CPU Organization

The internal organization of part of a simple von Neumann CPU is shown  below.
This part is called the data path and consists of the registers, the ALU (Arithmetic Logic Unit), and several buses conecting the pieces.
The register feed into two ALU input registers, labeled A and B in the figure. These register hold the Alu input while the ALU is performing some computation.

The ALU itself perform addition,subtraction and other simple operation on its inputs, thus yielding a result in the output register. This output register can be stored back into a register. Not all design have the A, B and output register. In the example, addition is illustrated, but ALU can perform other operation.

Instructions can be divided into one of two categories :

# register - memory
allow memory words to be fetched into registers, where they can be used as ALU inputs in subsequent instructions.

# register - register
fetches two operands from the registers, brings them to the ALU input register, perform some operation on them and store result back in one of the registers.