Systems architecture

What you need to know

• ✓The CPU contains the Arithmetic Logic Unit (ALU), the Control Unit (CU), the Clock, Registers, and Buses.
• ✓The ALU carries out arithmetic operations (such as addition and subtraction) and logical decisions (such as comparing numbers using AND, OR, NOT).
• ✓The Control Unit (CU) coordinates CPU operations, manages the execution of instructions, and sends control signals to other components.
• ✓The Clock generates continuous electrical pulses at a fixed frequency to synchronise all CPU operations.
• ✓Registers are tiny, extremely fast temporary storage locations located directly inside the processor.
• ✓Buses are sets of parallel wires that carry signals between the CPU and memory (address bus, data bus, control bus).
• ✓The Fetch-Execute cycle repeats continuously: Fetch (retrieve next instruction from main memory/RAM), Decode (CU works out what the instruction means), Execute (ALU or CU carries out the action).
• ✓CPU performance is affected by Clock Speed (more cycles per second = more instructions processed), Number of Cores (multiple processors running simultaneously), and Cache Size (larger cache reduces delays waiting for data from slower RAM).

What you don’t need

• Names and roles of specific registers (MAR, MDR, PC, Accumulator) — the AQA 8525 specification explicitly states: 'Knowledge of specific registers is not required' (this is required by OCR J277, but NOT AQA).
• The term 'Von Neumann architecture' — not named in the AQA 8525 specification.
• Pipelining and parallel processing architectures beyond basic multiple cores.

Worked examples

1. Describe what happens during the Fetch stage of the Fetch-Execute cycle. (2 marks)

   Answer: The CPU sends a memory address along the address bus to main memory (RAM). The instruction held at that address is transferred back to the CPU along the data bus and copied into a CPU register.

2. Explain why increasing the cache size of a CPU can improve computer performance. (2 marks)

   Answer: Cache memory is located directly on or near the CPU and is much faster to access than main memory (RAM). Increasing cache size allows more frequently used instructions and data to be stored close to the processor, reducing the time the CPU spends waiting for data from RAM.

Common mistakes

• ✕Memorising MAR, MDR, PC, and Accumulator for AQA 8525: These specific registers are in OCR's specification, but AQA 8525 does not test specific register names.
• ✕Assuming doubling the number of cores automatically doubles performance: Software must be written to take advantage of parallel processing, and tasks often depend on previous results.
• ✕Confusing clock speed with internet connection speed or bus speed.
• ✕Thinking the CPU stores programs permanently: The CPU only holds instructions temporarily while actively processing them; permanent storage is secondary storage (SSD/HDD).