Flynn's Classification categorizes computer architectures based on the number of concurrent instruction streams and data streams. There are four categories: SISD refers to a traditional von Neumann architecture with a single instruction stream and single data stream. SIMD uses a single instruction stream to process multiple data streams, as seen in vector processors. MIMD allows multiple instruction streams and data streams, as in most parallel computers today. MISD, with multiple instructions operating on a single data stream, is theoretically possible but no systems have been built this way.

1. Flynn’s Classification
Prepared by
Dr.S.Raja Ratna
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• Michael J Flynn classified computers on the basis of multiplicity of
instruction stream and data streams in a computer system.
• Flynn's Classification refers to a classification of parallel
computer architectures.
• Parallel computers can be classified by the concurrency in
processing sequences (streams), data, or instructions from the
perspective of an assembly language programmer.

3. • Flynn uses the stream concept for describing a machine's
structure A stream simply means a sequence of items (data or
instructions).
• It gives how sequence of instructions or data will be executed
upon a single processor.
• Instruction stream: is the sequence of instructions as executed
by the machine.
• Data Stream: is a sequence of data including input, or partial or
temporary result, called by the instruction Stream.
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4. • The four classifications defined by Flynn are based upon the
number of concurrent instruction and data streams available in the
architecture.
Flynn’s Classification
• Instructions are decoded by the control unit and then ctrl unit send
the instructions to the processing units for execution.
• Data Stream flows between the processors and memory
bidirectional.
Single
Instruction
Multiple
Instruction
Single Data SISD MISD
Multiple data SIMD MIMD
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5. Flynn Classification of Computer architectures
SISD: Single Instruction Single Data
– Classical von Neumann architecture
SIMD: Single Instruction Multiple Data
MISD: Multiple Instructions Single Data
– Non existent, just listed for completeness
MIMD: Multiple Instructions Multiple Data
– Most common and general parallel machine
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SISD (Singe-Instruction stream, Singe-Data stream)
• SISD corresponds to the traditional mono-processor (von
Neumann computer).
• A single data stream is being processed by one instruction
stream
• It represents the organization of a single computer containing a
control unit, a processor unit, and a memory unit.
• Instructions are executed sequentially, and the system may or
may not have internal parallel processing capabilities.

7. SISD (Singe-Instruction stream, Singe-Data stream)
where, CU = Control Unit, PE = Processing Element, M = Memory
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8. SIMD (Single-Instruction stream, Multiple-Data streams)
• Each instruction is executed on a different set of data by different
processors i.e multiple processing units of the same type process on
multiple-data streams.
• All processors receive the same instruction from the control unit but
operate on different items of data.
• The shared memory unit must contain multiple modules so that it can
communicate with all the processors simultaneously.
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9. SIMD (Single-Instruction stream, Multiple-Data streams)
SIMD is mainly dedicated to array processing machines. However, vector
processors can also be seen as a part of this group
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MIMD (Multiple Instruction and Multiple Data Stream)
• In MIMD, all processors in a parallel computer can execute different
instructions and operate on various data at the same time.
• In MIMD, each processor has a separate program and an
instruction stream is generated from each program.
• Each processor has a separate program. An instruction stream is
generated from each program.
• Each instruction operates on different data.

11. • Several processing units operate on multiple - data streams.
• Programmers normally write a single program that runs on all
processors of MIMD computer.
MIMD (Multiple Instruction and Multiple Data Stream)
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12. MISD (Multiple Instruction and Single Data stream)
• MISD structure is only of theoretical interest since no practical
system has been constructed using this organization.
• In MISD, multiple processing units operate on one single-data
stream.
• Each processing unit operates on the data independently via
separate instruction stream.
• Systolic array is one example of an MISD architecture.
• Systolic array is a homogeneous network of tightly coupled data
processing units (DPUs) called cells or nodes.
• Each processor executes a different sequence of instructions. 12

13. where, M = Memory Modules, CU = Control Unit, P = Processor Units.
• In case of MISD computers, multiple processing units operate on
one single-data stream
• In practice, this kind of organization has never been used.
MISD (Multiple Instruction and Single Data stream)
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