Bitwise math/Rotate with Carry
Moving on now to rotate with carry. Rotate with carry occurs on a computer when a number is too large to fit into a single register. For example: On 32 bit systems the largest value an unsigned variable can hold is 2^32 - 1. An 8 bit system is being used just for sanity:
| 2^8 | - | 1 | = | 255 |
|---|
That would be the largest value that can be held. If wanting to hold 256, it would need two pieces of processor memory to do this so rotate with carry. There's a bit on the CPU called CF or carry flag. It is just a little bit, 1 or 0. So on an 8 bit system a 16 bit number would want to be rotated.
| 16 bit number | 1011 1000 1010 0011 |
|---|
Now, the registers only contain 8 bits, so:
- register 1 = 1011 1000
- register 2 = 1010 0011
To connect these during the shift operation rather the rotate operation, there is a small bit called the carry flag (CF) on a CPU, that will store the value as the bits are shifted between the two registers. It will take the first "1" from register 2 into the CF bit then move all of register2 1 bit to the left, and so on and so forth.
