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what is the basic difference between Intel (CISC) and ARM (RISC) architecture?

Hennes
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Dzn
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  • The differences between x86 and ARM is widely known. I cannot agree that ARM is beating Intel in the mobile market. – Ramhound Feb 27 '15 at 11:12
  • Besides ARM isn't a CISC architecture. [Here](http://stackoverflow.com/questions/13071221/is-x86-risc-or-cisc) nor is x86 MIPS. – Ramhound Feb 27 '15 at 11:20
  • More [research](http://superuser.com/questions/640243/is-it-possible-for-an-x86-processor-to-match-an-arm-processor-in-terms-of-perfor?rq=1) – Ramhound Feb 27 '15 at 11:47

1 Answers1

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In the 70's and early 80's:

  • RAM was very very expensive
  • it ran at the same speed as the CPU.
  • programming by hand in assembly language was common

So - simplifying very much here - it made sense to design CPUs where each instruction performed a lot of work, was easy to translate from high-level languages, and where programs were expected to use memory as a scratchpad, as opposed to internal CPU registers. This makes the CPU design complicated and power-hungry.

The RISC principle advocates making instructions simple and lightweight, recognizes that compilers are the ones usually generating assembly language and not humans, and provides lots of registers (faster than RAM) for use of intermediate calculations. This makes the CPU design simpler and requires less power. The assembly language is more complicated and you usually need more instructions to do things - but as RISC was becoming prevalent, RAM prices were falling.

RISC looked like it was going to win out in the late 80's/early 90's, but Intel started putting RISC-like features into it's CISC-like CPUs - and moved forward with additional performance features such as caching, branch prediction, register renaming, etc., and today's 64-bit CPUs from Intel and AMD can be considered a hybrid.

However, ARM CPUs:

  • are still much more simpler internally.
  • ARM licenses its CPU core to chipset manufactuers. So it's easy for companies such as Qualcomm or Apple to integrate a CPU core into a phone chipset.
  • the above two contribute to power savings which are very important on mobile devices, even though they do not perform as well as Intel hardware.
LawrenceC
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    The delta between the power savings is starting to smaller and smaller. Intel has spend considerable R&D money to get Atom to match ARM not only performance but to reduce its power requirements. They used this same R&D money towards their server and desktop SKUs as well. – Ramhound Feb 27 '15 at 11:46