---
title: "Flynn's classification of parallel computers"
subject: "Microprocessor"
chapter: 6
key: "semester_4/microprocessor:flynn-classification"
source: https://www.bctengineer.com/q/semester_4/microprocessor/flynn-classification
site: BCT Engineering
---

# Flynn's classification of parallel computers

Microprocessor · Chapter 6: Advanced Topics

- Asked in: 2073 Magh, 4 marks; 2070 Bhadra, 4 marks; 2069 Poush, 4 marks; 2068 Bhadra, 5 marks (asked 4 times)
- Video: [Flynn's Classification in Computer Organization and Architecture: SISD, SIMD, MISD, and MIMD](https://www.youtube.com/watch?v=BHD7qhZRoXQ) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/microprocessor/flynn-classification
- Whole chapter: https://www.bctengineer.com/md/semester_4/microprocessor/chapter-6

## Question

Discuss Flynn's classification.

## Short answer

Flynn's classification has four classes: SISD (one instruction stream, one data stream), SIMD (one instruction, many data streams; array processors, GPUs), MISD (many instructions, one data stream; fault-tolerant systems), and MIMD (many instructions, many data streams; multicore, clusters).

## Answer

Flynn's classification (1966) groups computers by the number of **instruction streams** and **data streams** that they process at the same time. There are four classes.

#### 1. SISD (Single Instruction, Single Data)

One control unit executes one instruction at a time on one data item. It is the conventional serial computer (older single-core uniprocessors such as the 8085/8086).

```text
 IS          IS        DS
 CU -------> PU <----> MM
```

#### 2. SIMD (Single Instruction, Multiple Data)

One control unit sends the same instruction to many processing elements, and each works on its own data at the same time. Used for vector/array processing, graphics, image processing. Examples: array processors, GPUs, MMX/SSE extensions.

```text
        +--> PU1 <--> MM1
 CU ----+--> PU2 <--> MM2
        +--> PUn <--> MMn
```

#### 3. MISD (Multiple Instruction, Single Data)

Several processing units run different instructions on the same data stream. It is rare and mostly theoretical; examples are fault-tolerant systems that compare results (such as flight control computers) and some systolic arrays.

#### 4. MIMD (Multiple Instruction, Multiple Data)

Several processors, each with its own control unit, run different instructions on different data at the same time. Used in multiprocessors and multicore PCs, clusters and distributed systems. Types: shared-memory (tightly coupled) and distributed-memory (loosely coupled).

| Class | Instruction streams | Data streams | Example |
|---|---|---|---|
| SISD | 1 | 1 | Single-core CPU |
| SIMD | 1 | Many | Vector processor, GPU |
| MISD | Many | 1 | Fault-tolerant systems |
| MIMD | Many | Many | Multicore, cluster |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. On what basis does Flynn classify computers?
   A. The number of instruction streams and data streams
   B. The clock speed and word length
   C. The size of memory and cache
   D. The number of registers
   Answer: A. Flynn's classes depend on the instruction and data streams processed at once. (Key point: Basis)

2. Which describes SIMD?
   A. Many instructions on a single data item
   B. One instruction applied to many data items by many processing elements
   C. One instruction on one data item
   D. Many instructions on many data items
   Answer: B. SIMD is used for vector and array processing, graphics and GPUs. (Key point: SIMD)

3. Which is an example of MIMD?
   A. A multicore processor or cluster
   B. An 8086 uniprocessor
   C. A GPU running one instruction on arrays
   D. A single-core CPU
   Answer: A. MIMD has multiple processors with their own control units on different data. (Key point: MIMD)

4. Which statement about MISD is correct?
   A. It is the common PC architecture
   B. It has one instruction stream and many data streams
   C. It is rare, with several instruction streams on one data stream, e.g. fault-tolerant systems
   D. It is the conventional serial computer
   Answer: C. MISD is mostly theoretical; fault-tolerant systems that compare results are examples. (Key point: MISD)

## Related in other subjects

- [Flynn's classification](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_organization_and_architecture%3Aflynn) — Computer Organization and Architecture, chapter 4