CPU & the Fetch-Execute Cycle Flashcards
All 17 cards in this deck
What is the purpose of the CPU?
To fetch, decode and execute program instructions.
True or false? The Fetch-Execute cycle repeats continuously while the computer is running.
True. As soon as one instruction has been executed, the next one is fetched. The cycle only stops when the computer is switched off.
What is the function of the Arithmetic Logic Unit (ALU)?
It performs all arithmetic and logical operations on data, e.g. addition, subtraction and comparisons.
What is the function of the Control Unit (CU)?
It decodes instructions and coordinates their execution, sending control signals to the other components to control the flow of data and instructions.
What are registers?
Small amounts of extremely fast memory inside the CPU that hold the data, instructions and memory addresses being used while an instruction is executed.
What is a bus in a computer system?
A collection of wires through which data and signals are transmitted from one component to another, e.g. between the CPU and main memory.
What effect does increasing the clock speed have on CPU performance?
More Fetch-Execute cycles are carried out per second, so more instructions are processed in a given time.
What effect does a larger cache have on CPU performance?
More frequently used data and instructions are held close to the processor, so it spends less time waiting for them to be fetched, improving speed.
What is a CPU core, and what effect does having more cores have on performance?
A core is a processing unit that can fetch, decode and execute its own instructions; more cores allow more instructions to be processed simultaneously.
What are the steps to decide which of two CPU specifications will perform better for a stated purpose?
e.g. CPU A: 4 cores, 2.0 GHz; CPU B: 2 cores, 3.2 GHz — for video editing software that uses all cores- Compare each characteristic: A has more cores, B has the higher clock speed.
- Match them to the task: video editing splits work across cores.
- Choose and justify: CPU A, as more instructions are processed simultaneously.
True or false? The CPU with the higher clock speed will always be the better performer.
False. Cache size and number of cores also affect performance, so a slower CPU with more cores or a larger cache may perform better.
What happens in the fetch stage of the Fetch-Execute cycle?
The next instruction is fetched (copied) from main memory into the CPU.
What happens in the decode stage of the Fetch-Execute cycle?
The control unit interprets the fetched instruction to work out what it means and what needs to happen to carry it out.
What happens in the execute stage of the Fetch-Execute cycle?
The CPU carries out the decoded instruction, e.g. the ALU does a calculation or comparison, or data is read from or written to main memory.
What is the role of the clock in the CPU?
It sends out a regular electrical pulse (timing signal) that synchronises the operations of the CPU's components.
What is clock speed, and what unit is it measured in?
The number of clock pulses (cycles) per second, measured in hertz (Hz), usually gigahertz (GHz): billions of cycles per second.
Why might a CPU with twice as many cores not run a program twice as fast?
Extra cores only help if the program is written to share its work between them. Even then, some instructions need the results of earlier ones, so they cannot run at the same time.