[libre-riscv-dev] IEEE754 FPU turning into ALU with Reservation Stations

[Aleksandar Kostovic]

Hey everyone, i am finally back and ready to do this. So Luke, can you give
me a super fast TL;DL of things that are done(i tried my best to stay up to
date, but a quick overview would be very much appreciated!) and how could i
help?

:)

On Mon, Mar 11, 2019 at 11:42 AM Luke Kenneth Casson Leighton <lkcl at lkcl.net>
wrote:

> i've made a start on an "InputGroup" module which is the front-end of
> what mitch alsup terms a "concurrent computation unit".  the idea is,
> to put an array of input latches for the input operands and a
> corresponding array of output latches for the results onto a single
> pipelined ALU.
>
> the minimum array width of the input-output latches has to be equal or
> greater to the pipeline depth, otherwise it's pretty much a complete
> waste of time having the stages of the pipeline.  sometimes however
> that's unavoidable, such as for DIV operations where the completion
> time will be extremely long.
>
> the InputGroup module has an array of pairs of operands as input and
> has a single pair of operands as output, plus a "Mux ID" which is the
> array index of the operands that are ready.
>
> i spent last week putting in the exact required "Mux ID" into the
> FPADD module, ready for this, so that the Mux ID, when received from
> the InputGroup, can be passed through the pipeline, and will come out
> the other side so that it can be used to identify the correct output
> in the Output Latch Array.
>
> this identification is essential for a variable-length pipeline with
> early-out capability (which is what FPADD, FPMUL and so on are, with
> the Special Cases logic being able to jump out).  it's also a
> complication as potentially two results may be ready in a given cycle,
> i'll deal with that when it comes to it.
>
> btw just to reiterate (jacob?), if there is only one ALU and it
> arranged with this structure surrounding it, it becomes a single-issue
> in-order design.  in such an in-order design, the array of
> input-output latches helps deal with the early-out capabilities of any
> pipeline.
>
> l.
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