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What’s the Difference Between AS400 and Mainframe?

Anna Thorsen

Anna Thorsen

There’s a lot of confusion surrounding the differences between IBM AS/400 and mainframe. We’re here to clear the air and explain in simple terms what those differences are.

In short,IBM AS/400 (rebranded as the iSeries) is a mid-range server.

A mainframe> (such as one from the IBM Z systems) is a high-end server.

What is a mid-range server?

Now referred to as iSeries (previously known as AS/400), this mid-range server is a more affordable but less powerful option than a high-end server like mainframe.

It was first produced with small-to-medium-sized businesses in mind. They needed less processing power than a mainframe, but more so than a standard commodity server that you may be familiar with today.

Perhaps the biggest difference between the two systems comes down to scale. As precisely puts it, “a mid-range server is a standalone system” whereas a mainframe is “a distributed network of components which interact and form a massive computing platform”.

What is a high-end server?

A high-end server comes from IBM (although not all do) and it is used to store large amounts of data and run centralization applications. Most banks and financial organizations still use high-end servers (mainframes) because they are reliable and secure.

Other organizations that have been operational for the last 30+ years will also likely be operating on a mainframe for the same reason.

They’ve been around for the longest, and they use a different operating system (Linux or z/OS) than mid-range servers.


To learn more about the different terminology used when talking about green screen or mainframe test automation, check out our blog post. In the post on what is mainframe automation, you'll find information on the difference between mainframes, mid-range servers, mainframe emulation software, and green screen/terminal emulation software.

Why should you test a mainframe and midrange server?

As with any business reliant on software or hardware for running their business, they want to minimize risk. Potentially with test automation.

Mainframe or mid-range servers, while secure, are still prone to risk whenever systems are updated, especially when they are operating alongside modernized applications like Salesforce.

This makes testing a crucial step to ensure the continued quality of your system.

You can learn more about testing mainframes and mid-range servers with automation in the linked post, or you can download our guide on how to get started with test automation for mainframe and mid-range servers.

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