System Level Architecture

CSS Optional — Paper-II, Computer Organization & Architecture

1. Definition

Flynn's Taxonomy classifies computer architectures by two independent dimensions: the number of concurrent instruction streams and the number of concurrent data streams a machine can process.

2. Classification — Flynn's Taxonomy ⭐ (high-yield)

Class	Meaning	Example
SISD	Single Instruction, Single Data — traditional uniprocessor, strictly sequential	Classic single-core CPU
SIMD	Single Instruction, Multiple Data — one instruction applied across many data elements at once	GPUs, vector processors
MISD	Multiple Instruction, Single Data — rare, mostly theoretical	Fault-tolerant / redundant systems
MIMD	Multiple Instruction, Multiple Data — independent processors on independent data	Multicore CPUs, clusters

3. Classification — Bus-Based System Architecture

Single-bus architecture — one shared bus for everything; simple but prone to contention.
Multiple-bus architecture — separate address, data, and control paths (or a bus hierarchy); higher throughput, more hardware.

4. Classification — Layered System View

Hardware — physical circuits and devices.
Instruction Set Architecture (ISA) — the machine-code interface.
Operating System — resource management, abstraction.
Application Software — the programs users actually run.

🗺️ Architecture Diagram — Flynn's Taxonomy

graph TD
    F["Flynn's Taxonomy"] --> SISD["SISD<br>1 instr stream, 1 data stream"]
    F --> SIMD["SIMD<br>1 instr stream, N data streams"]
    F --> MISD["MISD<br>N instr streams, 1 data stream"]
    F --> MIMD["MIMD<br>N instr streams, N data streams"]

🧮 Worked Example

A GPU adding a brightness value to 1,000 pixels simultaneously using one instruction = SIMD.
A multicore CPU running a browser tab and a music player independently = MIMD.

🎯 CSS Exam Angle

Flynn's taxonomy is one of the highest-yield short questions in Computer Organization — it appears across multiple attempts almost every 2–3 years.