What is the purpose of the CPU?
The CPU (Central Processing Unit) fetches, decodes, and executes instructions stored in memory. It is the “brain” of the computer.
What does CPU stand for?
Central Processing Unit
Name the three stages of the instruction cycle (in OCR spec)
Fetch → Decode → Execute
What happens during the fetch stage?
The address of the next instruction is placed in the MAR; that address is used to fetch the instruction from memory into the MDR; the Program Counter is incremented.
What happens during the decode stage?
The Control Unit decodes the instruction, understanding what action it must perform, and prepares the necessary control signals.
What happens during the execute stage?
The instruction is carried out: data may be fetched, calculations done by the ALU, results stored, etc.
What is the ALU and what does it do?
The ALU is the Arithmetic Logic Unit. It performs arithmetic operations (e.g. addition, subtraction) and logic operations (e.g. comparisons).
What is the Control Unit (CU) and what is its role?
The Control Unit manages and coordinates the CPU’s operations, issues control signals, decodes instructions, and orchestrates the fetch-decode-execute cycle.
What is cache in a CPU?
Cache is a small, very fast memory inside the CPU used to store frequently accessed instructions or data so the CPU can access them quickly.
What are registers?
Registers are very small storage locations inside the CPU used to hold data, instructions, addresses temporarily during processing.
Name the four registers in Von Neumann architecture required by OCR
MAR, MDR, Program Counter (PC), Accumulator (ACC)
What is the MAR (Memory Address Register)?
MAR holds the memory address of the location to fetch data from or to write data to.
What is the MDR (Memory Data Register)?
MDR holds the data or instruction that has been fetched from memory, or is about to be written to memory.
What is the Program Counter (PC)?
The PC holds the address of the next instruction to be fetched. After fetching, it is typically incremented to point to the next instruction.
What is the Accumulator (ACC)?
The Accumulator holds intermediate results of calculations by the ALU.
What is “Von Neumann architecture”?
A design in which data and program instructions share the same memory and buses. The CPU follows the fetch-decode-execute cycle using shared memory.
Why is the distinction between instructions and data important in Von Neumann architecture?
Because instructions tell the CPU what to do (opcode) and data is what is acted on (operand). They are stored in the same memory, so the architecture must distinguish them at run time.
Name three characteristics of a CPU that affect its performance
Clock speed, cache size, number of cores
How does clock speed affect performance?
Higher clock speed means more fetch-decode-execute cycles per second, so instructions are processed faster. However, too high a speed may cause overheating or instability.
How does increasing cache size impact performance?
More cache allows more data/instructions to be stored nearer the CPU, reducing the time to fetch from RAM, thus speeding up processing.
How does having more cores influence performance?
Multiple cores enable parallel processing: more than one instruction stream can run simultaneously if software is designed to use them.
What is an embedded system?
A computer system built into a larger device for a dedicated function (not a general-purpose computer).
Give three characteristics of embedded systems
Often small, dedicated, efficient, built for reliability, low cost, specific task, not easily reprogrammed.
Give examples of embedded systems
Washing machines, microwaves, digital cameras, car engine control units, thermostats etc.
