Single Instruction Single Data (SISD)

Single Instruction and Single Data Stream (SISD) is a computer architecture where a central processing unit (CPU) executes one instruction at a time on a single data piece. 

In a SISD setup, a single processor performs operations sequentially on memory-stored data. The sequence of operations is linear and involves the processor receiving an instruction from memory, applying it to a data point, and then proceeding to the next instruction. The sequence is linear, without any simultaneous tasks occurring.

While SISD systems provide straightforward, predictable processing, they cannot execute multiple instructions concurrently, a feature known as parallelism. This limitation results in slower processing speeds than more sophisticated models like SIMD (Single Instruction Multiple Data) or MIMD (Multiple Instruction Multiple Data), which can execute multiple instructions or process multiple data points simultaneously.