Re: [myhdl-list] Bit-wise inversion issue

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

Ok, let's try to reach a decision.

I believe there are 2 solutions that can be defended.

Possible solutions
------------------

1) "Pure Python"
Bit inversion for intbv's behaves like for Python int's.

2) "Practical Hardware"
Make an exception for intbv's whose allowed values are
a finite set of positive values. Let's call these p-intbv's.
For a p-intbv, bits outside the bit range of an unsigned
representation, are assumed to remain zero upon bit-inversion.

Analysis
--------
Both solutions have advantages and drawbacks.

The major drawback of the "Pure Python" solution is that it
may be counter-intuitive to hardware designers. Many designs
mainly use p-intbvs, e.g. created by the form intbv(0)[n:].
But with this solution it would not be possible to represent
the bit-inversion of a p-intbv by another p-intbv. Negative
values would have to be introduced just for this purpose.

The major drawback of the "Practical Hardware" solution is
that the result of a bit-inversion would not just depend
on the intbv value, but also on its constraints. It is the
only operator for which this would be so. This is disturbing.

Conclusion
----------

The drawback of the "Pure Python" solution is probably fatal
for practical purposes. Therefore, I propose to keep the
"Practical Hardware" solution. This is in line with the
feedback from this thread thus far.

Jan

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