Re: [f-cpu] magnetude comparison

["gaetan@xeberon.net" <gaetan@xeberon.net>]

Michael Riepe a écrit :

> Hi again,
>
> On Tue, Mar 02, 2004 at 01:13:56AM +0100, gaetan@xeberon.net wrote:
> [...]
>  
> > vhdl world is so wonderful...
> >    
> Yes, definitely. ;)
>
> [...]
>  
> > > > (damned, why there isn't mathml supported by all email reader)
> > > >        
> > > Because some of them still run in text mode?
> > >
> > >      
> > arg!!! some people still use 'mail' command?
> >    
> Mutt - the best e-mail client money can't buy :)
>
> [...]
>  
> > > If you use a shifter that requires a binary encoded shift count :)
> > >
> > > My omega network shifter can also be controlled by bit vectors that
> > > represent numbers like (2**shift_count)-1.  They are much easier to
> > > generate in this case: a single left-to-right cascade_or is sufficient.
> > > That will take d=3/t=3 for a 64-bit operand -- half a stage.
> > >
> > >
> > >
> > >      
> > i don't know what an omega network it, i though my LOP was a good thing...
> >    
> It is, because it's still faster (no late shift count calculation at all).
>
> [...]
>  
> > > > but if i do
> > > > f = (a and b) or (c and e)
> > > > then i should estimate it d=2/t=2 (right?)
> > > >        
> > > That depends on the target, but d=2/t=2 -- meaning separate AND and
> > > OR gates -- is the safest choice.
> > >
> > >      
> > that's not very funny, i have some control logic (with some "complex" 
> > boolean equations)....
> >    
> How complex?  How many input variables, at most?
>  
i have something like:
sel =  C and (not(g) or (MSB and L) )
d=3/t=3
it's the output selector of compound adder in close datapath (rounding 
to nearest)
since MSB, L and C comes from the A+B output of the adder, is a full 
length delay.
+ mux4 to select beween the 4 rounding mode selectors (d=1/t=1)
+ mux2 to select the output
=> i have the rounded result ~d=5/t=5 after the adder.

one of  the most complex signal is in the far datapath:
seladd = (not (c) and g and (LSB or r or s) ) or
        ( c and L and ( [LSB-1] or g or r or s) )
selsub = (c and (   (not(g) and not(r) and not(s)) or
                         ( g and r) or
                         ( MSB and g and (LSB or s)
                    )
         )
delay: d=3/t=3 (if i could have cound an additionnal d/t for not(g) and 
not(r) and not(s), but it can be computed before)
and then
if effective addition then
   sel = seladd
else -- effective substraction
  sel = selsub
end;
d=4/t=4
it's the compound adder selector in the far datapath (for rounding to 
nearest)
=> so in this datapath i will have the rounded result d=6/t=6 after the 
end of the added... :(

> [...]
>  
> > i will take some nights to digest it i think... but it's very interesting...
> > but why not writing it down (for example in the wiki), then any new 
> > rooky that may come will not
> > ask you that again and again...
> >    
> Because I'm a lazy *censored*? ;)
>
>  
and you write me huge mails?

++

--

~~ Gaetan ~~
http://www.xeberon.net


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