# How to do signed operations on intbv

**URL:** <https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419>\
**Category:** Support\
**Created:** [March 17, 2020, 11:39am UTC](https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419 "2020-03-17T11:39:26Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![saudalwasly](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/saudalwasly/32/130_2.png) [@saudalwasly](https://discourse.myhdl.org/u/saudalwasly)\
**Post date:** [March 17, 2020, 11:39am UTC](https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419/1 "2020-03-17T11:39:26Z")

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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

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<div class="post-metadata">

**Author:** ![josyb](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/josyb/32/5_2.png) [@josyb](https://discourse.myhdl.org/u/josyb)\
**Post date:** [March 17, 2020, 8:54pm UTC](https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419/2 "2020-03-17T20:54:58Z")

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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

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<div class="post-metadata">

**Author:** ![saudalwasly](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/saudalwasly/32/130_2.png) [@saudalwasly](https://discourse.myhdl.org/u/saudalwasly)\
**Post date:** [March 18, 2020, 5:17am UTC](https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419/3 "2020-03-18T05:17:04Z")

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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.

```auto
@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.

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<div class="post-metadata">

**Author:** ![josyb](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.myhdl.org/josyb/32/5_2.png) [@josyb](https://discourse.myhdl.org/u/josyb)\
**Post date:** [March 18, 2020, 4:44pm UTC](https://discourse.myhdl.org/t/how-to-do-signed-operations-on-intbv/419/4 "2020-03-18T16:44:18Z")

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> [@saudalwasly](#):
>
> `outX = intbv(0, min=-2 **31, max=2** 31-1)`

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