A computer system is a complete set of hardware and software working together to perform tasks and process data. It includes input devices (like a keyboard or mouse), output devices (like a monitor or printer), storage (like a hard disk), a processor (CPU), and software (both system and application software). A computer system takes input, processes it, and produces output.

A subsystem is a smaller part of a larger computer system that performs a specific function. For example, within a computer system, there are subsystems like the input system, processing system, memory system, and output system. These subsystems can also be broken down into smaller parts—for instance, the input subsystem might include the keyboard handler and mouse handler.

Decomposition means breaking down a complex system or problem into smaller, more manageable parts or sub-tasks. This makes it easier to understand, design, and solve. In computer science, decomposition helps programmers handle complicated tasks by focusing on one smaller task at a time. For example, a game program might be decomposed into sound control, graphics, user input, and scoring systems.

Decomposition and the use of subsystems offer several key benefits in computer system design. Breaking down a complex system into smaller parts makes it much easier to understand and manage. This approach allows each subsystem or task to be designed and developed independently, promoting better focus and planning. It also supports teamwork, as different programmers can work on separate subsystems simultaneously, which speeds up development and improves collaboration. Additionally, decomposition enhances testing and debugging, since errors can be isolated and fixed more easily within individual components. Subsystems also improve code reusability, as modules can be reused in other programs. Maintenance becomes simpler, too, because updates can be made to specific parts without affecting the entire system. Overall, decomposition leads to cleaner, more structured, and well-organized code that is easier to read and manage.

Despite the advantages, there are several drawbacks to decomposition and subsystems. The design phase can be time-consuming, as breaking down a system into sub-systems requires careful planning, which demands extra effort. Once the individual sub-systems are developed, combining them into a fully functioning system can be challenging and prone to errors, increasing the complexity of integration.Additionally, if decomposition is not done properly, there is a risk of overlapping tasks or missing functionalities, which can result in inefficiencies. Effective communication within teams is crucial, as team members working on different parts of the system must coordinate to avoid inconsistencies. Furthermore, while individual sub-systems might work correctly on their own, debugging can become difficult when errors occur during interactions between the sub-systems, making it harder to track and resolve bugs.