Re: [myhdl-list] essay about integer arithmetic

[Pieter <pie...@gm...>]

That's funny, only yesterday I saw someone write this:

some_signal <= '0'
if ((a-b) = 1 or (b-a) = 1) then
  some_signal <= '1';
end if;

The arithmetic rules in Verilog are confusing and just wrong. The
rules in VHDL are hard to grasp for students, but there's some logic
behind it. If only the result of an operation would be as wide as the
target, that would solve many issues. It doesn't feel right to change
the size of the operand, to get the right size for the result. (and of
course you should be able to add a signed and an unsigned operand
without having to cast the unsigned to signed + adding a zero to the
front)

@ Michael Baxter
I don't have a lot experience in hardware design. For me this doesn't
feel like automating bit-widths. You still define max and min values
for the intbev, so you declare the bit width explicitly. Isn't it
about the arithmetic rules that are counter-intuitive? I can't see
there's anything wrong with those of MyHDL, they are intuitive and
create the hardware you want. Can you explain why you believe this
automates bit widths and this should never be done?

kind regards,

pieter

2009/3/6 Michael Baxter <ma...@cr...>:
> IMHO, though informed with years of hardware implementation experience,
> automating bit-widths should be 100% a non-goal. It should never be done under
> any circumstances whatsoever.
>
> IMHO, this is a serious failing of MyHDL.
>
> Best,
> Michael
> California
>
> On Fri, 06 Mar 2009 07:21:50 -0500, Neal Becker wrote
>> Jan Decaluwe wrote:
>>
>> > This is an essay that I wanted to write for a long time.
>> > It describes what I think is wrong with integer arithmetic
>> > in VHDL and Verilog, and why MyHDL provides a solution.
>> >
>> > Before releasing it to the general public, I'm interested
>> > to hear what you think about it.
>> >
>> >     http://www.jandecaluwe.com/hdldesign/counting.html
>> >
>> >
>> >
>>
>> If you really want to automate the bit widths, perhaps some kind of
>> interval arithmetic is wanted?
>>
>> python mpmath and pyinterval both supply some interval arithmetic,
>> but these are over reals, not integers.
>>
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