Sitting above the drivers is the operating system and other system programs. The operating system, or OS, communicates with the drivers through a standard set of application programming inter-faces, or APIs. The APIs provide a structure by which the next level of up in the abstraction level can communicate with the layer below it. Thus, in order to read a character from the keyboard of your computer, there is a low-level driver program that becomes active when a key is struck. The operating system communicates with this driver through its API. At the next level, the application software communicates with the OS through system API’s that once again, abstract the lower levels so that the individual differences in behavior of the hardware and the drivers may be ignored. However, we need to be a bit careful about taking the viewpoint of Figure 1.3 too literally. We could also argue that the Application and Operating System layers should be reversed because the user interacts with the application through the Operating System layer as well. Thus, mouse and keyboard inputs are really passed to the application through the Operating System and do not go directly from the User to the Application. Anyway, you get the picture. The computer hardware, as represented by a desktop PC, can be thought of as being comprised of four basic parts:1. Input devices can include components such as the mouse, keyboard, microphone, disks, modem and the network.2. Output devices are components such as the display, disk, modem, sound card and speakers, and the network.3. The memory system is comprised of internal and external caches, main memory, video memory and disk.4. The central processing unit, or CPU, is comprised of the arithmetic and logic unit (ALU), control system and busses.