AS400 OS
The AS400 OS was originally called OS/400. Today the operating system in that same platform lineage is IBM i, running on IBM Power Systems hardware.
The AS400 OS was originally called OS/400. IBM later renamed the operating system i5/OS, then IBM i. Today, IBM i is the current operating system for the AS400 platform lineage, and it runs on IBM Power Systems hardware.
What Is the AS400 OS Called Today?
The current name is IBM i. Older terms still show up in documentation, job postings, support tickets, and search queries:
- OS/400: the operating system name used with the original AS/400 era
- i5/OS: the name used during part of the iSeries and System i era
- IBM i: the current operating system name, introduced when IBM separated the OS name from the hardware brand
When someone says AS400 OS today, they usually mean IBM i, even if the system is running on modern IBM Power hardware rather than an original AS/400 machine.
Why the AS400 OS Is Different
The AS400 OS lineage was built as an integrated business operating environment. It combines the operating system, object model, security model, work management, and DB2 for i database in a tightly integrated platform. That is different from a typical server stack where the database, security model, job scheduling, and application runtime are assembled from separate products.
This integration is a major reason the platform persists. Applications can run for decades because the OS, database, and application model were designed as one controlled environment.
How IBM i Runs Day to Day
IBM i organizes work through jobs, job queues, subsystems, libraries, object types, user profiles, authority settings, and PTF maintenance. Users may still access applications through 5250 green-screen sessions, but the same system can also expose APIs, web interfaces, SQL access, Java, Python, PHP, Node.js, and modern integration services.
For a deeper daily-operations view, read IBM i Operating System Explained. For release history, read IBM i Release History.
AS400 OS vs IBM i
AS400 OS is the legacy phrase. IBM i is the current product name. They belong to the same lineage, but the wording matters when you are buying support, checking compatibility, planning an upgrade, or reading IBM documentation.
If a vendor asks for your AS400 OS version, provide the IBM i release and Technology Refresh level if possible. For example, IBM i 7.4 TR12, IBM i 7.5 TR8, or IBM i 7.6 TR2 tells a technical support team much more than simply saying AS400.
What Hardware Runs the AS400 OS Today?
Modern IBM i runs on supported IBM Power Systems hardware. IBM's current Power platform supports IBM i, IBM AIX, and Linux on Power. The exact IBM i release and PTF level needed depends on the server generation and model.
For hardware context, read IBM Power Systems Overview. For the terminology distinction, read AS400 vs IBM i.