Re: [myhdl-list] intbv with min -1, max 1 has 2 bits?

[Jan D. <ja...@ja...>]

Günter Dannoritzer wrote:
> Hi,
> 
> I tried to create a signed intbv with one bit in the form:
> 
>  >>>a = intbv(-1,-1,1)
>  >>>print "a: %d, min: %d, max: %d, _nrbits: %d"%(
>            a, a.min, a.max, a._nrbits)
> 
> a: -1, min: -1, max: 1, _nrbits: 2
> 
> It turns out that intbv uses 2 bits for it. Looking in the 
> intbv.__init__() function there is this code section starting at line 46:
> 
>             if max is not None and min is not None:
>                  _nrbits = maxfunc(len(bin(max-1)), len(bin(min)))
>                  if min >= 0:
>                      _nrbits = len(bin(max-1))
>                  elif max <= 0:
>                      _nrbits = len(bin(min))
>                  else:
>                      # make sure there is a leading zero bit in positive 
> numbers
>                      _nrbits = maxfunc(len(bin(max-1))+1, len(bin(min)))
> 
> 
> What is interesting about this is that the first assignment to _nrbits 
> right after the if clause is actually overwritten by any of the 
> following assignments.
> 
> So the reason for the -1,1 range to have 2 bits seems to lay in the 
> comment about having a leading zero bit for positive numbers.
> 
> I might be oblivious and the reason is obvious, but I don't see it at 
> the moment. Anyone can explain why I need a leading zero bit and cannot 
> create a 1 bit intbv?

1) why a leading zero bit
Well, for a number that can be both positive or negative you need to
have room for a sign bit, right? that's what the leading zero bit does.

2) why not a 1 bit signed intbv
That seems to be a bug. The one bit can function as a sign bit in
this (corner) case. I guess the test should really be:

                 ...
                 elif max <= 1: # not 0
                     _nrbits = len(bin(min))
                 ...


A sidenote: To get insight in this, I used the brand new web interface
to the brand new mercurial repository at:

     http://myhdl.sourceforge.net/hg/myhdl

If you go to _intbv.py through "files", and then "annotate", you see the changeset
when these lines were introduced. Click on it and then go to "changeset" to review
that changeset. Great isn't it :-)

Jan

-- 
Jan Decaluwe - Resources bvba - http://www.jandecaluwe.com
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