Is it hard wired CU or microprogramed?


closed as unclear what you're asking by Yuval Filmus, Evil, David Richerby, Juho, Tom van der Zanden Nov 14 '17 at 11:09

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  • $\begingroup$ I'm not sure computer engineering is on-topic here. At any rate, I believe that the question can be answered by browsing Wikipedia. $\endgroup$ – Yuval Filmus Nov 9 '17 at 14:20

x86 are famous for using microcode, because there are a few complex operations, such as call gates or how the x87 calculates sines or exponentials, which cannot be reasonably constructed with a simple finite state machine. x86 also have very extensive patching resources, which allows to fix some bugs on released silicon, and which is probably handled through microcode (Some hints are probably in the patents from Intel and AMD).

Small ARM CPUs don't need and don't use microcode. The "R" and "M" classes.

High end ARMs such as Apple's... Who knows ? Besides the visible instruction set, there are also many hidden stuff, for example for debugging, self-testing the hardware and for power management, which may be based on microprograms.

The general trend nowadays, is that, even in x86, the instructions that need performance are decoded directly which is faster and more power efficient that fetching microcode memory.


Have a look at this article from 2010: https://www.realworldtech.com/sandy-bridge/ describing a popular real-world processor.

Even in 2010, "hardwired control unit vs. microprogrammed" doesn't make sense anymore. The model where the processor reads an instruction, then executes it, isn't how processors are implemented anymore.

  • $\begingroup$ One point that's worth emphasising is that as well as global instruction handling, modern CPUs have a lot of local control problems. For example, floating point functional units may need extra control to handle unusual cases such as underflow. No modern CPU designer would turn away microcode if it turned out to be the best solution in these situations. $\endgroup$ – Pseudonym Nov 11 '17 at 23:24

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