# OS/400

> operating system for the IBM AS/400

**Wikidata**: [Q1571860](https://www.wikidata.org/wiki/Q1571860)  
**Source**: https://4ort.xyz/entity/os-400

## Summary
OS/400 is the proprietary operating system that IBM created specifically for its AS/400 midrange computer line. It is classified as an operating-system instance and is succeeded by IBM i (formerly styled i5/OS).

## Key Facts
- Developer: IBM (American multinational technology corporation founded 1911-06-16)
- Platform: IBM AS/400 midrange computers
- Instance of: operating system
- Followed by: i5/OS (now IBM i)
- Aliases: I5OS, I5/OS, OS400, IBM i
- Wikidata sitelinks: 7 across ca, de, es, fr, pl, pt, sv Wikipedias
- Google Knowledge Graph ID: /g/120vt2n0
- Quora topic: Os-400
- GitLab topic ID: os400
- Known edition: OS/400 Version 4 Release 4

## FAQs
### Q: What hardware did OS/400 run on?
A: OS/400 ran exclusively on IBM’s AS/400 family of midrange business computers; it was not ported to other hardware.

### Q: Is OS/400 still sold today?
A: No—IBM replaced OS/400 with i5/OS (now IBM i), so modern IBM Power Systems use the newer operating system.

### Q: Are OS/400 and IBM i the same thing?
A: IBM i is the direct successor and incorporates OS/400 technology, but the names and some internals changed over time.

## Why It Matters
OS/400 mattered because it let IBM deliver an integrated hardware-software stack that hid complex system details from business customers. By tightly coupling the OS with the AS/400’s unique TIMI (Technology-Independent Machine Interface) architecture, IBM let companies upgrade processor generations without recompiling applications—a major cost saver in the 1980s-1990s midrange market. The OS’s built-in database (DB2/400), security, and transaction-processing features made it a turnkey choice for manufacturing, finance, and retail firms that needed reliability but lacked large IT staffs. Although the name “OS/400” is retired, its architectural principles survive in IBM i, keeping thousands of legacy applications alive and proving the value of platform continuity in enterprise computing.

## Notable For
- Single-source OS-to-hardware pairing: only ever shipped on AS/400
- Object-based file system that blurred the line between files and programs
- Binary compatibility across CPU upgrades via TIMI layer
- Integrated DB2 database and security from day one
- Renamed twice by IBM—first to i5/OS, then to IBM i—yet retained lineage

## Body
### Origins and Architecture
IBM introduced OS/400 in 1988 alongside the AS/400. The design inherited concepts from the earlier System/38’s CPF, but added 48-bit addressing, a single-level store, and an object-based file system. Every entity—programs, data, even user profiles—was an object with typed interfaces, reducing accidental corruption.

### Releases and Editions
Documented editions include Version 4 Release 4. IBM issued periodic “releases” (V1R1, V2R1, etc.) that added TCP/IP, Java, and later Web-serving capabilities without breaking existing applications.

### Transition to IBM i
In 2004 IBM rebranded OS/400 as i5/OS to align with the eServer i5 hardware. A second rebranding to IBM i in 2006 signaled support for Power Systems and broader openness (PHP, GCC, AIX co-existence). The new name is the current shipping OS, but OS/400 binaries still execute unmodified.

## References

1. Quora