An Operating Systems Vade Mecum

Perspective 2533.4 CommunicationThe trend of the last few years is toward the use of computers as tools for allowing peopleto communicate with one another. Programs for mailing notes between users of thesame interactive computer have been available for years. These notes are collected infiles that serve as mailboxes and can be read by a program that the recipient can invoke.Notification that mail has arrived is often by a short message to the terminal of the recipientor by a note when the recipient starts a session with the operating system. Thecommand interpreter can also check the mailbox and tell the user if new mail has arrived.Programs also allow two users to direct notes to each other’s terminals without using amailbox.A different sort of communication involves several computers at once. Computersthat are connected for transfer of data are said to belong to a network. Small ‘‘satellite’’computers can be linked to large central computers either by permanent lines or by occasionaltelephone connections. The satellite computers might be used for everyday computation.The central computer might be used for archival data storage and large-scalecomputation that exceeds the capacity of the smaller machines. An installation mightconnect its several computers using a local area network to allow sharing of data andcomputation. Programs are often provided for data transfer and remote computation. Asingle operating system might control all the machines, making the existence of the networkinvisible to typical users. The Locus variant of Unix is a particularly successfulimplementation of this idea.In the last few years, the two ideas of computer mail and computer networks havebeen combined. Many national and international computer networks exist. The UUCPnetwork connects perhaps 10,000 installations, mostly in North America, but includingKorea, Israel, and Europe. There is a similar network in Australia. The Arpa Internetcovers thousands of installations in North America and Europe. BITNET has over 1500computers at university computing centers, mostly in North America, Europe, Japan, andIsrael. CSNET connects dozens of computers in computer science departments inuniversities in the United States and Canada as well as computer-industry researchlaboratories. Computer corporations often have internal networks connecting their sites,such as IBM’s VNET and Digital Equipment Corporation’s Easynet.These networks are used mainly for computer mail, although more general datatransfer and even remote computation are possible in some cases. We will return to thetopic of networks in Chapter 9 when we discuss inter-process co-operation.4 PERSPECTIVEAlthough the range of computers and their operating systems is quite large, certain commonthemes apply to all of them. The User Principle leads to similar solutions on bothlarge and small computers. For example, a personal-computer user needs a commandinterpreter just as much as a batch user on a central mainframe computer. Both havetasks that must be submitted to the computer. The convenience of having several