One's Complement Math

General
1

How todo 8 bit one’s complement addition with carry added back to the result?

2

Not sure exactly what you’re trying to do. Could you provide an example?

3

not sure if this is right as its been 40+ years since I cared about 1’s or 2’s complement values

Private Function one_comp(x as int8, y as int8) as int16
  dim z as int16
  if x<0 then x=not x
  if y<0 then y=not y
  z=x+y
  if z>&HFF then z=z+1 // carry
  z=z and &HFF
  return z
  
End Function
4

Thanks Dave.
I will give it a try!

Robert: I have this checksum calculation i need this kind of math in.
My original function works in 99 % of the messages, great to see a different approach.

5

// return the "one's complement sum" of n 8 bit values Function OnesCompSum(paramArray uInts As UInt8) As UInt8 Dim n As UInt16 = 0 For Each x As UInt8 in uInts n = n + x // unsigned sum If n>&h0FF Then n = (n+1) And &h0FF // Add carry back and keep it 8 bit Next n = (not n) And &h0FF // one's complement is done just after the final sum Return n End Function

6

Thanks Rick, even better! Will fit right in my checksum calculation routine.