[Libre-soc-dev] daily kan-ban update 24nov2020
Luke Kenneth Casson Leighton
lkcl at lkcl.net
Wed Nov 25 12:49:58 GMT 2020
crowd-funded eco-conscious hardware: https://www.crowdsupply.com/eoma68
On Wed, Nov 25, 2020 at 7:55 AM Jacob Lifshay <programmerjake at gmail.com> wrote:
> Finished the 16-bit compressed instructions demo:
> I ended up writing an assembler in bash for the demo:
> I also wrote an example predecoder that works with O(log N) gate depth
> that handles the full details of the proposed 16/32/48/64-bit
ah excellent. nicely done. i notice the key is to use something akin
to carry look-ahead. this makes a lot of sense given that the modes
are mutually-exclusive. neat.
btw fascinatingly carry look-ahead was the basis of one of the Aspex
ASP inter-processor instructions, allowing them to do literally
unlimited-length arbitrary-precision arithmetic.
> It outputs an array corresponding to 16-bit instruction
> memory units with a flag for each 16-bit unit indicating if that
> particular unit is the start of an instruction as well as the mode
> that instruction should be decoded in. I'm assuming it's relatively
> easy to figure it out from there.
should be yes. a sinple naive version would perform an add-cascade (2
bits for operand 1, steadily increasing result accumulator) to produce
the indices needed to extract instructions from the correct location.
following that, in combination with the length/mode-detection those
indices can be used to extract the instructions for further (parallel,
that's going to be an arbitrary-shift-in shift register with an
absolutely massively-high number of read ports, and it'll definitely
result in a 2-stage decode. first stage length second stage what's
currently PowerDecoder2 (and others).
> instruction decode modes are assumed
> to be kinda part of the PC instead of like VLE where they are set in
> the page table.
we're still going to need to mark pages as "variable-length" (with a
single bit) in order to distinguish them from pages that purely
contain ppc64le because 16/48 is, as you showed yesterday(?)
incompatibly-mixable with pure ppc64le word-aligned instructions.
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