Re: [f-cpu] back to VHDL

[Michael Riepe <michael@stud.uni-hannover.de>]

On Sun, Feb 10, 2002 at 02:17:50AM +0100, Yann Guidon wrote:
[...]
> I have one question however : Can we "link" the SHL unit to the
> LSU ? This would allow us to perform both endian and alignment
> with few overhead and some flexibility : it would even allow
> us to do "signed" loads, where the data is first aligned and
> then sign-extended... This is often used for CPUs like ALPHA
> and MIPS where data are handled only in a word-sized way.

That's not very RISCy, isn't it?

We should duplicate the byterev part and add it to the LSU (for
loade/storee), but the rest is IMHO overkill. In general, data should
be properly aligned. Applications that violate this rule are supposed
to do their bit fiddling on their own.

Sign-extension (that is, the `widen' instruction) is currently not handled
by the SHL unit (sign-extended right shifts are, but that's a totally
different operation). We can implement it in the SHL, the LSU, or both.
Or in another, separate unit.

-- 
 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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