From Compsci1
COMPLEMENT
We use complement to help store negative numbers in the computer. We use + and – to mark positive and negative numbers. However, computers can only read information in bits, 0s or 1s, so we use a new method to store negative numbers.
The 1’s complement (ones complement) of a binary numbers is found by changing all the 1s to 0s and all the 0s to 1s.
1001 0011
1s complement 0110 1100
The 2’s complement (twos complement) of a binary number is found by getting the ones complement first and then adding 1.
EG:
Binary number 1100 0101
1’s complement 0011 1010
+ 1
_______
2’s complement 0011 1011
Negative numbers are the 2’s complement of the corresponding positive number. To subtract a number, find the 2’s complement and add it.
Sign bit:
The left most bit of a binary number is the sign bit:
0 means the number is positive
1 means the number is negative.
Twos complement overflow flag:
If we add or subtract numbers and part of the answer overflows the available capacity of the storage space this can cause a problem. If we add 2 negative numbers in twos complement the answer can overflow beyond the capacity and give the impression of a positive number. To catch this problem the computer has the TWOS COMPLEMENT OVERFLOW FLAG which can be written into a program and checked after an operation to see if this has happened.
