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I want to know what is microcoded microarchitecture of an instruction set architecture (ISA) and why is it used? What is the difference between microcoded architecture and single cycle microarchitecture?

Please clarify with some examples.

Thanks

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    \$\begingroup\$ This sounds suspiciously like a homework assignment, and this info is easy to find by googling. What have you learned so far? \$\endgroup\$ – Wouter van Ooijen Jan 11 '14 at 19:15
  • \$\begingroup\$ This is not a homework question. I did google but i could not find any good resource. If you are a better googler, you are welcome to give informative comment. \$\endgroup\$ – user2979872 Jan 11 '14 at 19:17
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Not all of the CPU instructions have the same complexity. When instructions are particularly complex (like some x86 instructions), it is more convenient to implement them as a series of elementary instructions that are contained in a very fast internal memory. When a microcoded instruction is executed the microprocessor split it into a series of simple hardware coded instructions and execute them in series.

So microcode is used in more complex instructions, while simple instructions are directly executed. For example today Intel microprocessors uses microcode in order to execute some old legacy instructions. The x86 microprocessors, that on the paper are CISC, internally uses an architecture that is more like RISC in order to boost the performance and they offer support to the legacy instructions through microcode.

You can find many RISC CPUs that don't use microcode. This is essentially due to their reduced instruction set, that makes more convenient implement all the instructions in hardware. An example of microprocessor that doesn't use microcode is the MOS 6502.

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  • \$\begingroup\$ can you provide any links or references to second your thought.I also request you to see L2s1 of coursera computer architecture course at class.coursera.org/comparch-002/lecture/10 because it says something else \$\endgroup\$ – user2979872 Jan 11 '14 at 19:19
  • \$\begingroup\$ You can find a lot of information here: en.wikipedia.org/wiki/Microcode \$\endgroup\$ – Oceanic815 Jan 11 '14 at 19:25
  • \$\begingroup\$ Could you please provide some reference which supports the second paragraph of your answer? While some instructions in IA are decoded by microcode into a series of instructions (especially those which are rarely used/legacy), this by itself does not imply that Intel's processors are "more like RISC". \$\endgroup\$ – Vasiliy Jan 11 '14 at 19:46
  • \$\begingroup\$ You can find some information on the Intel architecture in this question stackoverflow.com/questions/5806589/… and in this post sunnyeves.blogspot.it/2009/07/… \$\endgroup\$ – Oceanic815 Jan 11 '14 at 19:55

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