HxTrinity
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I am a bit confused on the Fetch - Decode - Execute Cycle and how it relates to clock speed

I know it's one of these answers but I have no idea which one.

Is it the first one

1) So Fetch is 1 tick, Decode is another tick of the clock and execute is another tick. This means that in one FDE Cycle, the clock ticks 3 times.

Example
Fetch
CLOCK TICK
Decode
CLOCK TICK
Execute
CLOCK TICK

OR

2) The Whole FDE Cycle is classified as one tick of the clock.
Example
Fetch
Decode
Execute
CLOCK TICK
Fetch
Decode
Execute
CLOCK TICK

I think it's the second one but I just wanna be sure. So one FDE Cycle is One tick of the Clock
Last edited by HxTrinity; 2 days ago
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AcseI
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So this is one of those silly questions, where the answer actually changes depending on how deep into the concept you go. I'd recommend checking out material for your course and asking your teacher to ensure you get the answer that the exam board wants.

The way things actually work is sort of a combination of two. Modern CPUs will execute different stages of the FDE cycle on instructions concurrently, in a process called pipelining. This is quite simplified, but imagine you have five instructions. Different parts of the CPU complete different parts of the cycle. So instruction 1 is fetched in the first clock cycle. Then instruction 1 is decoded and instruction 2 is fetched in the second clock cycle. Then instruction 1 is executed, instruction 2 is decoded and instruction 3 is fetched in the third clock cycle. And so on. This ensures that every part of the CPU is busy with something; without pipelining we wouldn't be able to fetch the next instruction until the current one has been executed.

So answer one is correct, in the sense that each stage (fetch, decode and execute) take one clock cycle (tick) and therefore the entire process takes three clock cycles. However answer two is also correct, because we want every stage of the FDE cycle to be completed every clock cycle but on different instructions. If this doesn't make sense, check out the Wikipedia article linked above and the diagram at the top should make it clear.

Hence why I recommend checking with your course leader and the material, because the answers here are two different simplifications of what is actually happening, and we have no idea what your exam board actually wants.
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HxTrinity
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(Original post by AcseI)
So this is one of those silly questions, where the answer actually changes depending on how deep into the concept you go. I'd recommend checking out material for your course and asking your teacher to ensure you get the answer that the exam board wants.

The way things actually work is sort of a combination of two. Modern CPUs will execute different stages of the FDE cycle on instructions concurrently, in a process called pipelining. This is quite simplified, but imagine you have five instructions. Different parts of the CPU complete different parts of the cycle. So instruction 1 is fetched in the first clock cycle. Then instruction 1 is decoded and instruction 2 is fetched in the second clock cycle. Then instruction 1 is executed, instruction 2 is decoded and instruction 3 is fetched in the third clock cycle. And so on. This ensures that every part of the CPU is busy with something; without pipelining we wouldn't be able to fetch the next instruction until the current one has been executed.

So answer one is correct, in the sense that each stage (fetch, decode and execute) take one clock cycle (tick) and therefore the entire process takes three clock cycles. However answer two is also correct, because we want every stage of the FDE cycle to be completed every clock cycle but on different instructions. If this doesn't make sense, check out the Wikipedia article linked above and the diagram at the top should make it clear.

Hence why I recommend checking with your course leader and the material, because the answers here are two different simplifications of what is actually happening, and we have no idea what your exam board actually wants.
Yeah, what would be the GCSE Level Response to this. Not going so deep into Computer Science Yet but I will definitely look into it.
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AcseI
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(Original post by HxTrinity)
Yeah, what would be the GCSE Level Response to this. Not going so deep into Computer Science Yet but I will definitely look into it.
I can't answer that, because either one could be correct. They're both overly simplified answers from different points of view. You'll need to refer to your GCSE material or ask your teachers
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