The purpose and main components of the CPU (the ALU, the control unit and registers) and the von Neumann stored-program architecture.

A Unit 1 hardware question. The CPU (central processing unit) is the part of the computer that fetches, decodes and executes instructions, carrying out the processing that runs programs (1 mark). The arithmetic logic unit (ALU) performs all the arithmetic operations, such as addition and subtraction, and logic operations, such as comparisons and AND/OR decisions (1 mark, and 1 mark for both arithmetic and logic). The control unit (CU) manages and coordinates the operation of the processor: it directs the fetch-decode-execute cycle, sends control signals to the other components and tells them what to do and when (1 mark). Markers reward the processing role of the CPU and the distinct jobs of the ALU and CU. A common error is to swap the ALU and CU, or to say the CPU stores programs, which is the job of memory.

A Unit 1 explain question. In the von Neumann architecture both the program instructions and the data they work on are stored together in the same main memory (1 mark for shared memory). The CPU fetches instructions from memory one at a time and executes them in order (1 mark). Because instructions are stored just like data, the program can be loaded, changed or replaced easily, which is why it is called a stored-program computer (1 mark). Markers reward the idea of instructions and data sharing one memory and being fetched one at a time. A common error is to say instructions and data are in separate memories, which describes the Harvard architecture instead.