The file system often includes security features to protect against file access by unauthorized users. The syster may be connected to other computers via a network and thus provide access to remote system resources Operating systems are often categorized by the major functionality they provide. This functionality includes distributed computing, fault tolerance, parallel processing, real-time, and security. While no operating syster incorporates all of these capabilities, many have characteristics from each category. An operating system does not need to contain every modern feature to be useful. For example, MS-DOS is a single-user system with few of the features now common in other systems. Indeed, this system is little more than a program loader reminiscent of operating systems from the early 1960s. Unlike those vintage systems, there are numerous applications that run under MS-DOS. It is the abundance of programs that solve problems from word processing to spreadsheets to graphics that has made MS-DOS popular. The simplicity of these stems is exactly what makes them popular for the average person Systems capable of supporting multiple users are termed time-sharing systems; the system is shared among all users, with each user having the view that he or she has all system resources available. Multiuser operating systems provide protection for both the file system and the contents of main memory. The operating system must also mediate access to peripheral devices. For example, only one user may have access to a tape drive at Fault-tolerant systems rely on both hardware and software to ensure that the failure of any single hardware component, or even multiple components, does not cause the system to cease operation. To build such a system quires that each critical hardware component be replicated at least once. The operating system must be able to dynamically determine which resources are available and, if a resource fails, move a running program to an operational unit. Security has become more important during recent years. Theft of data and unauthorized access to data are prevented in secure systems. Within the United States, levels of security are defined by a government-produced document known as the Orange Book. This document defines seven levels of security, denoted from lowest to highest as D, C1, C2, B1, B2, B3, and Al. Many operating systems provide no security and are labeled D. Most time-sharing systems are secure enough that they could be classified at the Cl level. The C2 and Bl levels are similar, and this is where most secure operating systems are currently classified. During the 1990s B2 and B3 systems will become readily available from vendors. The Al level is extremely difficult to achieve, although several such systems are being worked on In the next several sections we expand upon the topics of distributed computing, fault-tolerant systems, parallel processing, and real-time systems. 96.3 Distributed Computing Systems The ability to connect multiple computers through a communications network has existed for many yea Initially, computer-to-computer communication consisted of a small number of systems performing bulk file transfers. The 1980s brought the invention of high-speed local area networks, or LANs. A LAn allows hundreds of machines to be connected together. New capabilities began to emerge, such as virtual terminals that allowed a user to log on to a computer without being physically connected to that system. Networks were used to provide remote access to printers, disks, and other peripherals. The drawback to these systems was the softwar it was not sophisticated enough to provide a totally integrated environment. Only small, well-defined interac tions among machines were permitted. Distributed systems provide the view that all resources from every computer on the network are available to the user. What's more, access to resources on a remote computer is viewed in the same way as access to resources on the local computer. For example, a file system that implements a directory hierarchy, such as UNIX, may have some directories on a local disk while one or more directories are on a remote system. Figure 96 1 illustrates how much of the directory hierarchy would be on the local system, while user directories (shaded directories)could be on a remote system IMS-DOS is a trademark of microsoft. Inc. 2UNIX is a trademark of UNIX Software Laboratories(USL) e 2000 by CRC Press LLC© 2000 by CRC Press LLC The file system often includes security features to protect against file access by unauthorized users. The system may be connected to other computers via a network and thus provide access to remote system resources. Operating systems are often categorized by the major functionality they provide. This functionality includes distributed computing, fault tolerance, parallel processing, real-time, and security. While no operating system incorporates all of these capabilities, many have characteristics from each category. An operating system does not need to contain every modern feature to be useful. For example, MS-DOS1 is a single-user system with few of the features now common in other systems. Indeed, this system is little more than a program loader reminiscent of operating systems from the early 1960s. Unlike those vintage systems, there are numerous applications that run under MS-DOS. It is the abundance of programs that solve problems from word processing to spreadsheets to graphics that has made MS-DOS popular. The simplicity of these systems is exactly what makes them popular for the average person. Systems capable of supporting multiple users are termed time-sharing systems; the system is shared among all users, with each user having the view that he or she has all system resources available. Multiuser operating systems provide protection for both the file system and the contents of main memory. The operating system must also mediate access to peripheral devices. For example, only one user may have access to a tape drive at a time. Fault-tolerant systems rely on both hardware and software to ensure that the failure of any single hardware component, or even multiple components, does not cause the system to cease operation. To build such a system requires that each critical hardware component be replicated at least once. The operating system must be able to dynamically determine which resources are available and, if a resource fails, move a running program to an operational unit. Security has become more important during recent years. Theft of data and unauthorized access to data are prevented in secure systems. Within the United States, levels of security are defined by a government-produced document known as the Orange Book. This document defines seven levels of security, denoted from lowest to highest as D, C1, C2, B1, B2, B3, and A1. Many operating systems provide no security and are labeled D. Most time-sharing systems are secure enough that they could be classified at the C1 level. The C2 and B1 levels are similar, and this is where most secure operating systems are currently classified. During the 1990s B2 and B3 systems will become readily available from vendors. The A1 level is extremely difficult to achieve, although several such systems are being worked on. In the next several sections we expand upon the topics of distributed computing, fault-tolerant systems, parallel processing, and real-time systems. 96.3 Distributed Computing Systems The ability to connect multiple computers through a communications network has existed for many years. Initially, computer-to-computer communication consisted of a small number of systems performing bulk file transfers. The 1980s brought the invention of high-speed local area networks, or LANs. A LAN allows hundreds of machines to be connected together. New capabilities began to emerge, such as virtual terminals that allowed a user to log on to a computer without being physically connected to that system. Networks were used to provide remote access to printers, disks, and other peripherals. The drawback to these systems was the software; it was not sophisticated enough to provide a totally integrated environment. Only small, well-defined interac￾tions among machines were permitted. Distributed systems provide the view that all resources from every computer on the network are available to the user. What’s more, access to resources on a remote computer is viewed in the same way as access to resources on the local computer. For example, a file system that implements a directory hierarchy, such as UNIX,2 may have some directories on a local disk while one or more directories are on a remote system. Figure 96.1 illustrates how much of the directory hierarchy would be on the local system, while user directories (shaded directories) could be on a remote system. 1 MS-DOS is a trademark of Microsoft, Inc. 2 UNIX is a trademark of UNIX Software Laboratories (USL)