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Comment by gumby | Hacker News Reader

A lot of this is done in software (microcode). But even with that case, your statement still holds: "Can you imagine having to make all this logic work faithfully, let alone fast, in the chip itself?" Writing that microcode must be fiendishly difficult given all the functional units, out of order execution, register renaming...

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The crazy parts that were mentioned in the parent comment are all part of the hot path. Microcode handles slow paths related to paging and segmentation, and very rare instructions. Not necessarily unimportant (many common privileged instructions are microcoded) but still rare compared to the usual ALU instructions.

But it's not a huge deal to program the quirky encoding in an HDL, it's just a waste of transistors. The really complicated part is the sequencing of micro operations and how they enter the (out of order) execution unit.

> A lot of this is done in software (microcode).

No, that's not the case, since >30 years. Microcode is only used for implementing some complex instructions (mostly system instructions). Most regular instructions (and the rest of the core) don't use microcode and their expansions into uOps are hardwired. Also the entire execution unit is hardwired.

There are typically some undocumented registers (MSRs on x86) that can control how the core behaves (e.g., kill switches for certain optimizations). These can then be changed by microcode updates.

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