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Re: Re: [f-cpu] 2R2W SHL


>De: Michael Riepe <michael@stud.uni-hannover.de>
>On Mon, Oct 08, 2001 at 11:08:52AM +0200, whygee@club-internet.fr wrote:
>> hello,
>> IIRC michael is designing the SHL unit. I have a little
>> suggestion : what about an instruction that does a 2R2W shift ?
>> that is : one register gets the "normal" output of the shift,
>> and the 2nd register gets the "shifted out" bits.
>> the ROR/ROL instructions would be a OR of the two results.
>That's the way I do rotates now -- I stuff the input vector with zeroes,
>do a double-size shift, and OR the upper and lower half (it's a little
>more complex when SIMD is involved, but that's it, basically).

there's no secret :-)

>The alternative approach is to use rotates as the basic element,
where do you fetch these ideas ?...

> and mask the unused bits when you're doing a shift operation.  In this case,
>you can get the `double width shift' result with two separate masks.
>BTW: did somebody synthesize the alternative SHL core (the Rotate64
>entity) I postet on Sep 28?  I'm still interested in some numbers...
me too :-)

>> This is useful when a lot of bitblt is done, for graphics and
>> bit string insertion/extraction... And there is no need to
>> specify the shift direction because it is swapped when the destination
>> registers are swapped :-)
>But the interpretation of the shift count is also changed ;)
not that much...

>> any comment ?
>Yep... I like the idea.  How do we call it, dshiftl? shiftd? shiftx?
no idea...
what about sh ? :-) [why make it complex ?]

> 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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