MyHDL

How to do signed operations on intbv

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#1

I am trying to implement a simple ALU with add and subtract operations. I got an error once the resulted value is less than 0!

Can anyone provide me with an example of how to do it correctly in MyHDL?
Thank you in advance

#2

I guess you have declared your accumulator as Signal(intbv(0)[8:])
This implies that only positive values can be assigned to this signal.
In order to allow for negative values you need to declare it this way: Signal(intbv(0, min=-128, max=128))
This may not the effective solution for an 8-bit accumulator as you probably also want to accommodate 8-bit unsigned values. So it is better to resolve this by using unsigned two’s arithmetic and declaring the accumulator as Signal(modbv(0)[8:]) and work with this.
I hope this sets you on your way.

Regards,
Josy

#3

Here is my code. It seems that the intbv is okay, but when the value is assigned to the Signal of type intbv the problem is araised.

@block
def ALU(rs1, rs2, op, out):
  outX = intbv(0, min=-2**31, max=2**31-1)
  print(outX.min, len(outX))

  @always_comb
  def logic():
    if op == 0b0000:
      outX = rs1 + rs2
    elif op == 0b0001:
      outX = rs1.val - rs2.val
    else:
      outX = 0b0
    
    out.next = outX   #The problem arises in this line
  return logic

@block
def Stimulus():
  clk    = Signal(bool(0))
  rs1    = Signal(intbv(0, min=-2**31, max=(2**31)-1)[32:])
  rs2    = Signal(intbv(0, min=-2**31, max=(2**31)-1)[32:])
  op     = Signal(intbv(0)[4:])
  Result = Signal(intbv(0, min=-2**31, max=(2**31)-1)[32:])

  ALU_1 = ALU(rs1, rs2, op, Result)

  @always(delay(10))
  def clk_driver():
    rs1.next = rs1 + 1
    rs2.next = rs2 + 10
    print('Results = %d'% Result)
    op.next = 0b1

  return instances()
if __name__ == '__main__':
    inst = Stimulus()
    inst.run_sim(300)

Thank you again for helping out.

#4

you instantiate it as: outX = intbv(0, min=-2**31, max=2**31)
The max value excludes the upper value