Re: [myhdl-list] Slicing an unsigned intbv to a signed one

[Günter D. <dan...@we...>]

Jan Decaluwe wrote:
...
> I'm now thinking aloud for the case you describe, sign-extension.
> 
> The ideal solution might be if we could somehow give an addional
> parameter to slicing, e.g. asking for "signed" slicing instead
> of the default, unsigned. But I don't see how.

We actually do that in the DeFixedInt class

  http://www.dilloneng.com/documents/downloads/demodel/

Similar to intbv for an instance of that class a bit width can be 
specified. The msb based on the specified width is considered the sign 
bit. The decision is made whether the msb is included in the slice or 
not. To quote from the doc string of the __getitem__ function:

     When the slice includes the sign bit it is taken over to the return
     value. If the sign bit is excluded the bits are taken as is with the
     sign bit set to 0.

     For example using the 4-bit number -6 = b1010, slicing bits 3:1 -->
     b101 includes the sign bit, the result is -3.

     Now using the 4-bit number -3 = b1101, slicing bits 2:1 --> b10,
     however, the slice excludes the sign bit, hence the result is 2.


However, for the case of intbv I would not focus on where the bits come 
from, but rather where they should go to. So if the destination is 
supposed to be signed, then don't worry about where the bits come from. 
With a slice there is a fixed number of bits and depending on how this 
slice gets assigned to the destination should make the decision about 
the sign of the destination. So let's say the destination is a signed 
intbv with _nrbits set and the slice gets assigned to it. If that slice 
matches _nrbits and in the slice the msb is set, that should turn the 
value into a negative value.

> 
> Another interesting way might be to introduce methods to intbv's
> at this point. We could do this like for python strings: the
> methods don't modify the object in-place, but instead return a
> new object with the modified value. (In MyHDL tradition,
> those intbv methods would return integers.) The advantage
> of this approach (as opposed to creating new functions)
> is that adding new methods doesn't pollute the myhdl
> namespace further.

In a way the sign extension could be done through the __setitem__ 
function. Basically the _min, _max attributes decide about how the value 
  assigned is interpreted. If _min=0 the intbv instance is seen as 
unsigned. If _min = -_max the intbv instance is seen as signed and the 
assigned value is interpreted as signed with _nrbits specifying the msb 
and if that bit in the assigned value is set, the value is considered 
negative.

That would allow to assign bit slices that come actually from unsigned 
values and convert to a signed value.

I am probably overlooking something, but maybe some ideas are a starting 
point?

Guenter