CPU Registers and Computation
The CPU has direct access to registers.  If A and B are already in the registers then the CPU can perform the addition directly (via the Arithmetic Logic Unit) and store the output in one of the registers.   No access to memory is needed.  However, you may want to move your data A and B from memory or the stack into the registers and vice-versa.  These are separate operations.
Registers can be of different sizes 8 to 64 bits, this depends on your CPU architecture. On a x86-64 CPU registers are 64bit thus addition of two 64bit numbers is a single operation.