🧠 Recap on the CPU
In this part of the lesson we are going to explore how the CPU works.
Remember from last lesson we described how it is the brain of the computer
🐏 Primary Memory
You may also remember from last lesson how the CPU fetches and processes each instruction from the Primary Memory, one at a time.
🔄️ The Fetch Decode Execute Cycle
The CPU and Primary Memory work together in order to make sure each instruction gets processed properly.
We call the process through which this happens the Fetch - Decode - Execute Cycle
We will now explore each stage of this process...
🦴🐕 Fetching
A special part of the CPU called the Control Unit sends a signal to the primary memory, asking for the next instruction in the sequence
The instruction is then sent on a special communication line called a bus
Once the instruction is received the CPU gets ready for the next stage
🔎 Decoding
Once the instruction has been received by the CPU it needs to work out what to do with it...
This process is known as decoding and is carried out by the Control Unit (the same part of the CPU that asked for the instruction).
Decoding is important because without it the CPU wouldn't know exactly how it must carry out the instruction.
🪓 Execution
Once the Control Unit has decoded the instruction it can now be carried out by another part of the CPU called the Arithmetic Logic Unit
Carrying out an instruction is called execution (excuse the silly image below).
Execution involves either arithmetic (calculating numbers) or logic (comparing one value to another)
🔲 CPU Performance Factors
In this part of the lesson we will learn about some important factors around the performance of the CPU.
We will also consider how these factors affect the speed and efficiency of the Fetch-Decode-Execute cycle.
🕰️ Performance Factor - Clock Speed
Each instance of the CPU fetching, decoding and executing an instruction is called a cycle.
- There is a part of the CPU called the clock and this regulates how many cycles can occur each second.
- More cycles per second means a set of instructions will get carried out over a shorter time-scale.
- The clock speed is usually measured in Hertz
- Gigahertz (Ghz) meaning billions of times per second!
🚌 Performance Factor - Bus Width
As you learned earlier, instructions travel along the bus from the primary memory to the CPU.
The number of bits (1s and 0s) that can travel along the bus at any given time is called the bus width.
Even with a fast CPU, a computer with a narrow bus width would be slow, because only a smaller chunk of each instruction could be fetched each cycle.
🚄 Performance Factor - Primary Memory Speed
If a computer has a fast CPU with a wide bus, its performance can still be hindered by the speed of the primary memory.
Imagine having a super-charged processor that has to wait for instructions from an incredibly slow primary memory!
