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OCR GCSE Computer Science (J277) · Paper 1

🧠 Systems Architecture

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

The CPU & embedded systems

The CPU carries out all the processing in a computer by running the fetch–decode–execute cycle billions of times a second. This is possible because of the Von Neumann architecture, in which instructions and data share one memory, and the CPU pulls each one from it in turn using its registers.

CPUControl UnitALUcacheregistersPC MAR MDR ACCmemory (RAM)instructions + datafetchresultsVon Neumann: one memory feeds the CPU — fetch, decode, execute, repeat
ALUIt carries out arithmetic and logic operations, such as calculations and comparisons.
Control UnitIt decodes each instruction and coordinates the rest of the CPU as it runs.
CacheIt is a small amount of very fast memory built into the CPU, holding frequently used data so that the CPU makes fewer slow trips to RAM. It is expensive to produce, which is why there is only a small amount of it.
RegistersThe PC (program counter) holds the address of the next instruction. The MAR (memory address register) holds the address currently being accessed. The MDR (memory data register) holds the data that has just been fetched. The ACC (accumulator) holds the ALU's current working result.

In one full cycle, the PC's address is copied to the MAR, the memory returns the instruction into the MDR, and the PC increments ready for next time. The CU then decodes the instruction and it is executed: the ALU might calculate a result into the ACC, data might move, or a jump might overwrite the PC, which is how if-statements and loops exist at machine level.

Performance depends on clock speed (measured in GHz, the number of cycles per second), the number of cores (parallel workers, though doubling the number of cores only helps if the software can split the task between them), and the size of the cache (a bigger cache means fewer trips to RAM). Embedded systems — the computer inside a device such as a washing machine or a traffic light — trade flexibility for being cheap, low-power, reliable and dedicated to a single job.

🎯 Examiner tip: Fetch–decode–execute answers earn marks for every register you name in its correct role. Practise saying the cycle through in one breath: PC, then MAR, then MDR, then CU, then ALU or ACC.
Turn these notes into memory. 11 flashcards cover this topic. Short daily sessions, spaced repetition up to your exam date, quizzes with friends. Free during the beta.Practise Systems Architecture in the app ↗
Memory & Storage ›