Early microprocessors had an instruction cycle consisting of these stages:
A. Fetch, Decode, and Execute B. Fetch, Execute, Interrupt C. Fetch, Decode, Write Back D. All of the above
The correct answer and explanation is:
The correct answer is A. Fetch, Decode, and Execute.
Early microprocessors followed a basic instruction cycle consisting of three main stages: fetch, decode, and execute. This cycle is crucial for processing instructions in a sequential manner.
- Fetch: The first step of the cycle is fetching the instruction from memory. The program counter (PC) keeps track of the address of the next instruction to be executed. The instruction is retrieved from memory and placed into the instruction register (IR).
- Decode: Once the instruction is fetched, it is decoded by the control unit. During this stage, the instruction is interpreted to understand what operation needs to be performed. The decoder breaks down the instruction into its components, such as the opcode (which defines the operation) and the operands (which indicate the data to be processed). The control unit then generates signals to direct other components, like the ALU or memory, to perform the required tasks.
- Execute: After decoding, the actual operation is carried out in this stage. If the instruction involves an arithmetic operation, the Arithmetic Logic Unit (ALU) performs the computation. If the instruction involves data transfer, it might interact with memory or input/output devices. This is the stage where the result of the operation is produced, which might then be written back to memory or a register.
The other options listed—B, C, and D—are not typically part of the basic early microprocessor instruction cycle. Option B (Fetch, Execute, Interrupt) involves the interrupt handling mechanism, which is not a part of the standard instruction cycle. Option C (Fetch, Decode, Write Back) might be used in more complex processors but does not represent the original simple cycle. Option D (All of the above) is incorrect because not all these stages are part of the basic instruction cycle. Therefore, A is the correct answer.