Arm

The ARM architecture (previously, the Advanced RISC Machine, and prior to that Acorn RISC Machine) is a 32-bit RISC processor architecture developed by ARM Limited that is widely used in a number of embedded designs. more...

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Because of their power saving features, ARM CPUs are dominant in the mobile electronics market, where low power consumption is a critical design goal.

Today, the ARM family accounts for approximately 75% of all embedded 32-bit RISC CPUs, making it one of the most widely used 32-bit architectures in the world. ARM CPUs are found in all corners of consumer electronics, from portable devices (PDAs, mobile phones, media players, handheld gaming units, and calculators) to computer peripherals (hard drives, desktop routers). Important branches in this family include Marvell's XScale and the Texas Instruments OMAP series.

History

The ARM design was started in 1983 as a development project at Acorn Computers Ltd.

The team, led by Roger Wilson and Steve Furber, started development of a compact RISC CPU. A key design goal was achieving low-latency input/output (interrupt) handling like the MOS Technology 6502 used in Acorn existing computer designs. Using the 6502 Acorn had managed to produce powerful, flexible, machines without resorting to costly direct-memory access hardware. A powerful chip that allowed the same style of design could represent a significant advantage for the company.

The team completed development samples called ARM1 by April 1985, and the first "real" production systems as ARM2 the following year. The ARM2 featured a 32-bit data bus, a 32-bit address space, (although the program counter was limited to 26 bits as the top 6 bits served as the status flags), giving a 4 Gbyte address space, although only the first 64 Mbyte address range could be used for executing code, and sixteen 32-bit registers. One of these registers served as the (word aligned) program counter with its top 6 bits and lowest 2 bits holding the processor status flags. The ARM2 was possibly the simplest useful 32-bit microprocessor in the world, with only 30,000 transistors (compare with Motorola's six-year older 68000 model with around 70,000 transistors). Much of this simplicity comes from not having microcode (which represents about one-fourth to one-third of the 68000) and, like most CPUs of the day, not including any cache. This simplicity led to its low power usage, while performing better than the Intel 80286. A successor, ARM3, was produced with a 4KB cache, which further improved performance.

In the late 1980s Apple Computer started working with Acorn on newer versions of the ARM core. The work was so important that Acorn spun off the design team in 1990 into a new company called Advanced RISC Machines Ltd.. For this reason, ARM is sometimes expanded as Advanced RISC Machine instead of Acorn RISC Machine. Advanced RISC Machines became ARM Ltd when its parent company, ARM Holdings plc, floated on the London Stock Exchange and NASDAQ in 1998.

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