Practise CPU components and the fetch–decode–execute cycle. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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State the purpose of the clock in a CPU.
It produces regular pulses that keep the CPU's operations in step.
Explain why a CPU with a higher clock speed can usually run programs faster.
More cycles per second means more instructions are processed per second.
Give one item, other than the address of the next instruction, that is held in a register during the fetch–decode–execute cycle.
The instruction currently being executed (or the data being processed).
A CPU has a 16-bit address bus. Each memory address holds 1 byte. State the number of memory locations the CPU can address.
65,536
State the component of the CPU that decodes each instruction.
control unit
Sample questions
Written for this site in the style of Edexcel exam questions. They are not taken from real past papers.
Question 1Easy3 marks
(a) State the purpose of the clock in a CPU.[1]
(b) Identify the bus that carries memory addresses from the CPU to main memory.[1]
Address bus
Control bus
Data bus
Expansion bus
(c) State the unit in which clock speed is measured.[1]
Show the answer and mark scheme
(a)Answer: It produces regular pulses that keep the CPU's operations in step.
it sends out regular electrical pulses that synchronise / time the operations of the CPU's components
(b)Answer: Address bus
(c)Answer: Hertz (usually GHz)
hertz (Hz) / gigahertz (GHz)
Question 2Medium4 marks
Describe what happens during the fetch stage of the fetch–decode–execute cycle.[4]
Show the answer and mark scheme
Answer: The PC's address goes on the address bus, a read signal is sent, the instruction returns on the data bus into a register, and the PC is incremented.
the address of the next instruction (held in the program counter) is placed on the address bus
the control unit sends a read signal along the control bus
the instruction stored at that address is sent from main memory to the CPU along the data bus
the instruction is stored in a register in the CPU (e.g. the current instruction register)
the program counter is increased so that it holds the address of the next instruction
Question 3Hard4 marks
The next instruction to be processed is: add the value stored in memory location 21 to the value in the accumulator. The accumulator is a register in the CPU.
Describe what happens during the decode and execute stages when this instruction is processed.[4]
Show the answer and mark scheme
Answer: The CU decodes it; address 21 goes on the address bus with a read signal; the value returns on the data bus; the ALU adds it to the accumulator, which stores the result.
the control unit decodes the instruction to work out the operation (add) and the address (21) that it uses
the address 21 is sent along the address bus
a read signal is sent along the control bus
the value stored in location 21 is sent to the CPU along the data bus
the ALU adds this value to the value in the accumulator