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by duskwuff 3904 days ago
> You're forgetting the memory elements – they're crucial to the functionality of FPGAs.

I'd argue that an even more critical feature of FPGAs is configurable routing. ROMs don't have that either. :)
1 comments
adwn 3904 days ago
True. But then again, that's like arguing over which organ is more important – the heart or the lung – when you can't survive without either of them ;-)
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duskwuff 3903 days ago
You can build memory elements - inefficiently, but it's possible - out of recurrent logic. You can't build routing.

Actually, now I wonder whether it'd be possible to build memory out of ROM by looping outputs back to inputs...

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adwn 3903 days ago
> You can't build routing.

You actually don't need configurable (programmable) routing: We can model every synchronous circuit as a Mealy machine [1], therefore we need two functions and a register. Every function can be constructed from a hierarchy of programmable, fixed-size LUTs which are statically connected (for example, you can build a 5-input LUT from three 4-input LUTs). Now you don't need to build routing, it's implicit in the next-state and output functions.

[1] like this: http://electrosofts.com/verilog/mealy.gif

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