Inside the CPU
CPUs are very complex, with lots of specialised components. You only need to know…
- Program Counter (PC)
- Control Unit (CU)
- Cache
- Four registers (we'll look at those on the next page)
Revision Tip
Learn the full names of components, not the abbreviations. They're named after what they do and this will help you to remember their purpose.
Data Flow Through the CPU
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Input
Data and instructions are fetched from RAM to be processed by the CPU.
This is the fetch part of the fetch-decode-execute cycle.
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Control Unit (CU)
Controls the flow of data throughout hardware.
Ensures data goes to the correct place.
Handles the decode part of the fetch-decode-execute cycle.
Registers
Stores the data and instructions currently being used by the CPU.
Each register temporarily stores one instruction or memory address
Think of each register as being like a tiny drawer in a desk. They don't hold much, but we can access them really quickly!
Arithmetic Logic Unit (ALU)
Does the arithmetic (maths) and logic (comparisons) on the data.
Handles the execute part of the fetch-decode-execute cycle.
Output
The results of the processing are returned back to RAM.
They may then be transferred to secondary storage (e.g. a hard disk) or to an output device (e.g. a monitor) so the user can see the results of processing.
When complete, the fetch-decode-execute cycle begins again.
CPU Cache
CPU cache is an area of super-fast memory on the CPU.
- Frequently used instructions are kept in the cache — it’s quicker than getting them from RAM
- Cache memory is small and very expensive
- CPU cache comes in three levels: Level 1 is the fastest but holds the least; Level 3 holds the most but is the slowest. Level 2 is somewhere between the two.
CPU cache can improve the performace of the CPU.
Misconception
Do not confuse CPU cache with web browser cache. CPU cache only stores data for the CPU. Browser cache stores frequently accessed web pages on the computer's secondary storage. It is not part of the CPU.