Binary Addition & Subtraction (OCR A Level Computer Science)
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Binary Addition
What is Binary Addition?
Binary addition involves summing numbers in base-2, which uses only the digits 0 and 1
Like denary addition, start from the rightmost digit and move towards the left
Carrying over occurs when the sum of a column is greater than 1, passing the excess to the next left column
Example addition
Binary addition example
Overflow Errors
Overflow occurs when the sum of two binary numbers exceeds the given number of bits
In signed number representations, the leftmost bit often serves as the sign bit; overflow can flip this, incorrectly changing the sign of the result
Overflow generally leads to incorrect or unpredictable results as the extra bits are truncated or wrapped around
An overflow occurring after a binary addition
Binary Subtraction
As well as adding binary numbers, we can also subtract binary numbers
One method of doing this is to use two's complement
Example 1
Subtract 1001 (9) from 0011 (3)
1. Given numbers
Number 1 | 1 | 0 | 0 | 1 |
Number 2 | 0 | 0 | 1 | 1 |
2. Two's complement
Convert the number to subtract (0011) to its two's complement
Invert bits to get 1100
Add 1 to get 1101
Number 1 | 1 | 0 | 0 | 1 |
Number 2 (Converted) | 1 | 1 | 0 | 1 |
3. Addition Operation
Now add 1001 and 1101
1001 + 1101 = (1) 0110 - including an overflow
Carry | 1 |
|
| 1 |
|
Number 1 |
| 1 | 0 | 0 | 1 |
Number 2 |
| 1 | 1 | 0 | 1 |
Addition | 1 | 0 | 1 | 1 | 0 |
4. Remove Overflow
The result is 10110 with overflow
Remove the overflow bit to get 0110
In two's complement arithmetic, the overflow bit does not contribute to the actual value of the operation but is more of a by-product of the method.
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