A CPU Is a Compiler

Re:A CPU Is a Compiler by Arrath on Thursday March 23, 2023 @02:54PM (#35279577)

> The alternative is to have 16 decoders, each decoding what an instruction /would/ be if it started at a given byte. These all complete in parallel, but many of them might produce garbage. So we need to find the first five of these that have non-garbage outputs, and /then/ mux those sixteen possible outputs down to five channels. This is doable (and done), but it's a lot more hardware in both decoders and muxing than you might think at first.

Well that just sounds hideously complex.

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Re:A CPU Is a Compiler by jcranmer on Thursday March 23, 2023 @03:06PM (#35279759)

Length decoding is a finite state machine, and the transition function of finite state machines is associative, which means you can actually compile them into a reduction tree to do it in parallel.

I don't think the x86 decoder is actually a particularly complex state machine (I haven't attempted to actually build it), so it's probably overall far less complex than most people assume it is.

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