ITPEC IP October 2023 Question 10

Topic: Computer ComponentAlso asked in March 2018 and April 2025

Which of the following is an appropriate combination of words that are inserted into blanks A and B in the description below concerning the execution sequence of instructions in a computer?

In a computer, the instructions are executed in the sequence shown below.

 (1)Refer to the program counter. (2)A the instruction. (3)Set the program counter to the main memory address of the next instruction. (4)B the instruction. (5)Execute processing according to the instruction. (6)Return to (1).
OptionAB

Explanation

A: Read, B: Decode — the instruction cycle fetches an instruction from memory, then interprets it, then executes it.

  1. Refer to the program counter for the address of the instruction.
  2. Read (fetch) the instruction from that main memory address into the instruction register.
  3. Update the program counter to the address of the next instruction.
  4. Decode the instruction: the control unit works out the operation and the operands.
  5. Execute it.
  6. Repeat.

Answer (b)

Why not others:

  • •(a) — reverses the order: an instruction cannot be decoded before it has been read from memory
  • •(c) — writing is not a step on the instruction itself; after reading, the instruction must be decoded before it can be executed
  • •(d) — the instruction is read from main memory, not written to it, at step 2

Key rule: The instruction cycle is fetch (read), update the program counter, decode, execute; results are written back only during execution.

Computer Component · AI-written, may contain errors

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