The checksum is a byte.
If you XOR a number with any other number, there can never be an overflow.

The XOR operation tests each bit of the two bytes operated on.
If the first two bits of each value are the same, the first bit of the new byte is 0.
If the first two bits of each value are different, the first bit of the new number is 1.

0000111 XOR
1010101 =

1010010