In microprocessor execution, do all instructions take the same number of machine cycles, or can different instructions require different numbers of machine cycles?

Introduction:
Microprocessors execute instructions through a sequence of well-defined phases (fetch, decode, execute, memory/IO access, write-back). Different instructions stress these phases differently. This question checks whether you know that machine-cycle counts vary by instruction type.

Given Data / Assumptions:

• Context: general-purpose microprocessors (e.g., 8085/8086-era and beyond).
• Options claim fixed one-cycle execution or even sub-cycle execution versus variable cycles.
• Machine cycle is a basic timing unit comprising several clock periods (T-states).

Concept / Approach:

Simple register-to-register operations may complete in fewer cycles than memory or I/O operations, which require additional bus transactions. Therefore, machine-cycle counts are instruction-dependent. No real ISA guarantees that every instruction finishes in exactly one machine cycle, and the notion of 'less than one machine cycle' is not meaningful because cycles are the smallest architected timing unit for bus/control sequencing.

Step-by-Step Solution:

Verification / Alternative check:

Consult any instruction timing table (e.g., 8085 MOV vs. LDA/JMP): memory-referencing and branching typically take more cycles than register-only operations.

Why Other Options Are Wrong:

'all instructions need only one machine cycle' – contradicted by memory/I-O ops and branches.

'all instructions need only a fraction of one machine cycle' – cycles are atomic timing blocks; cannot have 'fraction' in this context.

'some instructions need less than one machine cycle' – not meaningful; cycles are indivisible architectural units.

Common Pitfalls:

Confusing 'clock period' with 'machine cycle' and assuming superscalar/pipelined overlap implies sub-cycle completion (it does not change per-instruction cycle accounting).

Final Answer:

some instructions need only one machine cycle while some other instructions need more than one machine cycle