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Re: [f-cpu] Manual 0.2.6
On Mon, Jul 29, 2002 at 11:43:11PM +0200, Yann Guidon wrote:
[...]
> * if you want to shift, use the shifter unit (SHL) : it's its job.
> The purpose of Xbar is not to shift.
That's right. `loadconsp' shifts are supposed to happen elsewhere, though.
> * a shifter takes more room than you would think. More
> room means decreased frequency and higher power consumption.
> Even though the shift is only 16 bits, remember that
> the "cost" is proportional to N*M (if M is the number of
> bits in the word and N is the shift amount).
> And even though there are several metal layers and better
> synthesizers, you can't modify this basic topological rule.
> i think that Michael's shifter will not match the speed of
> other units if he keeps wanting a single cycle shift.
I remember that the synthesis report was encouraging.
> * The Xbar is already very loaded :
> - 4 bypass (2 write ports with 2 bypass nets each)
> - 3 read ports that must feed half a dozen of inputs
> - roughly 10 execution units (and still no FP in sight)
> - CIP, NIP; TLB, immediate data
> and i probably forget many things.
> In the beginning, Xbar was a place where we could do
> whatever we wanted but now it's proably as heavy and
> slow as the register set. Adding a shifter in this
> critical datapath would slow down the whole circuit.
The shifter itself is outside the CDP. There's only a MUX inside the
CDP, and you'll need that for partial writes as well.
> * Show me the proposed instruction form.
Didn't I?
--
Michael "Tired" Riepe <Michael.Riepe@stud.uni-hannover.de>
"All I wanna do is have a little fun before I die"
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