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Software System Components

A computer system actually consists of seven layers of software (top to bottom): Programs; System Utilities; Command Shell; System Services; User Interface; Logical Level; and Hardware Level. A Graphics Engine stradles the bottom three layers. Strictly speaking, only the bottom two levels are the operating system, although even technical persons will often refer to any level other than programs as part of the operating system (and Microsoft is trying to convince the Justice Department that their web browser application is actually a part of their operating system). Because this technical analysis concentrates on servers, Internet Facilities are specifically separated out from the layers.

Examples

The following are examples of each category:

Programs: Examples of Programs include your word processor, graphics programs, music software, games, etc.
System Utilities: Examples of System Utilities include file copy, hard drive repair, and similar items. On the Macintosh, all the Desk Accessories (calculator, key caps, etc.) and all of the Control Panels are examples of System Utilities.
Command Shell: The Command Shell on the Macintosh is the Finder and was the first commercially available graphic command shell. On Windows, the Command Shell is a poorly integrated comination of the File Manager and the Program Manager. The command line (C:\ prompt) of MS-DOS or Bourne Shell of UNIX are examples of the older style text-based command shells.
System Services: Examples of System Services are built-in data base query languages on mainframes or the QuickTime media layer of the Macintosh.
User Interface: Until the Macintosh introduced Alan Kay’s (inventer of the personal computer, graphic user interfaces, object oriented programming, and software agents) ground breaking ideas on human-computer interfaces, operating systems didn’t include user interfaces. The Macintosh user interface is called the Macintosh ToolBox and provides the windows, menus, alert boxes, dialog boxes, scroll bars, buttons, controls, and other user interface elements shared by almost all programs.
Logical Level of Operating System: The Logical Level of the operating system provides high level functions, such as file management, internet and networking facilities, etc.
Hardware Level of Operating System: The Hardware Level of the operating system controls the use of physical system resources, such as the memory manager, process manager, disk drivers, etc.
Graphics Engine: The Graphics Engine includes elements at all three of the lowest levels, from physically displaying things on the monitor to providing high level graphics routines such as fonts and animated sprites.

Components of a software system:

examples of how operating systems are organized:

organization of Macintosh operating system

Inside Macintosh, Volume I, page I-10^b4b

     “The routines available for use in Macintosh programs are divided according to function, into what are in most cases called “managers” of the feature they support. … Most are part of either the Operating System or the User Interface Toolbox and are in the Macintosh ROM.
    “The Operating System is at the lowest level; it does basic tasks such as input and putput, memory management, and interrupt handling. The User Interface Toolbox is a level above the Operating System; it helps you implement the standard Macintosh user interface in your application. The Toolbox calls the Operating System to do low-level operations. …
    “RAM-based software is available as well. In most cases this software performs specialized operations (such as floating-point arithmetic) that aren’t integral to the user interface but may be useful to some applications.” —Inside Macintosh, Volume I, page I-9^b4b

“The Macintosh User Interface Toolbox provides a simple means of constructing application programs that conform to the standard Macintosh user interface. By offering a common set of routines that every application calls to implement the user interface, the Toolbox not only ensures familiarity and consistency for the user but also helps reduce the application’s code size and development time. At the same time, it allows a great deal of flexibility: An application can use its own code instead of a Toolbox call wherever appropriate, and can define its own types of windows, menus, controls, and desk accessories.” —Inside Macintosh, Volume I, page I-9^b4b

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