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04 - Fetch Decode Execute Cycle

 1. Ever wondered how your brain is involved in executing an instruction? For instance, if someone says "Pass the salt"....what happens?
Voxel-man-brain.jpg

  The instruction is executed immediately

  The instruction is first fetched (received), decoded (made sense of) and then executed.

  The instruction is decoded, and then it is fetched

  The instruction is executed and then it is decoded and fetched

 2. The fetch-decode-execute cycle is the sequence of steps that the CPU follows to process instructions. This is also sometimes called the ...

  instruction cycle

  computer cycle

  brain cycle

  michelin star cycle

 3. Here is a diagram (Source: BBC Bitesize) of the Fetch Decode Execute cycle. Out of the following options, what happens first?
fde.png

  The instruction is immediately decoded and executed

  The processor checks the program counter to see which instruction to run next

  The program counter gives an address value in the memory of where the next instruction is

  The processor fetches the instruction value from this memory location

 4. The next steps that occur in the Fetch Decode Execute cycle are shown in the excerpt below. Can you fill in the blanks?
The program counter gives an address value in the memory of where 
the next instruction is.

The processor fetches the instruction value from this memory 
location.

Once the instruction has been fetched, it needs to be decoded and 
executed. For example, this could involve taking one value, putting 
it into the ALU, then taking a different value from a register and 
adding the two together.

Once this is complete, the processor _________________________________
_____________________________________________________________________

This cycle is repeated until the program ends.

  goes back to the program counter to find the next instruction

  goes to the ALU to check to see if there are any more calculations to perform

  goes back to execute an instruction

  goes back to the CU (Control Unit) to find the next instruction

 5. The three stages of the FDE cycle are 'Fetch', 'Decode' and 'Execute'. What happens in the fetch stage?
#1. In the fetch stage, the ALU provides a pointer to the program
counter. The program counter then executes the instruction directly
by sending instructions via the data and address bus. 


#2. In the fetch stage, the program counter provides the memory
address of the next instruction. This is transferred through the 
address bus to the memory and the instruction is transferred to the
instruction register of the CPU through the data bus.

  Both #1 and #2 could be correct

  Neither are correct

  #1 is correct

  #2 is correct

 6. What happens in the decode stage of the FDE cycle?

  The decoder held in the program counter dissolves the instruction in order to understand its true meaning

  The decoder in the control unit works out what the instruction means (decodes the instruction)

  The decoder sends the instruction to the program counter to be counted

  The decoder in the ALU executes the instruction

 7. What happens in the 'execute' part of the FDE cycle?

  The ALU executes the instruction and sends it to the printer

  The control unit decides whether or not to execute the instruction by counting the number of bits the data has

  The registers register and execute the instruction and then send it to the program counter to be executed

  The control unit instructs the ALU to perform the computations required and the result is stored in one of the registers

 8. Why is the fetch decode execute cycle called a cycle?
fde2.png

  Because it looks a little like a bicycle and hence its name

  Because it cycles around at millions of bits per second and this is the whir you hear when you switch on your PC

  Because the cycle is repeated over and over as each instruction in the program is executed and complete

  Because a cycle refers to something that cannot repeat itself

 9. Describe the role of the 'program counter' in the fetch decode execute cycle.

  This is a register that programs the CPU

  This is a register in the CPU which contains the memory address of the next instruction to be processed

  This is a register that executes each instruction as it deals with programming

  This is a register that decodes the instructions as they come in

 10. The program counter stores the memory address of the next instruction to be fetched. What happens when that address has been used?

  The progam counter explodes and no longer functions correctly

  The program counter transforms into the ALU

  The program counter is incremented by 1 so it now has the address of the next instruction

  The program counter switches off automatically until it is manually switched on