The relationship between AS400 and iSeries often causes confusion for professionals entering the IBM midrange ecosystem. Many assume these terms describe identical technology, but a closer examination reveals distinct historical and architectural contexts. Understanding the lineage from the original Application System/400 to the modern IBM Power Systems is essential for making informed infrastructure decisions.
Defining the IBM Midrange Lineage
To clarify the confusion, one must look at the chronological evolution of IBM naming conventions. The AS400, which stands for Application System/400, was the original product name launched in 1988. This specific hardware and software bundle represented a revolutionary integrated platform where the operating system, database, and hardware were tightly coupled to deliver exceptional reliability.
From AS400 to iSeries
The term iSeries emerged in 2000 as a marketing rebranding of the AS400 platform. This transition marked a shift toward aligning the midrange servers with the broader IBM branding strategy seen in the pSeries (RISC) and xSeries (x86). While the name changed, the core technology—specifically the OS/400 operating system evolved into IBM i—remained the consistent element that provided the platform's legendary stability.
Technical Continuity and Divergence
Technically speaking, an AS400 running OS/400 is functionally the same logical platform as a modern iSeries running IBM i. The object-based architecture, library list system, and database file structures remain largely compatible. For administrators, the day-to-day operations and command structures feel identical, ensuring a seamless transition for legacy applications.
Modern Context with Power Systems
Following the iSeries, the next evolution was the IBM System p, which utilized POWER processors. This hardware refresh retained the IBM i software, meaning the platform remained distinct from standard IBM xSeries (x86) servers. Today, the hardware is branded as IBM Power Systems, but the logical partition (LPAR) running IBM i is the direct successor to the original AS400 logic, not a completely new system.
Addressing the Core Question
So, is AS400 the same as iSeries? The answer requires nuance. They are not identical physical machines, as the hardware components, rack configurations, and processor technologies differ significantly across decades. However, they are identical in logical function and operational philosophy. The iSeries did not replace the AS400; it inherited and modernized it, ensuring backward compatibility that allows vintage applications to run unmodified on current iron.
Why the Distinction Matters
Understanding that AS400 refers to the historical hardware and iSeries refers to the modern continuation helps businesses plan for migration and support. Calling a current IBM Power server an "AS400" is technically inaccurate in a hardware sense, but referring to the operating system environment as "AS400-like" accurately describes the user experience. This knowledge prevents miscommunication with vendors and ensures that IT departments procure the correct service contracts for the actual hardware platform in use.
