cannot find the reason for such a strange behavior

[richard]

Richards code below shows (I think) the ~operator working but not really, at least not that I can see, in the exact way you're using it

thanks for that henrik lets see if we can agree to the problem definition it's possible the badgers have moved the goalposts.

my assumption is
the elements of a byte array [of less than 256 bytes] are summed from the first element to the penulitmate element into a word var
the low byte of this word var is inverted and placed into the last element.
the allegation is that :-
{note .w0 is the word sum of the array}

array[z] = ~w0   ;  where  w0 ia a word and the [byte]array is of z+1 elements

can produce and incorrect result in that, when read back array[z] is always $fd
or that

IF I DO NOT INVERT THE BYTE VALUE THE CRC RECEIVED IS CORRECT

which i assume to be

array[z] = w0.lowbyte  ; not inverted

is correct where as

array[z] = w0.lowbyte
array[z] = ~array[z]  ; inverted

can produce and incorrect result in that, when read back array[z] is always $fd