The CPU and the Fetch Decode Execute cycle - Answers
Q1. Students should draw lines to link the words with their description.
| Words | Description | |
| Decode. | This comes before 'Execute'. | |
| The CPU. | The brain of the computer. | |
| 1 Hertz. | 1 cycle every second. | |
| Accumulator. | You do all the calculations here. | |
| Hard disk. | The CPU doesn't work directly with this to run programs. | |
| Core. | A mini processor within the main processor. | |
| Busses. | These are used to send addresses and data between the CPU and RAM. | |
| Program Counter. | The place where you can find the address of the next instruction to fetch. | |
| Central | The C in CPU. | |
| RAM. | The place where you can find programs you are currently using. |
Q2. A series of steps how the CPU executes an instruction:
a) To 'run' a program, the CPU has to get the first instruction in the program.
b) It puts the address of the instruction it wants to fetch on the address bus.
c) The computer then fetches the instruction given by this address.
d) It fetches the instruction by putting it on the data bus.
e) The instruction is passed back to the CPU.
f) The CPU then 'decodes' the instruction.
g) What this simply means, is it decides what to do with the instruction.
h) Then, once it has decided what to do, it does it!
i) Carrying out an instruction is called 'executing' the instruction.
j) The CPU sends out signals to all of the other hardware components so that the instruction is executed.
Students need to have a clear idea of how instructions are stored and executed within a computer system so should be able to remember these simplifies steps.
Q3. The maximum number of instructions the CPU can work on at any one time is just one.
Q4. This type of processor design is known as a Von Neumann processor. Von Neumann was the person who came up with the design of this processor and the fact that you only need one RAM for both programs and the files you are using.
Q5. The unit used to measure 'cycles per second' is Hertz. This is used to measure lots of things that involve cycles per second e.g. 240 V ac 50 Hz.
Q6. At the end of a FETCH - DECODE - EXECUTE (FDE) cycle, the CPU checks the Program Counter and then does another FDE cycle.
Q7. The reasons why some processors can work faster than others: more cores, crystal works at a higher number of cycles per second, better quality materials. Note there are other things e.g. Word size but these are beyond KS3.
Extension tasks
a) Pipeline processors: In the conventional Von Neumann architecture, an instruction is fetched, then decoded and finally executed. This happens in sequence. It would be nice if an instruction could be fetched and then, while it is being decoded, another instruction is fetched, and while the decoded instruction is being executed, other instructions are being fetched and others still decoded - all at the same time. A processor that is designed to do just this is known as a 'pipeline processor'. As with parallel processing, programs need to be written to take full account of this capability.
b) Maths co-processor: This is a specialised processor that works in addition to the main processor. It has a design that can handle really large numbers. Whilst it is doing this, the main CPU is free to work on other things. Maths co-processors reduce the time that CPUs have to spend working on big Maths problems.