Add Two Long Numbers Together Without Causing an Overflow Error

' Add two numbers together, without overflow or signed bits
Function FullAdd(ByVal x As Long, ByVal y As Long) As Long
    Dim z As Long
    Dim z1 As Boolean, z2 As Boolean, c2 As Boolean
   
    ' First, add the two numbers without their top two bits (prevents overflow)
    z = (x And &H3FFFFFFF) + (y And &H3FFFFFFF)
    
    ' Perform a full addition on the 31st bit (carry is in z31)
    z1 = CBool(z And &H40000000) Xor (CBool(y And &H40000000) Xor _
   CBool(x And &H40000000))
    c2 = (CBool(y And &H40000000) And CBool(x And &H40000000)) Or _
   (CBool(z And &H40000000) _
   And CBool(x And &H40000000)) _
   Or (CBool(y And &H40000000) And CBool(z And &H40000000))

    ' Now perform a full addition for the 32nd bit (but we don't care about the _
   carry bit)
    z2 = c2 Xor (CBool(y And &H80000000) Xor CBool(x And &H80000000))

    ' If you wanted to test for an overflow you could test the carry bit here and _
   raise the appropriate error
    'If (CBool(y And &H80000000) And CBool(x And &H80000000)) Or _
   (c2 And CBool(x And
   &H80000000)) _
    '        Or (CBool(y And &H80000000) And c2) Then Err.Raise6 , , _
   "Overflow in FullAdd"


    ' Now we can chuck it all together
    FullAdd = (z And &H3FFFFFFF) Or (z1 And &H40000000) Or (z2 And &H80000000)

    
    ' nb. an alternative would have been to add the first and second 16-bit groups
    ' separately. Although neater, this would have need a 16-leftshift and 16-rightshift
    ' As shifts are not natively supported, this method is quicker

End Function