Instruction stream
- Sequence of instructions from memory to control unit
- When flow of instructions from main memory to the CPU is established. This flow of instructions is called instruction stream
Data stream
- Sequence of data from memory to control unit
- Flow of operands between processor and memory bidirectionally. This flow of operands is called data stream
Stream Example
x = 5
y = 5
print(x+y)x & y = Data Stream
print & + = Instruction Stream
Classifications
SISD
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Single Instruction Stream Single Data Stream
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A serial (non-parallel) computer
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Single Instruction: Only one instruction stream is being acted on by the CPU during any one clock cycle
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Single Data: Only one data stream is being used as input during any one clock cycle
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Deterministic execution
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Oldest type of computer
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Examples
- Older generation mainframes
- Minicomputers
- Workstations
- Single processor/core PCs
SIMD
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Single Instruction Stream Multiple Data Stream
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A type of parallel computer
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Single Instruction: All processing units execute the same instruction at any given clock cycle
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Multiple Data: Each processing unit can operate on a different data element
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Best suited for specialised problems characterised by a high degree of regularity, such as graphics/image processing
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Synchronous (lockstep) and deterministic execution
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Lockstep
All processors execute the same instruction at the same time (but on different data items); no synchronisation is required
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Two varieties: Processor Arrays and Vector Pipelines
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Most modern computers, particularly those with graphics processor units (GPUs) employ SIMD instructions and execution units
MISD
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Multiple Instruction Stream Single Data Stream
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A type of parallel computer
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Multiple Instruction: Each processing unit operates on the data independently via separate instruction streams.
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Single Data: A single data stream is fed into multiple processing units.
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Few (if any) actual examples of this class of parallel computer have ever existed.
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Some conceivable uses might be:
- Multiple frequency filters operating on a single signal stream
- Multiple cryptography algorithms attempting to crack a single coded message
MIMD
- Multiple Instruction Stream Multiple Data Stream
- A type of parallel computer
- Multiple Instruction: Every processor may be executing a different instruction stream
- Multiple Data: Every processor may be working with a different data stream
- Execution can be synchronous or asynchronous, deterministic or non-deterministic
- Currently, the most common type of parallel computer - most modern supercomputers fall into this category.
- Examples: most current supercomputers, networked parallel computer clusters and “grids”, multi-processor SMP computers, multi-core PCs.
- Many MIMD architectures also include SIMD execution sub-components