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The input subsystem contains the hardware that allows the user of the PC to provide direction and commands to the PC system. The subsystem is primarily comprised of several input devices that convert manual mechanical action by the PC's operator into electronic pulses that are interpreted by the computer:
These devices come in many different shapes and sizes; some are plain and others are fancy, and they range widely in price, quality and features. However, they all serve the same basic purpose: providing different types of input to the system. Keyboards are of course textual; mice are graphical, and joysticks and similar devices provide multi-dimensional positional input and are most often used in games and graphical software.
In addition to the input devices, the input subsystem includes the controllers these devices use; they are normally found on the motherboard, and sometimes, sound card. Interfaces and controllers for these input devices have become fairly standard over the years, and modern PCs all use the same connections for these input devices: mini-PS/2 ports for keyboard and pointing device, and game port for the joystick. Older PCs had a different-sized port for the keyboard, but adapters are available; similarly, some older mice used a serial port, and can still be used on modern systems through a serial port. Some newer mice and keyboards use the USB interface, but this is still not the standard.
These devices are important from the standpoint of the usability of the PC, so you are encouraged to pay attention to their feel and capabilities when shopping for them. At the same time, they don't really directly influence much in the PC in terms of capabilities or performance, nor do they impact the selection of other hardware. This, combined with their universality in terms of interface and the fact that their controllers are built into other hardware, means they are usually selected last when specifying a system. In some cases they are "scrounged" from other PCs. Even when buying an entirely new PC that comes with a mouse and keyboard, it's pretty easy to replace these stock input devices with others that you like better, if you so desire. I always do. :^)