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X.21:  International standard for very fast digital links. Supported in some PTT services (e.g., BT Kilostream in the UK).

X.25:  ITU-T international (non-IBM) standard for attachment to packet-switched networks. Supported by IBM (without any great enthusiasm) on several systems. See packet switching.

X.25 NCP Packet Switching Interface:  See NPSI.

X.400:  Series of draft international standards for message handling systems, including e-mail, fax, and image/graphics transmission. Ratified by the ITU-T in 1984. X.400 support is available from IBM through IBM Interchange Services for e-business, GTMOSI, OSME, X.400 Connection, X.400 MTF, IPMS, ONDS, and on the iSeries 400.

X.400 Connection:  IBM z/OS and z/VM software for handling IBM format messages, converting them to X.400 and routing them across an EDI¹ system through OSME. Supports PROFS, DISOSS, OfficeVision, PS/CICS. Withdrawn 1995.

X.400 MTA:  X.400 Message Transfer Agent. A sort of OSI equivalent of SNADS – both SNADS and MTA provide a general purpose, asynchronous, distribution service.

X.400 MTF:  X.400 Message Transfer Facility. First product announced by IBM to span all seven layers of the OSI model. Allows user applications running on z/OS systems to participate in message handling systems with other vendors’ systems that are X.400 compliant. Part of ONDS, withdrawn July 1995.

X.500:  Standard for directory services for electronic mail systems. IBM supports X.500.

X.509:  An international standard that defines the contents of digital certificates. See CRMF.

x/EFS:  xSeries Enabled For S/390. Withdrawn September 2001.

X/Open:  Originally a group of vendors (including IBM) which joined up to set up standards (particularly for Unix). Unlike UII and OSF, X/Open tried not to be partisan in the Unix battle. October 1993, Novell, the then owner of Unix, handed control of the Unix standard lock, stock, and barrel to X/Open, which then tested and branded implementations claiming to be Unix systems. November 1995, X/Open started cozying up to the OSF with the declared aim of making the two organizations appear as two sister divisions of a virtual corporation. The Open Group was the result. See also XPGn.

X/WITT:  See WITT.

X11R6:  X-Consortium release 6 of X. Defines how X-applications will work. See X protocol.

XA¹:  Extended Architecture. See also MVS/XA, VM/XA SF, VM/XA SP.

XA²:  An X/Open standard for distributed transaction processing which defines the interface between a database and a TP monitor. XA describes a standard protocol for transaction co-ordination, commitment, and recovery between a transaction manager and one or more resource managers (such as databases).

XAMO:  XA Migration Option.

X-architecture:  IBM design blueprint for the eserver xSeries product line.

XC:  See ESA/XC.

XCA:  eXternal Communication Adapter. A communication adapter that is not part of the host processor (such as the 3172).

XCF:  Cross System (coupling) Facility. Software, introduced in MVS/ESA SP4, which allows programs to communicate channel-to-channel, peer-to-peer, across a sysplex – i.e., it’s the basic sysplex-enabling bit of z/OS. XCF supports program communication, and sends program status and signal information between z/OS system images in a sysplex.

XDF:  See ESCON XDF.

XDR:  External Data Representation.

xDSL:  Any of the variations of DSL. See DSL².

XDSM:  X/Open Data Storage Management API specification.

XEDIT:  The much-loved z/VM CMS¹ editor (strictly it’s the name of the CMS command used to call up the editor). Different enough from the ISPF/PDF editor to be truly annoying to visitors from z/OS.

Xenix:  A Unix-alike developed by Microsoft. At one time available from IBM for the PC. SCO (Santa Cruz Operations, now part of Caldera International) ended up with Xenix, but have put all their efforts behind UnixWare, which they acquired from Novell.

Xerox:  Everybody else knows them from their photocopiers, but computer people are just as likely to think of Xerox PARC, founded in 1970, and still in operation. There, the mouse, the GUI, client/server architecture, laser printing and the Ethernet/LAN were all born. In a company with executives totally focused on paper and business machines, it took young outsiders like Steve Jobs and Bill Gates to visit PARC and take away the ideas for their own companies: Apple and Microsoft. Xerox did try. Their D series computers pre-empted the PCs of the 1980s by a decade. And a Xerox word processor had a paper shaped monitor that displayed an entire page at once, but no one would pay more the $10,000 per station for it. Today, few remember Xerox was ever in the computer field at all.

Xerox PARC:  See Xerox.

XGA:  eXtended Graphics Array. Enhanced VGA system first introduced for 486-based PS/2s. Resolution of 1024 x 768 pixels and graphics performance ten times that of the VGA.

XGA-2:  Upgraded version of the XGA standard announced October 1992. Claimed to be two and a half times as fast as the original XGA and four times as fast as SVGA.

XHTML:  Extensible Hypertext Markup Language, a reformulation of HTML in XML. A W3C recommendation, XHTML is simply HTML rewritten using XML syntax.

XI:  X.25 SNA Interconnection.

XL Fortran:  AIX¹ FORTRAN compiler that supports the Open MP FORTRAN API, SMP³, direct manipulation of the floating point status and control register, 64-bit architecture, asynchronous I/O, debug memory routines and the FORTRAN 95 standard.

XL High Performance Fortran:  AIX¹ FORTRAN compiler that supports parallel processing of large applications in a multiple workstation environment.

XL HPF:  See XL High Performance Fortran.

XMI:  XML Metadata Interchange, an OMG specification that provides a standard way of exchanging metadata between software tools, repositories and data warehouses. XMI is based on XML, UML and MOF.

XMIT:  Transmit.

XML:  eXtensible Mark-up Language. A subset of SGML and a W3C standard. Widely accepted as a replacement for traditional EDI¹ for exchanging data between applications over the Internet. Unlike HTML, XML is self-defining in the sense that, when you look at an XML file, most of what you see are field names in XML tags followed immediately by a field value.

XMP:  X/Open Management Protocol. A network/system management API developed by the OSF for the Distributed Management Environment (DME). Supported by IBM within NetView.

XMS:  Cross Memory Services. See CMS².

XNS:  Xerox Network Systems. A group of Internet protocols, similar to TCP/IP but developed by the Xerox Corporation. XNS uses different packet formats and terminology. See IPX.

X-off:  Transmitter off.

X-on:  Transmitter on.

X-Open Software License Manager:  Standard from The Open Group, which defines a software license use management system.

XPGn:  X/Open Portability Guide n. A standard for open systems. If X/Open believed that your product conformed to the appropriate standard, it gave the product X/Open branding.

X protocol:  The protocol rules for writing programs for with the X Window System.

XRC:  eXtended Remote Copy. See Remote copy.

XRF:  eXtended Recovery Facility. Feature available with DB2, IMS¹ and CICS, providing vastly increased availability for mission-critical applications. XRF creates a mirror system synchronized with the main system; the mirror image can take over immediately in the event of a failure in the primary system (in many, but not all, cases). Can also be used for planned machine maintenance by purposely switching to a backup system while working on the main system. See also mirroring.

xSeries:  IBM eserver series of Intel architecture servers running Linux, Novell NetWare and Windows operating systems. Announced October 3, 2000, with up to 8 SMP³ servers.

XSL:  eXtensible Stylesheet Language. A W3C standard used with XML. XSL comprises a language for transforming XML documents into another markup language format, together with a vocabulary for specifying formatting semantics. Commonly used to convert XML data to HTML for presentation in a Web browser environment. An XSL processor is one of the suite of XML application enablers delivered with OS/400.

XSLM:  See X-Open Software License Manager.

XSSO:  X Single Sign-On.

XSSO/PAM:  X Single Sign-On/Pluggable Authentication Mechanism.

Xstation:  Low-priced family of X-Windows terminals for attachment to a LAN (Ethernet or TRN). Sold for DTP¹, CASE¹, low-end CAD, and the like – not high-performance CAD. Based on technology bought in from X-terminal specialist Network Computing Devices. Early 1996, IBM stopped selling the things under its own name, and handed the business back to Network Computing Devices.

xterm:  A terminal emulation program for the X Window System. Allows the user to have several different invocations of xterm running at once on the same display, each of which provides independent input and output for the process running in it.

X-Terminal:  A type of terminal developed in the Unix world which provides a GUI type environment (usually X-Windows) without the need for a programmable workstation. The X-Terminal is responsible solely for painting the screen – a host decides what to paint (Note that the workstation is called the server, and that the host is called the client, which is the opposite of what everybody else (including common sense) says). The X-Terminal typically has no disk, but has a lot of memory (at least 500KB, and possibly 2MB) and a specialized processor. Sounds like a good idea but the problem then was that, in some applications, each X-Terminal seemed to need 2MB in the host as well, if it’s to perform adequately. By the mid 1990s the idea was going rapidly out of fashion. Early 1996 IBM passed all of its X-Terminal business over to Network Computing Devices. See also Xstation.

XTP:  Xpress Transport Protocol. Data transmission protocol for high-speed links (over 75Mbps) that reached its peak of popularity in the early 1990s.

X-Windows:  Software system for creating GUIs on X-Terminals. Developed originally at MIT (as part of the DEC/IBM/MIT Project Athena) but picked up by the wider Unix community. Note that X-Windows is not an actual GUI, it is a set of standards and tools from which GUIs can be specified and built. Has been an influence on the Motif GUI being developed by the OSF. IBM was involved in the evolution of X-Windows through the MIT X Consortium which provided a forum for discussion of the standard. See also X-Terminal, X Window System.

X Window System:  A specification for device-independent windowing operations on bit map display devices. It was developed initially at MIT as part of the DEC/IBM/MIT Project Athena and is now a de facto standard supported by the X Consortium. X uses a client/server protocol, the X protocol. The server is the computer or X terminal with the screen, keyboard, mouse and server program and the clients are application programs. Clients may run on the same computer as the server or on a different computer, communicating over Ethernet via TCP/IP protocols.

XY749:  Obsolete plotter for use with System/3x.

XY750:  Obsolete plotter for use with System/3x.

Xylan:  Switched Ethernet specialist which IBM all but took over mid 1996 in a five-year joint technology, development, sales, and marketing agreement. Alcatel bought Xylan in April 1999.

Xyratex:  Company formed in a management buyout (MBO) from IBM in 1994. Develops high-speed networking, digital broadcasting and data storage products for OEMs.