X Primer: Closing The User Knowledge Gap

Edith D. Myers
UNIX Today!

November 14, 1988

Judging from the staunch support it's getting from all corners of the industry, the X-Windows user interface may prove to be the key that opens the world of graphics computing to the UNIX desktop, observers say.

Given its significance in this regard, it behooves anyone with a gap in his or her knowledge base to learn as much as possible about this promising interface standard.

X-Windows was developed at the Massachusetts Institute of Technology as part of Project Athena, a three-way partnership of MIT, Digital Equipment Corp. and IBM.

The interface has already garnered the blessings-and all-out support-of 19 major computer companies, including DEC, IBM, AT&T, Sun Microsystems, Apollo Computer and Hewlett-Packard.

The system has become a de facto standard and has been submitted to the American National Standards Institute for formal standard status.

ANSI has created a dedicated subcommittee, X3H3.6, to investigate the inclusion of X-Windows in ANSI specifications.

In addition, MIT has formed a consortium of companies to monitor and evolve the standard. A related testing consortium is working on code test/verification suites to ensure functionality and compatibility of X products.

As if that weren't enough, X/ Open-the consortium of companies doing business in Europe-has adopted the X-Windows system and uses it in the publication of its specification documents.

The enormous interest in X-Windows stems, in part, from the fact that it is essentially an open, public-domain standard that provides a single programming and user interface to the networked, bit-mapped graphics that make up windowing systems.

This means that a programmer working with the standard doesn't have to worry about which hardware platforms are targets of the application. A user working with X-Windows gets transparent access to a variety of systems and applications.

"The ultimate advantage to the user," according to Brian Croxon, senior vice president of new products for Visual Technology, Lowell, Mass., "is that there's a software platform there to which he will see a large number of applications ported.

"It {X} plays to the requirement for portability in a heterogeneous environment. You can work with a network of different kinds of equipment," he explained. Visual introduced its V640-XDS, an X display terminal, in August.

X-Windows contains two basic software components: Xlib and X Server. Xlib resides with an application, while X Server resides in the user's display device.

The two software components communicate via packets over a local area network. Xlib passes screen-display information from a host application to X Server and passes user-input information from X Server back to the application host.

An X Primer: The User Gap

X Server is responsible for translating both incoming and outgoing information between Xlib and the specific workstation in which X Server resides. Thus, in a sense, the network itself becomes a user's system.

To optimize performance across a network, the X client/server architecture splits up the graphics-interface functions. It keeps functions that relate directly to the host application in the client address space, while window context data reside in the server address space on the user's workstation.

X-Windows separates window management functions from the core standard architecture, allowing users to set interface policy and change it when necessary. X-Windows essentially hides communications from the application and the user display, so that it can maintain open windows to applications on a variety of remote, heterogeneous computers on a single-user workstation.


Another interesting feature of X-Windows is that the system can use any error-corrected duplex byte stream with asynchronous communications, allowing use of a tree structure and thereby permitting an infinite nesting of windows.

As a result, the system establishes "parent-child" relationships under which a parent window sets the outer boundaries for its children. The children can be resized, reshaped and moved within the parent's borders. If the parent is resized, its children will change accordingly.

Industry observers are quick to caution users that windowing products such as Microsoft Windows and Windows/386 and IBM Presentation Manager are different from X-Windows because they are strictly PC-resident.

"They do not have the built-in ability to separate the client and the server function," noted Visual's Croxon.

"It's a matter of architecture," added Judy Estrin, executive vp of Network Computer Devices Inc., Mountain View. "X's architecture puts the network between the disk processor and the applications processor. What you have on the desktop is a lower-cost unit. Something like Presentation Manager requires an applications processor on the desktop."

To date, DEC has raised the loudest vendor voice in support of X-Windows, calling it "an important step in the transition from a timesharing- to a workstation-based product orientation."

To promote this transition, DEC licenses the software interface to its own X-based product-DECwindows-to independent software developers and other computer vendors, and offers ISVs and systems integrators its X User Interface Toolkit, consisting of such components as pop-up menus, scroll bars, icons and window borders.

"Little or no training is required to use DECwindows," said Phil Auberg, DECwindows product manager. "It's a network-transparent universal interface. It makes available to users applications that might not even run on their systems."

Visual's and NCDI's products will support DECwindows and, as Visual's Croxon said, "any other interfaces that come along."

Croxon said his firm has a lead on potential competitors but allowed "there will be a number of people coming into the market as it develops."

NCDI will be there. Estrin said her company's network display station-featuring a 16-inch display screen-will be formally announced in January, with volume production planned for March.


Copyright 1988 CMP Publications, Inc. All rights reserved.