The SISD computing system is like the von Neumann machine, which is a uniprocessor machine. As you can see in Flynn's taxonomy diagram, it executes a single instruction that operates on a single data stream. In SISD, machine instructions are processed sequentially.
In a clock cycle, the CPU executes the following operations:
- Fetch: The CPU fetches the data and instructions from a memory area, which is called a register.
- Decode: The CPU decodes the instructions.
- Execute: The instruction is carried out on the data. The result of the operation is stored in another register.
Once the execution stage is complete, the CPU sets itself to begin another CPU cycle:
The algorithms that run on this type of computer are sequential (or serial) since they do not contain any parallelism. An example of a SISD computer is a hardware system with a single CPU.
The main elements of these architectures (namely, von Neumann architectures) are as follows:
- Central memory unit: This is used to store both instructions and program data.
- CPU: This is used to get the instruction and/or data from the memory unit, which decodes the instructions and sequentially implements them.
- The I/O system: This refers to the input and output data of the program.
Conventional single-processor computers are classified as SISD systems:
The following diagram specifically shows which areas of a CPU are used in the stages of fetch, decode, and execute:
