CPU parts: ALU, CU, 5 registers, 3 buses
A microprocessor (an integrated circuit on one chip) is the CPU, which processes instructions and data. In Von Neumann architecture instructions and data share the same memory and buses. Two units: the ALU (calculations and logic) and the CU (decodes instructions, sends control signals). Five registers — PC (next-instruction address), MAR, MDR, CIR, ACC (ALU result). Three buses carry addresses, data and control signals.
The fetch-decode-execute (FDE) cycle
The fetch-decode-execute (FDE) cycle repeats to run a program; name the register at each step. Fetch: the PC address is copied to the MAR; the instruction is fetched along the data bus into the MDR, copied to the CIR, and the PC is incremented. Decode: the CU decodes the instruction in the CIR. Execute: it is carried out — e.g. the ALU calculates and stores the result in the ACC. Name the specific register at each stage to earn the marks.
CPU performance, instruction set, embedded
Clock speed — FDE cycles per second in hertz (/s); higher = more instructions/second. Cores — independent units; more cores run in parallel. Cache — small fast memory near the CPU for frequently used data; larger = fewer slow RAM fetches. Instruction set — all machine-code commands the CPU can run. Embedded system — a dedicated computer inside a device performing one function; microprocessor, low power, not user-reprogrammable.
Drawn from real examiner reports.
Clock speed = FDE cycles per second
"The speed at which the CPU runs" is circular and scores nothing. The mark-scheme form: the number of fetch-decode-execute cycles per second, in hertz (e.g. 3 GHz = 3 billion cycles/s). Asked why a higher clock speed helps, give the mechanism — more cycles per second = more instructions executed — not just "it is faster".
November 2023 Paper 2: "Few candidates were able to accurately describe what is meant by the clock speed. Many candidates gave a [vague] description such as it is the speed at which the CPU runs."
ALU and CU are units, not registers
The ALU and CU are units, not registers. The five registers are PC, MAR, MDR, CIR, ACC. Writing "arithmetic logic unit" in a list of registers loses the mark — the ALU performs calculations and logic and the CU decodes instructions, but neither stores a single addressed value the way a register does.
November 2023 Paper 2: in the register question "the most common incorrect answer was arithmetic logic unit".
Say which value each register holds
Saying only what a register "stores" is half an answer — state which value. The MAR holds the address being read/written; the MDR holds the data or instruction at that address; the PC holds the address of the next instruction; the CIR holds the current instruction; the ACC holds the result of an ALU calculation.
November 2023 Paper 2: register descriptions such as "the MDR stores data" or "the MAR stores addresses" lacked the "further detail" needed for full marks.
Instruction set ≠ mnemonics
The complete list of commands a CPU can run is the instruction set. Answering "mnemonics" is wrong — mnemonics (e.g. ADD, LDA) are the short codes used inside an assembly-language instruction, not the name of the whole set. The instruction set is hardware-specific: a program must use a CPU's own instruction set to run on it.
November 2023 Paper 2: candidates named the command list "mnemonics" instead of the instruction set.
Cache is not the same as RAM
Cache and RAM differ. Cache is a small amount of very fast memory inside/near the CPU holding frequently used data, cutting slow fetches from main memory. RAM is the larger, slower main memory holding the programs and data currently in use. Calling cache "the computer's memory" loses the mark — say it is small, fast and near the CPU.
Generic microprocessor/embedded answers
Vague answers about "data sent to the microprocessor and converted to binary" do not describe its role — it processes instructions and data to control the device. For an embedded system in a scenario, avoid a generic "it uses sensors and a microprocessor" answer: name the actual input (sensor), the decision made, and the output (actuator) for that specific device.
November 2023 Paper 2: thin descriptions of the microprocessor role. June 2023 Paper 2: examiners noted a "generic description of a system that uses sensors and microprocessors" rather than the specific context.
Walk the FDE cycle register-by-register
For any FDE question, name the register at every step in order: PC → MAR → fetch → MDR → CIR → PC incremented → CU decodes → ALU → ACC. Each named register/step is typically a mark, so "it fetches then runs the instruction" throws marks away.
Performance: Cause → Mechanism → Effect
Use a Cause → Mechanism → Effect chain: higher clock speed → more cycles/second → more instructions/second; more cores → each runs its FDE cycle → more work in parallel; larger cache → more data near the CPU → fewer slow RAM fetches.
Define-the-term: two elements + units
For "define / state what is meant by", give a two-element answer — what it is + what it does / key property (e.g. cache = small, fast memory near the CPU + holds frequently used data). State units where they exist (clock speed in Hz/GHz).
The central processing unit (CPU) is the part of the computer that processes the instructions and data that are input, so the result can be output. A microprocessor is an integrated circuit on a single chip that performs the role of the CPU. This topic covers what is inside the CPU, how it runs a program (the fetch-decode-execute cycle), what makes one CPU faster than another, the instruction set, and embedded systems.
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