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The Function of the CPU
[q]What is the CPU?
[a]The CPU (central processing unit) is a small computer component which is responsible for the processing of data and instructions.
[q]What are CPUs often known as?
[a]CPUs are often known as
‘the ‘Brain’ of the Computer’.
[q]What is the job of a CPU?
[a]The job of the CPU is to ‘Fetch’, ‘Decode’ and ‘Execute’ instructions.
In other words, ‘process data’.
[q]What examples are there of processing data?
[a]Processing means things like:
[q]When you open a program, where are its instructions and data loaded?
[a]When you open a program, all the data needed for the program to run is loaded onto the RAM.
[q]What happens during the ‘Fetch’ stage of the fetch-decode-execute (F-D-E) cycle?
[a]The CPU will fetch data/instructions from the RAM.
To do this it will call the address of the next item via the address bus.
The data/instructions are then sent back to the CPU via the data bus.
[q]What happens during the ‘Decode’ stage of the fetch-decode-execute cycle?
[a]The decode stage is where the CPU makes sense of the instructions and prepares for the next stage.
[q]What happens during the ‘Execute’ stage of the fetch-decode-execute cycle?
[a]The execute stage is where the data processing takes place.
Instructions are carried out on the data.
[q]What are the three main areas that together form the CPU architecture?
[a]The CPU consists of:
– Control Unit
– Immediate Access Store (cache)
– Arithmetic and Logic Unit
[q]Name 3 jobs that the control unit will carry out.
[a]1. Managing and monitoring hardware ensuring data goes to the correct hardware.
2. Managing and monitoring input / output signals, ensuring they are dealt with correctly.
3. Managing the Fetch – Decode -Execute cycle.
[q]What is the purpose of the IAS?
(Immediate Access Store)
[a]The RAM is far slower at delivering data than the ALU (arithmetic and logic unit) is at processing it. The IAS (cache) is far faster at delivering the data than the RAM. The IAS, therefore, will accept sets of instructions from the RAM so that it can supply the ALU at the same speed as the CPU can process. This means the CPU can work at top speed (not having to wait for deliveries of data from the RAM)
[q]What is the purpose of the ALU?
(Arithmetic & Logic Unit)
[a]The ALU is where the CPU actually carries out the maths and logic on the data (it processes it).
It has an arithmetic part (where it performs calculations on the data) and it has a logic part (where it deals with logical operators such as true/false/equal to etc)
[q]Describe the journey of an instruction as it makes its way to through the CPU.
[a]1. An input device (e.g. keyboard) sends data to the CPU. The Control Unit receives this data.
2. The Control Unit sends this data into main memory to be used later.
3. When the time is right, the data will be transferred from main memory into cache (IAS)
4. The data will then be sent to the ALU for processing
5. The control unit will send the processed data back (for example to an output device such as a screen or monitor).