Chapter 2

ARM

A company named Acorn Computers developed a 32-bit RISC architecture named the Acorn RISC Machine (later renamed to Advanced RISC Machine) in the late 1980s. This architecture proved to be useful beyond their limited product line, so a company named ARM Holdings was formed to license the architecture for use in a wide variety of products. It is commonly found in embedded devices such as cell phones, automobile electronics, MP3 players, televisions, and so on. The first version of the architecture was introduced in 1985, and at the time of this writing it is at version 7 (ARMv7). ARM has developed a number of specific cores (e.g., ARM7, ARM7TDMI, ARM926EJS, Cortex)—not to be confused with the different architecture specifications, which are numbered ARMv1–ARMv7. While there are several versions, most devices are either on ARMv4, 5, 6, or 7. ARMv4 and v5 are relatively “old,” but they are also the most dominant and common versions of the processor (“more than 10 billion” cores in existence, according to ARM marketing). Popular consumer electronic products typically use more recent versions of the architecture. For example, the third-generation Apple iPod Touch and iPhone run on an ARMv6 chip, and later iPhone/iPad and Windows Phone 7 devices are all on ARMv7.

Whereas companies such as Intel and AMD design and manufacture their processors, ARM follows a slightly different model. ARM designs the architecture and licenses it to other companies, which then manufacture and ...

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