[Open-graphics] Designing a CPU

[Timothy Normand Miller]

On 3/17/07, Daniel Rozsnyó <daniel at rozsnyo.com> wrote:
> >> Also, we need ways to handle writing to full fifos and reading from
> >> empty ones.  One way would be to have the instruction block, but being
> >> able to pause the pipeline adds extra logic and complexity that we
> >> might like to avoid.
> >
> > OK so do you want an error return or do you want to query for size, or
> > do you want interrupts? I think being able to query the size of a fifo
> > would be best.
> >
>
> I have an idea for empty fifos: forwarded ignore flag. Each data word
> (register, alu in/out) will have one more signal - a bit indicating if
> the data is valid or not. Then on the end, you might get an ignored
> write to memory/mmio. If you want to branch depending on such an
> optional value, then a default choice must be given by the instruction
> (to which direction jump, if the value is invalid). The flag will
> propagate as the null value does in sql expressions in databases (if
> either operand is null, the result is null).
>
> Pro:  pipeline is not stopped, no polling in nanocode
> Cons: power consumption, logic complexity?
>
> For the full fifos we'll have to propagate the impossibility of write
> back to the source, so maybe a near-full signal on the destination fifo
> could issue a "full" state on the source fifo's input side. The
> back-routing would be set ad-hoc by the nanocode, as the functional
> units will be linked. If it's too abstract, I can draw an image if it.


I like your ideas.  I'm also thinking we might want to consider making
it possible for MMIO to stall the pipeline.  I have a way to do it
where the busy signal is registered, so it's not terrible.  Having a
read or write block has certain advantages that MAY be worth the extra
logic.  It's also especially not so bad if only one stage is able to
cause a stall.

-- 
Timothy Normand Miller
http://www.cse.ohio-state.edu/~millerti
Favorite book:  The Design of Everyday Things, Donald A. Norman, ISBN
0-465-06710-7