Processor Taxonomies, SISD, SIMD, MIMD
As we already know the processor executes instructions which can process the data. We can consider two streams flowing through the processor. Stream of instructions which passes through the control unit, and stream of data processed by the execution unit. In 1966 Michael Flynn proposed the taxonomies to define different processors' architectures. Flynn classification is based on the number of concurrent instruction (or control) streams and data streams available in the architecture. Taxonomies as proposed by Flynn are presented in Table1
Table 1: Processor taxonomies by Flynn
| Basic taxonomies | Data streams | ||
|---|---|---|---|
| Single | Multiple | ||
| Instruction streams | Single | SISD | SIMD |
| Multiple | MISD | MIMD | |
SISD
Single Instruction Single Data processor is a classical processor with a single control unit and a single execution unit. It can fetch a single instruction at one cycle and perform single calculation. Mature PC computers based on 8086, 80286 or 80386 processors or some modern small-scale microcontrollers like AVR are examples of such an architecture.
SIMD
Single Instruction Multiple Data is an architecture in which one instruction stream can perform calculations on multiple data streams. Good examples of implementation of such architecture are all vector instructions (called also SIMD instructions) like MMX, SSE, AVX, and 3D-Now in x64 Intel and AMD processors. Modern ARM processors also implement SIMD instructions which perform vectorised operations.
MIMD
Multiple Instruction Multiple Data is an architecture in which many control units operate on multiple streams of data. These are all architectures with more than one processor (multi-processor architectures). MIMD architectures include multi-core processors like modern x64, ARM and even ESP32 SoCs. This also includes supercomputers and distributed systems, using common shared memory or even a distributed but shared memory.
MISD
Multiple Instruction Single Data. At first glance, they seem illogical, but these are machines where the certainty of correct calculations is crucial and required for the security of the system operation. Such an approach can be found in applications like space shuttle computers.
SIMT
Single Instruction Multiple Threads. Originally defined as the subset of SIMD. In modern construction, Nvidia uses this execution model in their G80 architecture [1] where multiple independent threads execute concurrently using a single instruction.
