The diagram shows a simplified view of a central processing unit (CPU). (a) State the purpose of the ALU. [2] (b) State the purpose of the Control Unit. [2]...

(a) Purpose of the ALU [2]

The ALU (Arithmetic Logic Unit) performs arithmetic operations such as addition, subtraction, multiplication, and division on numerical data. [1]

It also performs logical operations such as comparisons (AND, OR, NOT) and determining whether values are equal, greater than, or less than each other. These logical comparisons are essential for decision-making in programs (e.g. IF statements). [1]

(b) Purpose of the Control Unit [2]

The Control Unit (CU) controls and coordinates the activities of all other components within the CPU and the wider computer system. [1]

It fetches instructions from memory, decodes them to determine what operation is required, and sends the appropriate control signals to other components (ALU, registers, memory, I/O devices) to execute each instruction in the correct sequence. [1]

(c) Register purposes [4]

(i) Program Counter (PC): Holds the memory address of the next instruction to be fetched and executed. After each fetch, it is incremented (usually by 1) so it points to the following instruction. [1]

(ii) Memory Address Register (MAR): Holds the address of the memory location that is about to be read from or written to. This address is placed on the address bus. [1]

(iii) Memory Data Register (MDR): Holds the data that has been fetched from memory (during a read operation) or the data that is about to be written to memory (during a write operation). It acts as a buffer between the CPU and main memory. [1]

(iv) Accumulator (ACC): Holds the results of calculations and logical operations performed by the ALU. It is the primary working register for arithmetic and serves as a temporary store for intermediate results. [1]