IBM POWER SYSTEM evolved from the old AS400

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March 28, 2023

IBM POWER SYSTEM evolved from the old AS400

By NickLitten

March 28, 2023


The IBM AS/400 was one of the most robust operating systems of the late 20th century.

Fast forward to 2023 and the grandson of the AS/400 is the IBM Power System. This new generation of IBM BUSINESS SERVER is running the IBM i Operating System. IBM i is fully backwards compatible to all the old legacy applications from the old iSeries, and ancient AS/400, servers.

Ibm power system

How does the IBM Power Systems Work?

The AS400 first became famous for running an operating system (OS/400) which had an independent machine layer between the hardware and the processor. This means the language the programmers worked with, sits on top of that layer. This lower layer – is the machine code that controls how the computer hardware works.

This core design has stayed with all the models of IBM System that followed the AS400. The iSeries, System i and Power System are all designed with this machine layer separation at its core.

IBM Power System is a strategic asset and benchmark of reliability for thousands of organizations worlwide.

For heavy-duty data processing, organizations rely on IBM Power Systems running the IBM i Operating system.

What are the differences between AS400 and iSeries and IBM i?


The IBM Power System was preceded by the IBM System i, iSeries and AS/400

With lots of generations of hardware, and the operating system software that powers those boxes, there are many versions of this machine from the original giant creamy coloured AS400 systems (that were the size of a car) to modern black rack-mounted Power Servers that are thousands of times faster.

I personally wish IBM had stuck with the AS/400 name and perhaps incremented each new generation of machine with a new number rather than a new name so

  • AS400 – The OG
  • iSeries – AS/500
  • System i – AS/600
  • Power System – AS/700

There are actually many other models in that list (the AS400 eServer, Portable iSeries, etc) but for clarity this list explains the major hardware models. IMHO of course.


The original operating system was OS/400 from 1989 onwards

This was rebranded as i5/Os around 2004 but remained essentially the same, but with lots of new internet focused functions.

The current operating system is IBM i and its fully backward compatible to legacy OS400, i5/OS and other flavours of IBM operating system — it’s amazing that we can pick up an AS400 application and restore it on an IBM i System that is 40 years newer and it will just run. #amazeballs

Here in 2023, I still hear people incorrectly calling their Power System and iSeries. Some of the folks that are not in I.T. still call it an AS400 – because their clunky old greens screens have looked basically the same for twenty years 🙂

I have less time for people working in I.T. incorrectly calling their IBM Power System an AS400 or iSeries. That’s just ridiculous. Do they call call their Ford motor cars “The Model T” ?

How do we use the IBM Power System?

Ibm 5250 screen terminal

Before desktop PC’s were common – the AS/400 used networked terminals. Fat clunky things – the most famous was the IBM 5250 with a fantastic mechanical keyboard. These terminals were a black screen with green text only. After a few years, new models came with coloured text on the screen gasp

Roll forward 30 years and these clunky old terminals are dead.

We have now Terminal Emulator software that runs on any of our devices.

Commonly called 5250 Emulators – there are many to choose from. They all look very similar but offer slightly different functions.

These are my favourite three, which I used on a daily basis:

  • The Rational Developer Integrated Emulator – basic, functional and easy to do quick tasks while writing code in RDi
  • Mochasoft 5250 Emulator – I really like the font of the display. This is simple streamlined and quick to use
  • IBM i ACS – the multi-tabbed experience is excellent along with the integrated IFS & SQL functions
  • HI Nick, I am new to your team.
    Last 10 years I have worked with many different domain clients. Approx. 2 out of 10 are up to date with the Power System but rest of them still using the word AS/400. It’s not the fault with the developer/admin who do the ground work, it’s the fault of managers who even treat the Mainframe resource as IBMi resource and hire them and make their life horrible.
    Some of my clients don’t want to go beyond CLP & COBOL. Although I have given 2-3 RPGLE programs with 50-60 lines which if coded in COBOL and CLP go beyond 500 lines. Suggesting them to call the system as Power System is a very difficult task.
    I wish IBM sales team can do some eye catchy advertisement for this server, and it will reach the graduate level student.

    • (Former sysadmin for several companies and IBM Rochester Level2/Supportline rep)

      Several times, in and outside of IBM, the customers’ number one complaint against IBM was the marketing department. Just the fact you could run databases with tools, programs, and procedures that the companies have used for DECADES, but new tools could be added, we felt IBM marketing missed the boat.

      So many of us were chomping at the bit with the realization that even Microsoft was using AS/400’s and we could have used that as the best marketing message.

      Here it is, thirty plus years later, and the IBM i and Power Systems are still continuing onward.

      My first AS/400 was a B10 sitting under my desk, running pre-release V1R1M2.

  • To most of us, developers and also users, the system is still called as400.
    The users see still green screens, like they did 30 years ago.
    I too wished IBM would have used different names, like as500, as600,…
    For users, maybe they heard sometimes they was an upgrade planned for their system. But, after the upgrades, they still saw they same programs, the same screens, so why suddenly change the name. Probably the it department didn’t even mention the new name.
    Commercially, Ibm did a bad job. As400 is an amazing system, but IBM didn’t advertise it enough.
    Bad thing about compatibility: as a developer, I need to change even s36 programs on as400, in 2023! ? And a minute later on the same system, I’m working with Ile rpg…

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