Overview
This lecture explains the two most widely used classifications of computer architecture: the Von Neumann vs. Harvard models, and Flynn's taxonomy based on instruction and data streams.
Classification by Architecture Models
- Computer architecture is commonly classified as either Von Neumann or non-Von Neumann models.
- Von Neumann architecture contains three subsystems: CPU, main memory, and I/O system.
- It is a stored program computer, meaning both programs and data are kept in main memory.
- Instructions are executed sequentially, with only one operation handled at a time.
- The Von Neumann bottleneck arises due to a single path for both data and instructions between memory and CPU.
- Harvard architecture uses separate memory units for instructions and data, enabling parallel access and faster performance.
- Modified Harvard architecture introduces a cache that allows the processor to act as a Harvard or Von Neumann machine, depending on memory access.
Flynn’s Taxonomy
- Flynn's taxonomy classifies computers by the number of instruction and data streams.
- SISD (Single Instruction stream, Single Data stream): One CPU, one instruction at a time, one data item at a time—typical Von Neumann machines.
- SIMD (Single Instruction stream, Multiple Data streams): One control unit; multiple ALUs perform the same instruction on different data items simultaneously.
- MISD (Multiple Instruction streams, Single Data stream): Hypothetical class; different instructions operate on the same data, rarely implemented in practice.
- MIMD (Multiple Instruction streams, Multiple Data streams): Multiprocessor systems where each processor executes independent instructions on separate data—common in modern systems.
Key Terms & Definitions
- Von Neumann Architecture — Computer model with shared memory for program instructions and data, sequential instruction execution.
- Harvard Architecture — Computer model using separate memories and pathways for instructions and data, enabling parallel access.
- Von Neumann Bottleneck — Limitation caused by a single memory pathway for both data and instructions.
- Flynn’s Taxonomy — A system for classifying computer architectures by instruction and data stream counts (SISD, SIMD, MISD, MIMD).
- SISD — Single CPU, one instruction, one data path.
- SIMD — Single instruction, multiple data units processed in parallel.
- MISD — Multiple instructions, single data stream, rare in practice.
- MIMD — Multiple instructions and data streams; multiprocessor systems.
- Cache — Small, fast memory storage used to speed up access; central to modified Harvard architectures.
Action Items / Next Steps
- Review and compare the characteristics of Von Neumann, Harvard, and modified Harvard architectures.
- Study examples of systems using SISD, SIMD, and MIMD for deeper understanding.
- Prepare questions for next class discussion on computer architecture classifications.