Computer architecture classification (SISD): In a Single Instruction stream, Single Data stream (SISD) architecture, how many control units are present?

Introduction / Context: Flynn’s taxonomy classifies computers by the number of concurrent instruction and data streams: SISD, SIMD, MISD, and MIMD. Understanding SISD helps clarify how classic uniprocessor systems are organized and why they differ from vector or multiprocessor designs.

Given Data / Assumptions:

• We are analyzing SISD = single instruction, single data stream.
• Question asks specifically about the number of control units.
• Assume a conventional synchronous design where one control unit drives the datapath.

Concept / Approach: In SISD, a single processing element executes a single stream of instructions operating on a single stream of data. Correspondingly, there is a single control unit orchestrating fetch, decode, and execution. By contrast, SIMD has one control unit broadcasting to many processing elements; MIMD has multiple control units operating independently.

Step-by-Step Solution: Map SISD to a uniprocessor model.Identify that instruction sequencing and control are centralized.Conclude that only one control unit is present.Select “one.”

Verification / Alternative check: Textbook block diagrams for SISD show a single control unit attached to a single arithmetic/logic datapath and a single program counter and instruction decoder.

Why Other Options Are Wrong: Two / more than one: would imply parallel or distributed control (SIMD/MIMD).

Zero: nonsensical; execution requires control.

None of the above: incorrect because one is standard.

Common Pitfalls: Confusing physical cores with logical control units; SISD predates multicore and describes the conceptual model where only one instruction stream is active.

Final Answer: one