Home Â» Addition and Subtraction using 1’s Complement in Digital Electronics

# Addition and Subtraction using 1â€™s complement

In our previous section, we learned about different complements such as 1â€™s complement, 2â€™s complement, 9â€™s complement, and 10â€™s complement, etc.. In this section, we will learn to perform the arithmetic operations such as addition and subtraction using 1â€™s complement. We can perform addition and subtraction using 1â€™s, 2â€™s, 9â€™s, and 10â€™s complement.

## Addition using 1â€™s complement

There are three different cases possible when we add two binary numbers which are as follows:

Case 1: Addition of the positive number with a negative number when the positive number has a greater magnitude.

Initially, calculate the 1â€™s complement of the given negative number. Sum up with the given positive number. If we get the end-around carry 1, it gets added to the LSB.

Example: 1101 and -1001

1. First, find the 1â€™s complement of the negative number 1001. So, for finding 1â€™s complement, change all 0 to 1 and all 1 to 0. The 1â€™s complement of the number 1001 is 0110.
2. Now, add both the numbers, i.e., 1101 and 0110;
1101+0110=1 0011
3. By adding both numbers, we get the end-around carry 1. We add this end around carry to the LSB of 0011.
0011+1=0100

Case 2: Adding a positive value with a negative value in case the negative number has a higher magnitude.

Initially, calculate the 1â€™s complement of the negative value. Sum it with a positive number. In this case, we did not get the end-around carry. So, take the 1â€™s complement of the result to get the final result.

#### Note: The resultant is a negative value.

Example: 1101 and -1110

1. First find the 1â€™s complement of the negative number 1110. So, for finding 1â€™s complement, we change all 0 to 1, and all 1 to 0. 1â€™s complement of the number 1110 is 0001.
2. Now, add both the numbers, i.e., 1101 and 0001;
1101+0001= 1110
3. Now, find the 1â€™s complement of the result 1110 that is the final result. So, the 1â€™s complement of the result 1110 is 0001, and we add a negative sign before the number so that we can identify that it is a negative number.

Case 3: Addition of two negative numbers

In this case, first find the 1â€™s complement of both the negative numbers, and then we add both these complement numbers. In this case, we always get the end-around carry, which get added to the LSB, and for getting the final result, we take the 1â€™s complement of the result.

#### Note: The resultant is a negative value.

Example: -1101 and -1110 in five-bit register

1. Firstly find the 1â€™s complement of the negative numbers 01101 and 01110. So, for finding 1â€™s complement, we change all 0 to 1, and all 1 to 0. 1â€™s complement of the number 01110 is 10001, and 01101 is 10010.
2. Now, we add both the complement numbers, i.e., 10001 and 10010;
10001+10010= 1 00011
3. By adding both numbers, we get the end-around carry 1. We add this end-around carry to the LSB of 00011.
00011+1=00100
4. Now, find the 1â€™s complement of the result 00100 that is the final answer. So, the 1â€™s complement of the result 00100 is 110111, and add a negative sign before the number so that we can identify that it is a negative number.

## Subtraction using 1â€™s complement

These are the following steps to subtract two binary numbers using 1â€™s complement

• In the first step, find the 1â€™s complement of the subtrahend.
• Next, add the complement number with the minuend.
• If got a carry, add the carry to its LSB. Else take 1â€™s complement of the result which will be negative

#### Note: The subtrahend value always get subtracted from minuend.

Example 1: 10101 â€“ 00111

We take 1â€™s complement of subtrahend 00111, which comes out 11000. Now, sum them. So,

10101+11000 =1 01101.

In the above result, we get the carry bit 1, so add this to the LSB of a given result, i.e., 01101+1=01110, which is the answer.

Example 2: 10101 â€“ 10111

We take 1â€™s complement of subtrahend 10111, which comes out 01000. Now, add both of the numbers. So,

10101+01000 =11101.

In the above result, we didnâ€™t get the carry bit. So calculate the 1â€™s complement of the result, i.e., 00010, which is the negative number and the final answer.