Hardware · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 7% of your exam marks.
The fetch-decode-execute cycle and von Neumann architecture are tested almost every year.
The Central Processing Unit (CPU) is the part of a computer that runs every program. It is the chip that takes a stream of binary instructions and acts on them, one at a time, billions of times per second.
The job of the CPU is to fetch, decode and execute instructions. Everything else a computer does (showing text on a screen, playing audio, running a game, opening a web page) is built from these three steps.
The CPU sits between the input devices (keyboard, mouse, sensors) that feed it data, and the output devices (monitor, speakers, printer) that show the results. Memory (RAM) and storage (SSD or hard drive) hold the data and instructions the CPU works on.
These two terms get confused often:
A modern laptop has one microprocessor chip containing several CPU cores plus a lot of supporting hardware. For exam purposes, treat "CPU" as the logical processing unit and "microprocessor" as the physical chip.
A CPU's instruction set is the complete set of all the commands (operations) that the can execute, expressed in machine code.
Every program a CPU runs is ultimately built from commands chosen out of its instruction set. A CPU can only carry out the operations its instruction set defines, so two CPUs with different instruction sets cannot run the same machine-code program directly.
An embedded system is a computer system built into a larger device to perform one dedicated function, rather than the many general-purpose tasks of a PC or laptop.
The embedded computer is part of the product and usually cannot run arbitrary new software. Everyday examples include the controller in a washing machine, a car's engine-management system, the system inside a pacemaker, and the controllers in lighting systems, security systems and vending machines. This is different from a general-purpose computer (a PC or laptop), which is built to perform many different functions.
State the purpose of a CPU
What comes up: a 1-mark "state what the CPU does / the purpose of a core".
Watch out: name the fetch-decode-execute cycle (or say it "executes instructions"). Vague answers like "runs the computer" or "processes data" — with no mention of instructions or the FDE cycle — may not score.