Re: [myhdl-list] fixed-point thoughts : part two

[Christopher F. <chr...@gm...>]

On 9/5/13 12:05 AM, Angel Ezquerra wrote:
<snip>
>>
>> The proposed *resized* function will look like:
>>
>>       resize(val,format
>>               [,round_mode='convergent']
>>               [,overflow_mode='saturate'])
>>
>> Example:
>>
>>       z[:] = resize(x+y,z)
>
> Probably a basic question, but why do you need [:] here? Doesn't
> resize return a fixbv?
>

The *resize* function will return an /int/ the same
as the /intbv/ operations and the /fixbv/ operations.
The "[:]" is used because we really want to update the
value of /z/ and not have /z/ reference a different object.

This needs to be done: first, so it can be converted.  For
modeling only, we could be a little more flexible but then ...

Second, the bound checking would not be possible.

Typically, what you would see is:

     x = Signal(fixbv(0, min=-1, max=1, res=.125))
     y = Signal(fixbv(.25, min=-1, max=1, res=.125))
     z = Signal(fixbv(0, min=-1, max=1, res=.125))

     @always_seq(clock.posedge, reset=reset)
     def logic():
         z.next = resize(x+y,z)

As an object the /fixbv/ would work like the /intbv/.

> Also, I don't think there is a way for Python to pass the resize
> function z's size while using the assignment syntax. If there was
> resize() would not need the second parameter, which would be cool.
> Maybe the converter could know though... An alternative could be to
> add a "set" or "assign" method to the fixbv class. This method would
> be equivalent to a resize followed by an assignment? The result would
> be:
>
>      z.assign(x+y)
>
> Which would get rid of the need to use [:] (if there is any) and to
> use z both as a parameter to resize and as its output.
>

As mentioned, for /intbv/ and I would imagine with the /fixbv/
the bit assignment would not be used frequently, it commonly
would be handled by Signal assignments.

If you are using the /intbv/ and /fixbv/ for some modeling or
nonlocal variable, and a property/attribute is more Pythonic
than "[:]" we could propose a *val* public property that updates
/_val/ attribute and calls the check bounds.

     z.val = resize(x+y)

This would be consistent with the /Signal/.  If this change
was desired it would be made to the /intbv/ and the /fixbv/,
for now "[:]" will be used in the /fixbv/.

Regards,
Chris