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Summary The Structure of the CPU
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This document contains information of the details of the CPU of a computer which is needed for the OCR A-Level specification.
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Processor Components – The CPUThe CPU takes instructions from the memory and decodes them and then executes them millions of
times a second, it has multiple components which allows it to do this function
Control Unit:
Controls and coordinates CPU activities breaks instructions down into steps like getting addresses
and storing data and then executes these, then stores this data in memory/registers
Buses: overview
- A bus is a set of parallel wires connecting two or more components of the computer (comes
in sets of 8, 16, 32, or 64)
- There are 3 buses that connects the processor to the main memory: the Address bus, the
Data bus, and the Control bus (these 3 are all known as the System bus)
- The Address bus allows the CPU to request a particular part of the memory (the address is
sent along here)
- The Data bus allows the CPU to return the requested data from the memory back to the CPU
- The Control bus allows control signals to be sent
- Data and control signals along the two buses can be sent in both directions whereas the data
being sent along the address bus can only go one direction
Buses: The Control Bus
- As the data and address buses are shared by the computer, the control bus must provide
control to that access and share of data is conflicted by the various components
- The control bus transmits commands, timing and specific status information between the
system components
- Examples of control lines:
1. Clock: used to synchronise operations
2. Memory read: data found in the memory location is sent back to the CPU
3. Memory write: data on data bus is written to memory locations
4. Bus request / bus grant: device requests to use data bus and the CPU grants it
Buses: The Data Bus
- Provides a bi-directional line for moving data between the CPU and components
- 8, 16, 32, or 64 lines
Buses: The Address Bus
- The main memory is divided into Units called Words, each Word is a fixed group of digits (16,
32, or 64 bit)
- These units are handled by the processor (different processors have different word sizes)
- Use of these Words gives each unit its own specific memory address
- The address bus transmits the memory location of the Words that want to be used by the
CPU
- The Data bus then transmits the result of this operation back to the word and it is written
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