Bitstuff

For people interested in how computers work bottom-up.

Sunday, May 28, 2006

Epiphany

So I just realized that a multiple-issue superscalar processor is not at all efficient in an fpga. It simply will not provide the performance I'd like for one major reason: multiple write ports in FPGAs are prohibitively expensive.

So what I will do, is create a one-issue out of order processor that runs at a very high clock rate. The goal is 200MHz. We'll see. The scheduling hardware will be much cheaper this way. Also, it will be faster than a slower multiple-issue processor on an fpga.

Basically, my one choice for implementing multiple-write ports was to time-multiplex the register files. This creates some asynchronous fun. I figured as long as I was doing the time-multiplexing for the register files, I might as well time-multiplex the write back bus. Turns out, that is not such a bad idea. In fact, I should make things easy and run everything at the higher clock speed and pipeline where I have to. This would make a 200MHz scalar out of order cpu similiar to a 50MHz four-issue cpu. Tada! But it's even better, because many instructions will take only one cycle, while others, like addition, will take a couple.

Neato, right?

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