard core is optimised for a specific manufacturing process, while a soft core can be used in any process but is less optimised. The architecture license enables the licensee to develop their own processors compliant with the ARM ISA. ARM processors possess a unique combination of features that makes ARM the most popular embedded architecture today. First, ARM cores are very simple compared to most other general-purpose processors, which means that they can be manufactured using a comparatively small number of transistors, leaving plenty of space on the chip for application-specific macrocells. A typical ARM chip can contain several peripheral controllers, a digital signal processor, and some amount of on-chip memory, along with an ARM core. Second, both ARM ISA and pipeline design are aimed at minimising energy consumption — a critical requirement in mobile embedded systems. Third, the ARM architecture is highly modular: the only mandatory component of an ARM processor is the integer pipeline; all other components, including caches, MMU, floating point and other co-processors are optional, which gives a lot of flexibility in building application-specific ARM-based processors. Finally, while being small and low-power, ARM processors provide high performance for embedded applications. For example, the PXA255 XScale processor running at 400MHz provides performance comparable to Pentium 2 at 300MHz, while using fifty times less energy. This report is largely based on material from Steve Furber’s book about the ARM architecture [Fur00]. Other sources are referenced throughout the report.