Copyright Goodheart-Willcox Co., Inc. 41 Chapter 2 Operating Systems Linux fi rst appeared in 1991 as an alternative to commercial operating systems and quickly gained popularity in part because of its open-source model. Th ere are many versions of Linux OS available today, both free and commercial distributions, to meet computer users’ needs. Th ere are varieties of ways in which an operating system can be installed. Once the most common installation method, the bootable and installation OS images are stored on a CD or DVD. However, many new systems, especially mobile computers, do not include an optical drive reader. In these situations, the OS can be installed locally via USB or fl ash drive, solid-state drive, removable or hot-swappable drive via network connection or network server or remotely through a pre-boot execution en- vironment (PXE). For these to install correctly, the bootable and installation images of the operating system are stored on the respective device, which is loaded into the desired machine. When the computer starts, the user enters the BIOS and selects the desired device from the boot media option. Th e operating system could also be load- ed onto a partition of the internal hard drive, but these are often reserved as recovery or repair installations. Operating System Characteristics Th e way an operating system handles activities, such as storing data, interfacing with the user, and presenting information on the screen, can be referred to as operating system characteristics. Most operating systems appear similar when judged by their screen display. However, there are many diff erences in the way they handle activities, especially data storage. Multitasking Multitasking is the ability of an operating system to support two or more programs running at the same time. When multitasking, it seems to the user that both pro- grams are running simultaneously. However, in reality, they are not. Th e computer simply switches control between the programs, giving the illusion they are running at the same time. An example of multitasking is using the printer while also using e-mail, browsing the Internet, or running another computer application. Th e computer runs the soft- ware in between sending packets of data to the printer. Most operating systems support multitasking, but the hardware may not. Systems designed with minimal hardware, such as tablets and smartphones, do not support multitasking very well. If you attempt to perform more than one task, either the system will not allow you to run an additional program, or the system will run sluggishly because of the shared resources (CPU, RAM, and chipset). Th e two major classifi cations of multitasking are preemptive and cooperative. Preemptive Multitasking Preemptive multitasking, sometimes referred to as time slicing, is a process that allows multiple programs to share control of the operating system. For example, two or more programs can share the CPU for processing information. No single program can totally take charge of the computer system. All programs running in the pre- emptive mode of multitasking are allowed to run for a set period of time, called the time slice, by an operating system process known as the scheduler. At the end of the time slice, a process is interrupted so the next process in line can run. Th is way, all the processes on the computer can share the CPU fairly. Since each time slice is brief (around 20 milliseconds in Windows), the processes appear to be running in parallel.