Computer Organisation and Architecture - Fetch-Decode-Execute Cycle

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1
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What are the steps in the fetch part of the fetch-decode execute cycle (in words)?

  1. The contents of the Program Counter are copied onto the MAR
  2. The contents of the MAR are transferred to the memory through the address bus where it receives a control signal from the control unit indicating a memory read
  3. The contents of the memory at the received address are transferred to the MBR through the data bus
  4. At the same time, the PC is incremented by 1
  5. The contents of the MBR are copied onto the CIR
2
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What are the steps in the fetch part of the fetch-decode execute cycle (in register transfer notation)?

  1. MAR ← [PC]
  2. MBR ← [Memory]addressed
  3. PC ← [PC] + 1
  4. CIR ← [MBR]
3
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What are the steps in the decode and execute parts of the fetch-decode execute cycle?

  1. Control unit splits the instruction in the CIR into opcode and operand
  2. Control unit uses opcode to identify the instruction being executed and the part of the processor executing the instruction
  3. Control unit generates necessary control signals for execution
  4. Certain actions are performed depending on the instruction
4
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What are the possible actions the processor can take in the execute part of the fetch-decode execute cycle?

-Jump/Branch instruction -> PC is updated

-Data is required -> Memory or I/O read is issued

-Arithmetic/Logic operation -> ALU calculates results and stores in register(s) and the status register updates to show overflow or carry

5
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What happens in the interrupt part of the fetch-decode execute cycle if an interrupt signal of higher priority has been received by the processor?

  1. The current process being executed is suspended and the current values of the registers are pushed to the system stack
  2. The appropriate Interrupt Service Routine is found and called to handle the interrupt
  3. After the interrupt has been serviced, the original values of the registers are retrieved from the stack and execution continues with the instruction addressed by the PC
6
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What is an interrupt?

A signal sent to the processor that suspends execution of the current process until the interrupt is handled with an Interrupt Service Routine (ISR)

7
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What sends interrupts?

Hardware devices and software programs

8
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Why do the register contents have to be pushed to the system stack when an interrupt is received?

To save the volatile environment as the registers are going to be overwritten so the values would be lost otherwise

9
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What does the use of interrupts allow a processor to do?

Multitask (using time slices and timer interrupts)

10
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  1. The contents of the memory at the received address are transferred to the MBR through the data bus