Re: [myhdl-list] intbv addition

[Christopher L. F. <cf...@uc...>]

On Jun 6, 2009, at 4:32 PM, Christopher L. Felton wrote:

>
>> Dear Chris,
>>
>> Thank you for your reply. I am trying to add numbers in MyHDL, I  
>> tried
>> to add numbers modeled by 'intbv' in two stages; the first was simple
>> program with no 'next' command included, and the other with 'next'.
>> The first one worked as expected but the second one strangely gave
>> weird results, when the numbers are positive, but when they are
>> extended to negative ranges the interpreter complains about exceeding
>> the minimum range of 0.  Here is the code:
>>
>> from myhdl  import Signal, delay, always, intbv, Simulation, bin
>> from random import randrange
>> from math   import pow
>>
>> def AddNum(B):
>>
>>     #interval = delay(10)
>>     max_num = pow(2,B-1) - 1
>>     min_num = -1 * pow(2,B-1)
>>
>>     a = Signal(intbv(0)[B:])
>>     b = Signal(intbv(0)[B:])
>>     c = Signal(intbv(0)[B+1:])

I probably didn't explain very well.  Below is code that works.

from myhdl  import Signal, delay, always, intbv, Simulation, bin
from random import randrange
from math   import pow

def AddNum(B):

    max_num = pow(2,B-1) - 1
    min_num = -1 * pow(2,B-1)

    a = Signal(intbv(0, min_num, max_num))
    b = Signal(intbv(0, min_num, max_num))
    c = Signal(intbv(0, 2*min_num, 2*max_num))

    for i in range(10):

        a.next = randrange(min_num+1, max_num)
        b.next = randrange(min_num+1, max_num)
        c.next = a + b

        yield  delay(10)
        print "i: %s a: %s, b: %s sum = %s binary %s" %(i, a, b, c,  
bin(c, B))


def main():
    bitwidth = 9
    inst = AddNum(bitwidth)
    sim  = Simulation(inst)
    sim.run()


if __name__ == '__main__':
    main()

>>
> The error you are seeing is correct.  There is basically two modes  
> for the intbv.  Constraint integer (see Jan's essay for more info)  
> and the regular bit-vector.  When using the intbv for integer  
> representation, as in your case with negative and positive numbers,  
> you need to define the maximum and minimum.  Instead of defining the  
> number of bits you only need to define the expected max and min for  
> the design.
>
> Example
> a = Signal(intbv(0, min=min_num, max=max_num))
> b = Signal(intbv(0, min=min_num, max=max_num))
> c = Signal(intbv(0, min=min_num, max=max_num))
> This is a little different than what you maybe use to in Verilog or  
> VHDL but it can be very powerful.  When you define the bit width it  
> is expected that the intbv is used as a generic bit array (no  
> numeric value).  But the implicit numeric min is zero and the max is  
> the size of the bit vector.
>
> Hope that helps
>
>>     for i in range(10):
>>
>>         a.next = randrange(0, max_num)
>>         b.next = randrange(0, max_num)
>>         c.next = a + b
>>
>>         yield  delay(10)
>>         print "i: %s a: %s, b: %s sum = %s binary %s" %(i, a, b, c,  
>> bin(c, B))
>>
>>
>> def main():
>>     bitwidth = 9
>>     inst = AddNum(bitwidth)
>>     sim  = Simulation(inst)
>>     sim.run()
>>
>>
>> if __name__ == '__main__':
>>     main()
>>
>> As I said, when negative numbers are used, i.e.
>>
>>  a.next = randrange(min_num, max_num)
>>  b.next = randrange(min_num, max_num)
>>
>> Python complains. What do you think is the problem?
>>
>> On Sat, Jun 6, 2009 at 5:34 PM, Christopher L.
>> Felton<chr...@gm...> wrote:
>>
>>>> a=intbv(3)[2:]
>>>> b=intbv(1)[2:]
>>>> c=a+b
>>>>
>>>> the sum (c) is not 'intbv' class, its 'long' if I'm not mistaken.  
>>>> How
>>>> can we get the sum to be 'intbv' of the same bitwidth (in this case
>>>> 2)?
>>>>
>>>>
>>>>
>>> You would want to do the following (which is the same as other HDLs)
>>>
>>> a = intbv(3)[2:]
>>> b = intbv(1)[2:]
>>> c = intbv(1)[2:]
>>> c[:] = a+b
>>>
>>> As you noticed, the result of the intbv math operations returns an
>>> integer (long).  What you need to do is create an intbv type for the
>>> left hand side of the operation.  For the above example you will  
>>> get an
>>> error.  The intbv will check the limits of addition.  You will  
>>> need to
>>> create "c" to be large enough for the result of the addition.
>>>
>>> c = intbv(1)[3:]
>>>
>>>
>>> Hope that helps, for more background information see Jan's essay on
>>> constraint integer, http://www.jandecaluwe.com/hdldesign/counting.html 
>>> .
>>>
>>> Chris
>>>
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>>
>>
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> looking to deploy the next generation of Solaris that includes the  
> latest
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