Q34.Marks: +2.0UGC NET Paper 2: Computer Science 7th Dec 2023 Shift 2
In most general case, the computer needs to process each instruction with the following sequence of steps:
(A) Calculate the effective address
(B) Execute the instruction
(C) Fetch the instruction from memory
(D) Fetch the operand from memory
(E) Decode the instruction
Choose the correct answer from the options given below:
1.(A), (B), (C), (D), (E)
2.(A), (B), (C), (E), (D)
3.(C), (E), (A), (D), (B)✓ Correct
4.(C), (E), (D), (A), (B)
Solution
The correct answer is (C), (E), (A), (D), (B)
EXPLANATION:
Fetch instruction from memory (C):
The first step in the execution cycle of an instruction is the instruction fetch, wherein the instruction is retrieved from memory. This is necessary because the Central Processing Unit (CPU) needs to know what operation to execute.
Decode the instruction (E):
Once the instruction is fetched, it needs to be decoded or interpreted. During this step, the CPU's control unit identifies what operation is to be undertaken, based on the instruction's opcode.
Calculate the effective address (A):
If the instruction has an operand, it's not usually the actual data but rather its address that is specified in the instruction. During this phase, the address for the operands is computed. The calculated effective address will be used to fetch the operand in the next step.
Fetch the operand from memory (D):
Once the effective address has been calculated, the CPU can fetch the operand from memory. In other words, it retrieves the data that the operation will work on.
Execute the instruction (B):
Finally, the action specified by the instruction is performed, typically a mathematical or logical operation on the fetched operand. The result of this operation may be stored back into memory, written to a register, or outputted depending on the specifics of the instruction.
So, this explains why option 3) (C), (E), (A), (D), (B) is the correct sequence for these steps in the execution of an instruction.
