The Multi-User Computers

By David E. Sanger
The New York Times

August 23, 1984

IN the booming days of mainframe and minicomputers, users sat at terminals and shared the powers of a central brain. Then, because sharing was slow and cumbersome, came the personal computer: a single microprocessor serving a single master.

Now, personal computers have been installed by the thousands in corporate offices, often with more enthusiasm than planning. And the rush is on to find ways for these computers to share their files and programs at will, without losing the speed and flexibility that have made personal computers so attractive.

In fact, such multi-user systems have been on the market for several years, manufactured by small companies like Fortune Systems, Altos Computer Systems and Durango Systems Inc. But last week, the International Business Machines Corporation introduced its Personal Computer AT, which can support at least three - and ultimately 16 - users at one time.

The race is on to develop operating systems, mostly variations of the American Telephone and Telegraph Company's Unix system, that will make a far more complex generation of microcomputers as easy to use as their older cousins.

''Until now, the world has not really believed in Unix and multi-user systems,'' said James S. Campbell, chairman of Fortune Systems. ''Now it will.''

Operating systems are most frequently likened to a traffic cop, the program that tells a computer to pick up a piece of data through this disk drive, route it through this processor and send it to that printer. On ordinary personal computers, most users can ignore the operating; it works silently in the background, except when a new word processing program is loaded in, for example, or when a backup copy of a diskette is made.

''There's a saying at Berkeley,'' said Mike Dubrall, a managing analyst at Yates Ventures, a California consulting group, referring to the university where many of the refinements of Unix have been made in the last decade: ''It goes, 'Operating systems are like underwear; people don't want to see them when they are in use.' ''

Unfortunately, operating systems for multi-user computers are far more complex than for single- user machines, and more than a few have come to market half-dressed. Many versions of Unix - and scores of them have been developed by companies that have licensed the basic Unix technology from A.T.& T. - still require significant technical prowess on the part of the operator.

What makes the problem so complicated is that an effective multi-user operating system must allocate the scarce resource of the microprocessor's time and power. While a single-user system receives one instruction at a time, machines like the new Personal Computer AT juggle many at once.

''The trick is protecting each user's file, and each user's program, from fouling up somebody using the computer at the same time,'' said George Alexy, the marketing manager of high-performance microprocessors at the Intel Corporation, developer of the 80286 chip, which powers the new I.B.M. machine.

To accomplish the task, the 80286 included features not found on the Intel 8088, used in other I.B.M. machines. The newer chip can divide the computer's memory system into segments. Each user, and each program, is assigned a set of ''addresses'' in that memory space, and cannot go beyond the limit.

The system protects users from corrupting each other's programs, Mr. Alexy said. ''But you are also trying to protect the operating system itself,'' he added, because many programs written for a single-user microcomputer - like a word processor or financial spreadsheet - were designed to temporarily modify or bypass the operating system in order to accomplish a certain complex task. On a multi-user system, such a modification could bring about disaster because a change that could help one user could make another's program completely unusable.

Redesigning the microprocessor was only part of the trick. Unix itself, which was originally designed to run on minicomputers like the Digital Equipment Corporation's VAX systems, had to be ''ported,'' or adapted, for use on a variety of machines, yielding a variety of different versions of Unix.

Now some order may emerge, however. I.B.M. settled on one Unix derivative for its new AT, marketed by the Microsoft Corporation under the label Xenix, and many believe it has the best chance of emerging as the industry standard. Its main competition is Unix V, a new version of Unix released by A.T.& T., and the first that the telephone giant seems intent on marketing as a commercial product.

Both Microsoft and A.T.& T. make various claims about the superiority of their versions, and it is still too early to say which will emerge as the best. Xenix includes a system known as ''record and file locking,'' which prevents one user from changing the contents of a computer file while another is working on the same file - an invitation for someone's work to be lost. Unix V is said to be more powerful, allowing more users to work on the system at once, but does not include some features that commercial users are expected to insist on.

A.T.& T. says that will soon change. And it also promises to add ''virtual memory'' features to its system, which will allow users to run programs on microcomputers that previously seemed too large and complex for the limited memory capacity of smaller systems. Microsoft has vowed to match any Unix V improvements.

''In time, Unix V will run on every type of computer, from micro to mini to mainframe,'' said Otis L. Wilson, A.T.& T.'s manager of software and sales. ''We invented Unix, and we know we can be in the lead in developing the new versions.''

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Copyright 1984 The New York Times Company