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ARM is a family of instruction set architectures based on RISC architecture developed by a single company - ARM Holdings.
Because ARM is a family of architectures and not a single architecture, it can be found in large scale of electronic devices (from simple small embedded systems with ARM MCUs, through smartphones, tablets and MP3 players to low-power servers).
When you look at a device with ARM processing unit (term processing unit is more accurate because ARM can be found in microcontrollers as well as in microprocessors), there are two things that matter: architecture and core.
So far (Q3 2014) there have been 8 ARM architectures released or announced (where some of them have their extended versions), 7 of them being 32-bit, and the last one being 64-bit, but user-space compatibile with 32-bit instruction set (therefore making it possible to run 32-bit user processes, yet not 32-bit operating systems without virtualization). Very broadly said, with every new architecture version there have been added some new features to all cores (with exceptions) which had been already tried in some cores of a previous architecture. Not all features of previous architecture must be again available in the next one, and not all new versions of technologies added in previous architectures must be compatibile with the old ones. The simplest reason for this one could think of is that designing processors isn't like writing software: when a program decides whether to process old file format or a new one, it makes its decision and then exectues only one code, but backward compatibility of (for example instruction) formats may mean more transistors and more transistors usually (always?) result in more heat.
You must know you can't buy an ARM processor just like you would buy Intel abc or AMD xyz. ARM Holdings is a company that designs architecture, writes an '''Architecture Reference Manual''', then designs a core and writes a '''Technical Reference Manual'''. Finally, it sells designed core to a chip-making-company and releases manuals to public. From there, a chip-making-company designs a processing unit - a microcontroller (MCU), a System On a Chip (SoC), an FPGA, whatever - and it may manufacture the silicon itself or order n-thousands of chips from a semiconductor manufacturer.
All of processors you'll see in tablets or smartphones are SoCs which act as sort of motherboard with a processor. They contain logic for driving peripherals (ethernet, USB, SD/MMC cards, SPI, I2C, audio), they may contain GPU for graphics coprocessing or FPGA for custom logic.
Many uses means either few multifunctional (and theferefore complex and power-consuming) devices, or many simple devices suited just for that kind of operation. ARM processors as RISC devices chose simplicity over complexity, and therefore they are way too many cores with different instructions used. To have just ''one assembler to rule them all'', ARM defined Unified Assembly Language which can be translated for *any* of ARM cores.
ARM cores are divided in lastest versions to three main lines:
* Cortex-M cores, used for really small devices, usually with on-chip memory and simpler operations
* Cortex-R cores, used for real-time devices
* Cortex-A cores, used for applications in multifunctional devices like smartphones, TVs or maybe computers.
==Overview==
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