How Operating Systems Work

Computer Operating Systems

When you turn on the power to a computer, the first program that runs is usually a set of instructions kept in the computer's firmware called the boot ROM. For a typical PC, this can be the basic input output system (BIOS), or on newer machines, the unified extensible firmware interface (UEFI). This code examines the system hardware to make sure everything is functioning properly and in the case of UEFI, that the boot software is legitimate and secure. Once the test has successfully completed, the firmware continues the boot process.

The bootstrap loader, or boot loader, is a small program that has a single function: It loads the operating system into memory and allows it to begin operation. In the most basic form, the bootstrap loader sets up the small driver programs that interface with and control the various hardware subsystems of the computer. It sets up the divisions of memory that hold the operating system, user information and applications. It establishes the data structures that hold the myriad signals, flags and semaphores that are used to communicate within and between the subsystems and applications of the computer. Then it turns control of the computer over to the operating system.


The operating system's tasks, in the most general sense, fall into several categories:

  • Processor management
  • Memory management
  • Device management
  • Storage management
  • Application interface
  • User interface
  • System security management

These tasks define the core of nearly all operating systems. Next, let's look at the tools the operating system uses to perform each of these functions.