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I don't think that ARM implementations are usually microcoded.

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• monocasa 4y ago
  ARM was always a bit of a CISC/RISC hybrid. The author of this article actually went into the microprogram engine of the ARM1 in some detail previously here: https://www.righto.com/2016/02/reverse-engineering-arm1-processors.html https://www.righto.com/2016/02/reverse-engineering-arm1-proc... Even today, I would be shocked if ARM designs weren't partially microcoded as well. Something like the ERET instruction is begging to be.
  • Taniwha 4y ago
    The difference here though is that the ARM1 uses a PLA to directly decode an ARM instructions into a small number of clocks worth of things to do (enables to ALU stages/etc), while the x86 decodes an instruction into the address of a bunch of microinstructions in a ROM which then get executed with a micro PC (including subroutine calls). Sure the x86 microinstructions also directly control enables to ALU stages/etc, but the difference is that they also include what are effectively opcodes in their own rights. In short the ARFM has decode logic that makes a bunch of clocks worth of ALU controls directly, while the x86 has decode logic that makes an index into a ROM from where that information is read/generated