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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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