AS/400 (IBM i)

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Stands for: IBM Application System/400, the midrange computer of 1988, and its line to the iSeries, System i and IBM i on Power Systems

IBM's Application System/400, the midrange computer announced in June 1988 and released that August, whose operating system and architecture survive today as IBM i on Power Systems: a single-level store, an object-based machine interface and an integrated relational database that have kept forty-year-old applications running on current hardware.

The AS/400 was the Silverlake system, built by a skunkworks at IBM Rochester, Minnesota, named after a lake in the town and officially under way from December 1985 after the company-wide Fort Knox project to unify its midrange lines had failed; IBM announced it on 21 June 1988 with more than 1,000 software packages and released it in August as the successor to the System/36 and System/38. Its operating system, OS/400, descends from the System/38's and was released on 26 August 1988; it became i5/OS in 2004 and IBM i in 2008, the i standing for integrated. The architecture is the reason the line still exists: the Technology Independent Machine Interface presents applications with a platform-independent instruction set that is translated to native code below it, so that when IBM replaced the 48-bit IMPI processors with 64-bit PowerPC-derived chips in 1995 the applications did not notice, and again when POWER4 arrived in 2001. By the end of 1990 an estimated 111,000 systems were installed and the line earned $14 billion that year; about 500,000 had shipped by 1997. The hardware was renamed AS/400e in 1997, eServer iSeries in 2000, eServer i5 in 2004 and System i in 2006, and in 2008 it merged with the System p into IBM Power Systems. IBM i 7.6 shipped on 18 April 2025, with RPM and YUM packaging for open-source software and TN5250 still carrying the green screens.

The AS/400 is the computer that proved an architecture could outlive every piece of hardware it ran on. IBM's Application System/400, in the encyclopaedia's account, was a family of midrange computers announced in June 1988, on 21 June as the Silverlake system with more than 1,000 software packages available at launch, and released in August of that year as the successor to the System/36 and the System/38. It came out of IBM Rochester, in Minnesota, where a skunkworks named for Silver Lake in the town had been officially at work since December 1985 after Fort Knox, the company's attempt from 1982 to consolidate the System/36, System/38, 8100, Series/1 and 4300 lines on a single 801-based processor, had failed. The numbers that followed explain why the name still carries weight: an estimated 111,000 installations by the end of 1990 and annual revenue of $14 billion that year, 250,000 systems by 1994 and about 500,000 shipped by 1997.

The idea underneath is the Technology Independent Machine Interface, TIMI, inherited from the System/38. Applications are compiled to a platform-independent instruction set and translated to the processor's native code below that interface, so that the processor can change without the applications being recompiled or even knowing. The article records the proof. The first machines used the 48-bit CISC IMPI processors of the System/38 lineage; in 1995 IBM moved the line to 64-bit PowerPC-derived processors, the RS64 family, following Frank Soltis's 1991 proposal, made at Jack Kuehler's request, to adopt a PowerPC variant for the AS/400 after an in-house design had been cancelled; POWER4 followed in 2001, then POWER5 and POWER6; and through all of it the applications written for the 1988 machine kept running. Few platforms can make that claim across a change of word size and instruction set, and it is the reason so much of the world's inventory, banking and manufacturing still runs on this line.

The operating system carries the other half of the design. OS/400, released on 26 August 1988, is described in the IBM i article as an evolution of the System/38's Control Program Facility with compatibility layers for the System/36's SSP and for AIX; it exposes the Machine Interface for hardware independence, uses object-based addressing on a single-level store, in which memory and disk form one address space and every thing on the system is a typed object, and integrates a relational database so tightly that the database is not a product installed on the system but a property of it. It was renamed i5/OS in 2004, when the hardware became the eServer i5, and IBM i in 2008, "the i standing for integrated". Fixes arrive as PTFs, program temporary fixes; open-source software arrives through RPM packages and YUM; and the terminal protocol of the green screen is TN5250, which is why that number appears in firewall rules at companies that have never bought a terminal.

The hardware's names changed with IBM's marketing. The AS/400 Advanced Series came in 1994 and the AS/400e in 1997; in 2000 the line became the eServer iSeries, sharing the POWER4 processor with the pSeries; in 2004 the eServer i5; in 2006 the System i; and in 2008 IBM consolidated the System i and the System p into IBM Power Systems, so that IBM i and AIX share one hardware line. The infobox of the AS/400 article records the original line as discontinued on 30 September 2013, which is a statement about the name rather than the platform: IBM i 7.6 was released on 18 April 2025, and the RS/6000, announced in February 1990 as an internal competitor, became the sibling it now shares hardware with.

For a reader of this glossary the AS/400 sits beside AIX, OpenVMS and the Unixes as one of the long-lived enterprise platforms a network engineer meets rather than administers: the host behind a TN5250 session, the system whose batch jobs define the maintenance window, the box in the corner of the data centre that cannot be patched on the project's timetable because it is the business. IBM's industry record carries the company's side of the story.

Also known as: as400, as/400, iseries, system i, ibm i, os/400, i5/os, silverlake, tn5250, 5250

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