Starting the Application
Windows displays an initial screen with icons and menus, which allow the application to be selected by clicking on a shortcut. Windows converts this action into an operating system command which runs the executable file (WINWORD.EXE, etc.) stored on disk. The application program is transferred from disk to RAM, or as much of it as will fit in the available memory. The word-processor screen is displayed and a new document file can be created or an existing one loaded by the user from disk.
Data Input
The primary data input is from the keyboard, which consists of a grid of switches that are scanned by a dedicated microcontroller within the keyboard unit. This chip detects when a key has been pressed, and sends a corresponding code to the CPU via a serial data line in the keyboard cable, or wirelessly. The serial data is a sequence of voltage pulses on a single wire, which represent a binary code, each key generating a different code. The keyboard interface converts this serial code to parallel form for transfer to the CPU via the system data bus. It also signals separately to the CPU that a keycode is ready to be read into the CPU, by generating an ‘interrupt’ signal. This serial-to-parallel (or parallel-to-serial) data conversion process is required in all the interfaces that use serial data transfer, such as the keyboard, screen and network (see the appendices for more information on binary coding, and serial and parallel data).
The mouse is a convenient pointer controller for selecting options on screen and drawing graphics. The original mouse used two rollers set at right angles, with perforated disks attached. The holes were detected using an opto-sensor, sending pulses representing movement in two directions to the CPU. This mechanism has been replaced with direct optical sensing of variations in the surface under the mouse, using complex software to extract the direction and speed information. This also eliminates unreliable mechanical components.
Data input from a network or USB source is also in serial form, while the internal disk interface is traditionally in parallel, direct onto the peripheral bus. The parallel connection is inherently faster, since data bits are transferred simultaneously on all bus lines.