Architecture of the CPU: The purpose of the CPU The fetch-execute cycle

Welcome to this GCSE OCR Computer Science study breakdown on the Architecture of the CPU. In this video we will build a clear mental model of what the CPU is for, how it runs program instructions held in main memory, and how the fetch-decode-execute cycle repeats continuously while the computer is on. You will learn the precise roles of key registers such as the program counter, memory address register and memory data register, and you will see how the control unit and ALU work together. We will also tackle common mistakes, including confusing the program counter with the memory address register, and we will finish with exam-ready recap points. Let us map the core ideas. The CPU is the processor that runs program instructions held in main memory. Its purpose is to fetch each instruction, decode it, carry it out and store any result. The fetch-decode-execute cycle repeats continuously while the computer is on. In the fetch stage, the program counter holds the address of the next instruction. That address is copied to the memory address register, the instruction at that address is transferred to the memory data register, and the program counter is incremented ready for the next instruction. In decode, the control unit works out what the instruction means. In execute, the control unit sends control signals, the ALU may calculate, and results are written back to registers or memory. Turning to our core study notes, precision matters. The program counter, or PC, holds the address of the next instruction. During fetch, that address is copied to the memory address register, or MAR. The instruction at that address is then transferred to the memory data register, or MDR. The program counter is incremented during the fetch stage so that the next instruction is fetched in the following cycle. In the decode stage, the control unit decodes the instruction to work out what it means. In the execute stage, the control unit coordinates the components that carry out the instruction. Results may be written back to registers or main memory before the cycle repeats. Use these technical register names accurately in your exam answers. Now let us walk through the process as a continuous cycle. Step one is fetch. The program counter holds the address of the next instruction. That address is copied to the memory address register. The instruction at that address is transferred to the memory data register. The program counter is incremented ready for the next instruction. Step two is decode. The control unit works out what the instruction means. Step three is execute. The control unit sends control signals; the ALU may calculate, and results are written back to registers or memory. Then the cycle repeats. Remember, the fetch-decode-execute cycle repeats continuously while the computer is on, and it happens for every instruction. Link each stage to what happens next so your answer reads as a sequence rather than a list of parts. Let us work through an example. For an instruction such as ADD 5, first the CPU fetches the instruction. The address in the program counter is copied to the memory address register, the instruction at that address is transferred to the memory data register, and the program counter is incremented ready for the next instruction. Next, the control unit decodes the instruction to work out what it means. Then, in execute, the control unit sends control signals; the ALU may calculate, and results are written back to registers or memory. For ADD 5, this might mean loading a value, adding five and storing the answer. Notice how each stage links to the next, and notice that decode and execute remain separate stages. Watch out for common pitfalls. First, do not say the CPU stores all programs permanently. Programs are held in main memory or secondary storage, and the CPU only holds current instructions and data in registers and cache. Second, do not confuse the program counter with the memory address register. The program counter holds the address of the next instruction, while the memory address register holds the address currently being accessed. Third, do not believe the cycle happens once per program. The fetch-decode-execute cycle repeats for every instruction while the computer runs. Fourth, do not combine decode with execute. Decode is a separate stage that happens before execute. For exam success, use technical register names accurately, link each stage to what happens next, start purpose answers with processing instructions and data, then use the cycle as the explanation, and keep decode and execute separate. To recap: the CPU processes instructions and data by repeatedly carrying out the fetch-decode-execute cycle. In fetch, the program counter holds the address of the next instruction, which is copied to the memory address register and fetched into the memory data register, while the program counter is incremented. In decode, the control unit works out what the instruction means. In execute, the control unit coordinates the components that carry out the instruction, the ALU may calculate, and results may be written back to registers or main memory before the cycle repeats. Remember the cycle repeats for every instruction while the computer is on, keep decode and execute separate, and use the exact register names to secure your marks.