Direct subtraction

The four possible subtractions in binary are:

1. 0 – 0 = 0
2. 1₂ – 0 = 1₂
3. 1₂ – 1₂ = 0
4. 10₂ – 1₂ = 1₂ ( borrow 1 from the next most significant digit to make 0 become 10₂,

hence 10₂ – 1₂ = 1₂)

Subraction using ones compliment

The main purpose of using ones compliment in computers is to perform binary subtraction. For example to get the difference in 5 – 3, using the ones compliment, we proceed as follows:

1. Rewrite the problem as 5 + (-3) to show that he computer binary subtraction by adding the binary equivalent of 5 to ones compliment of 3.
2. Convert the absolute value of 3 into 8-bits equivalent i.e. 00000011₂.
3. Take the ones compliment of 00000011₂e. 11111100₂ which is the binary representation of -3₁₀.
4. Add the binary equivalent of 5 to ones compliment of 3 i.e.

00000101

+ 11111000

(1)00000001