Flynn's taxonomy, proposed by Michael J. Flynn in 1966, classifies computer architectures based on two properties: Instruction Stream and Data Stream. These properties describe the number of concurrent instructions and data streams that can be processed by a computer architecture. The two categories are as follows:

Instruction Stream: This property refers to the number of instructions processed simultaneously by the architecture. Flynn's taxonomy defines two possibilities:

Single Instruction, Single Data (SISD): In this category, the computer architecture executes a single instruction stream and operates on a single data stream at a time. This is the traditional sequential processing model found in most conventional computers, where each instruction is executed one after the other on a single set of data.

Data Stream: This property refers to the number of data streams processed simultaneously by the architecture. Flynn's taxonomy defines three possibilities:

• Single Instruction, Multiple Data (SIMD): In this category, the computer architecture executes a single instruction stream but operates on multiple data streams in parallel. SIMD architectures typically use vector processing or parallel processing units to perform the same operation on multiple data elements simultaneously. This is commonly found in graphics processing units (GPUs) or parallel computing systems.
• Multiple Instruction, Single Data (MISD): This category represents architectures that process multiple instruction streams but operate on a single data stream. While not commonly implemented in practice, MISD architectures have been explored in certain specialized applications, such as fault-tolerant systems where multiple redundant instruction streams are processed in parallel.
• Multiple Instruction, Multiple Data (MIMD): This category encompasses architectures that can process multiple instruction streams and operate on multiple data streams concurrently. MIMD architectures allow for independent instruction and data streams, and each processing unit can execute different instructions on separate sets of data. This category covers a wide range of parallel computing systems, including multi-core processors, clusters, and distributed systems.

Flynn's taxonomy provides a framework for categorizing computer architectures based on their instruction stream and data stream properties, allowing for a better understanding of different parallel processing models and their capabilities.