Introduction
The AS400 platform has been around for more than 30 years and like any long-lived technology, it has evolved through several major upgrades and rebrands.
Each stage in its journey, from AS400 to iSeries, and now IBM i, has brought important changes to both hardware and software, that have led to the continued use of the technology.
Although, AS400 started off as a closed system where users were “locked in” to IBM’s proprietary solutions, the platform has become open to integrating and supporting several third-party solutions.
IBM has played catch-up with the advancements, bringing in capabilities to accommodate modern technologies like cloud computing and AI as the platform’s prowess progressed.
AS400 has had many upgrades and rebrands, and this article is all about breaking down the different stages of technology.
Historical Background
The story of IBM’s midrange systems began in 1988 with the launch of the AS400 (Application System/400). It came with its own operating system, OS/400, and was designed as an all-in-one platform with integrated hardware, OS, database, and middleware, all devised by IBM.
AS400 was designed to provide businesses with a reliable and efficient platform for managing and processing data, especially in sectors like manufacturing, logistics, and finance.
In 2000, IBM rebranded AS400 as iSeries which came with major upgrade to its architecture that delivered better performance compared to the previous systems. Additionally, IBM prioritized investment protection with iSeries, meaning customers could keep using their existing AS/400 applications even on new hardware and OS versions.
The next rebrand came in 2006, when IBM combined server product lines AS400, RS600, xSeries and zSeries under the brand names System i, p, x and z respectively. This wasn’t just a naming exercise but reflected a deeper integration across product lines where technologies were shared across platforms.
Finally, in 2008, IBM consolidated the hardware platforms of system i and system p into a single line of servers, now called IBM Power Systems. Instead of maintaining separate physical servers for iSeries (AS400 lineage) and pSeries (UNIX lineage), IBM provided one server line that could run both workloads side by side.
Alongside this hardware shift, IBM introduced the IBM i operating system, an evolution of OS/400, designed to run on Power Systems. This modernization also introduced concurrent capabilities like cloud readiness and AI integration to Power systems while still supporting the legacy applications developed on AS400.
AS400 (Application System/400)
One of the defining traits of the AS400 was that the OS, database, application services, and hardware were tightly coupled. For users, this greatly reduced the need for integrating parts from third-party vendors and simplified system management.
It featured the OS/400 operating system, an object-based OS which became known for its impeccable security and ease of administration. AS400 quickly became a preferred platform for small and midsize businesses to run mission-critical workloads.
The system supported IBM’s proprietary programming languages such as RPG and COBOL. These languages were tailor-made for business logic which made AS400 a stable platform to build and maintain enterprise applications.
Despite its many strengths, AS400 isn’t conducive to accommodating the latest technologies. Classic AS400 systems (or even unmodernized IBM i environments) use proprietary data formats, green-screen UIs, and older communication protocols. These legacy features make it hard to connect with modern technologies like APIs, web applications, cloud platforms, and DevOps pipelines.
However, the architecture with which AS400 was built continues to be the foundation for all the upgrades that came later, be it iSeries or even Power Systems.
iSeries
When IBM launched the iSeries in 2000, as a successor to the AS/400, they upgraded the hardware architecture significantly, most notably the introduction of PowerPC-based processors (now called IBM Power processors) among many other improvements. These upgrades gave the iSeries increased processing power, memory, and storage capacity.
Like its predecessor, the iSeries too continued using OS/400 which was tightly integrated with the hardware and offered backward compatibility for AS/400 applications. Over and above, iSeries could support more modern programming languages (like Java, C++, and web technologies) in addition to RPG and COBOL.
A major strength of the iSeries was its seamless integration in which system components could work together without needing complex setup or third-party service. It could handle mixed workloads and multiple applications without compromising performance.
iSeries also marked a shift towards an open environment where users were no longer bound to IBM-only tools or custom solutions and could integrate with the broader IT world more easily. IBM also introduced support for open standards that made it more flexible, future-proof, and collaborative across IT landscapes.
IBM i
In 2008, IBM made an upgrade to the lineage of AS400 by introducing IBM i, the successor to the operating system OS400 designed to run on Power Systems. This marked a major leap forward.
One of the most significant enhancements in IBM i was the ability to create logical partitions (LPARs) that enable businesses to run different workloads or environments on a single server, which greatly improves resource utilization and cost-efficiency.
Some of salient features of IBMi are its object-based architecture, integrated middleware, integrated database (DB2 for i) and its support for modern programming languages and frameworks like Node.js, Python, and REST APIs.
Security has also been vastly improved with features like role-based access control, audit journaling, and encryption. In addition, IBM i is designed with cloud integration in mind, supporting hybrid IT environments and automation.
IBM i continues to offer backward compatibility, so older AS400 and iSeries apps still run. At the same time, it embraces open-source technologies. It is the foundation of IBM’s midrange strategy that continues to drive their technology and evolve with modern enterprise needs.
Key Differences
The transition from AS400 to iSeries and finally to IBM i has had numerous upgrades to not only hardware but even the system architecture and the underling operating system.
From a hardware perspective, AS400 systems were built on proprietary chips but when iSeries came about, IBM introduced faster processors and more memory. IBM i runs on IBM Power Systems, which use Power processors designed for high-performance computing and virtualization.
In terms of software, the AS400 used OS/400, was stable but had several limitations, especially its lack of support for modern development frameworks. The iSeries improved OS/400 to support Java and web integration while IBM i took a leap forward by facilitating integration with open-source technologies, DevOps tools, and offering seamless connectivity to cloud environments.
Performance and scalability improved significantly across generations. IBM i supports multi-tenant environments and hybrid cloud models, whereas AS400 was strictly on-premise. IBM i’s automation features and security tools far surpass those of its predecessors.
Another critical difference lies in application modernization. While AS400 and iSeries relied heavily on RPG and COBOL, IBM i supports modern languages and APIs which facilitate smoother integration with modern applications.
In essence, IBM i combines the trusted legacy of AS400 with the innovation required for today’s digital ecosystems making it a future-ready platform with a heritage of reliability.
Future Outlook
BM’s midrange systems remain rooted in the technology first introduced with the AS/400 decades ago and continue to evolve and stay relevant. IBM i remains a core offering in IBM’s enterprise portfolio, with ongoing support, updates, and a clear development roadmap.
One of the key strengths of IBM i is its backward compatibility which allows decades-old applications to still run on today’s Power Systems.
IBM i received its recent update in May 2025 with IBM 7.6 which had major security upgrades like MFA (multifactor authentication), QR-based enrollment, and CLI support.
Looking ahead, IBM i is expected to have deeper integration with hybrid cloud platforms, more robust automation features, and expanded support for AI and machine learning workloads. IBM is also strengthening its commitment to the open-source community by encouraging the use of languages like Python, Node.js, and technologies like containerization through Kubernetes.
Organizations that rely on AS400 or iSeries systems face increasing pressure to modernize. Yet, the path forward doesn’t require abandoning their current infrastructure. Instead, with IBM i, businesses can modernize at their own pace, blending stability with innovation.
