[libre-riscv-dev] building a simple barrel processor

[Luke Kenneth Casson Leighton]

On Fri, Mar 8, 2019 at 8:17 AM Jacob Lifshay <programmerjake at gmail.com> wrote:

> On Thu, Mar 7, 2019, 23:34 Luke Kenneth Casson Leighton <lkcl at lkcl.net>
> wrote:
>
> > On Fri, Mar 8, 2019 at 6:11 AM Jacob Lifshay <programmerjake at gmail.com>
> > wrote:
> >
> > > Just to clarify, I'm asking if you think it's a good idea to work on this
> > > since it will take some time.
> >
> >  at its heart a barrel processor is a single-core single-issue
> > timeslicing design, suited to real-time I/O processing.  if we were to
> > add multiple barrel 4-time-sliced SMP cores, it would result in
> > multiple proliferations of the massive dual/triple-ported 8k SRAMs.
> >
> actually, for barrel processors, because each instruction takes many cycles
> to execute, you only need a single-ported sram split into banks, since each
> hart has its own separate bank(s), each hart can read each instruction's
> arguments one at a time, then write at the end. if there are enough harts
> per core you can also run the sram at a lower clock rate & maybe lower
> voltage -- dedicating more pipeline stages to reading and writing the
> register file.

 as a parallel processor - a dedicated GPU - this would likely be
really good.  i remember older GPUs running at like only 300-400mhz.

 as a general-purpose processor however it would suck.  the latency
would be atrocious for anything that required single-process
performance.

 we'd therefore need to completely change the design strategy to a
dual (split) CPU + GPU, and have the kazan codebase modified to
include an IPC/RPC mechanism that was capable of packaging up all API
calls, shipping them over to the GPU and having it execute things
there.

l.