Question

Which of the following is the correct sequence of operations in a microprocessor?

• A. Opcode fetch, memory read, memory write, I/O read, I/O write
• B. Opcode fetch, memory write, memory read, I/O read, I/O write
• C. I/O read, opcode fetch, memory read, memory write, I/O write
• D. I/O read, opcode fetch, memory write, memory read, I/O write

Hint:

Microprocessor Machine Cycles. Every instruction starts with an Opcode Fetch cycle. Subsequent cycles depend on the instruction and can include Memory Read, Memory Write, I/O Read, and I/O Write.

Answer 1

The Correct Option is A

A microprocessor executes instructions fetched from memory
Each instruction execution cycle typically involves several machine cycles, which are sequences of basic operations
Common machine cycles include:
(1) Opcode Fetch: Fetching the instruction code (opcode) itself from memory
This is always the first cycle for any instruction

(2) Memory Read: Reading data or an operand from memory

(3) Memory Write: Writing data to memory

(4) I/O Read: Reading data from an input/output device
5
I/O Write: Writing data to an input/output device
An instruction might involve one or more of these cycles after the initial opcode fetch, depending on the nature of the instruction (e
g
, an instruction to load data from memory involves opcode fetch followed by memory read; an instruction to add a register to memory involves opcode fetch, memory read, and memory write)
Option (1) lists these fundamental machine cycles starting correctly with Opcode fetch
The subsequent order doesn't necessarily represent a single instruction but rather the types of operations available
Options (3) and (4) incorrectly start with I/O read
Option (2) incorrectly places memory write before memory read in the listing of possible operations
Option (1) provides the most logical listing of the basic operation types starting with the essential fetch cycle