[libre-riscv-dev] IEEE754 FPU

Luke Kenneth Casson Leighton lkcl at lkcl.net
Fri Mar 1 23:43:55 GMT 2019

Ok so I am making a complete pig's ear of the adder, slowly and painfully
transforming it to connect inputs to outputs instead of taking global
variables and extracting and dropping the stage data in and out of globals.

Each stage's logic I have split out into combinatorial module blocks with
input and output, no clock sync at all. This is deliberate so that it
becomes possible later to combine phases into a single cycle, reducing the
number of "states" and also when doing pipelines reducing the number of
pipeline stages.

The chain has a bypass on the add and normalisation, due to special cases
for zero, NaN and Inf, however the use of global variables made it
complicated to split the chain. I suceeded by creating a new state that
puts the intermediate z from the add stages into the output, separate from
a different z used to store the special cases, these two different stages
go into the same output.

This separation allowed me to remove at least one global variable.

The normalisation phase is currently the biggest hurdle as it is a
multi-cycle phase. My next efforts will be to work out how to get the
normalisation phase to cycle on an internally protected local temporary
variable, only outputting it when the result is ready.

The problem right now is that the data from the normalisation phase changes
AFTER it is suppised to be finished.

Still working on it.


crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68

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