Lower-cost but more powerful than its predecessors, the AS/400 was extremely successful at launch, with an estimated 111,000 installed by the end of 1990 and annual revenue reaching $14 billion that year,[1] increasing to 250,000 systems by 1994,[2] and about 500,000 shipped by 1997.

Early systems were based on a 48-bit CISC instruction set architecture known as the Internal Microprogrammed Interface (IMPI), originally developed for the System/38.

[4] In 1991, the company introduced a new version of the system running on a series of 64-bit PowerPC-derived CPUs, the IBM RS64 family.

The Fort Knox project proved to be overly ambitious, and ran into multiple delays and changes of scope.

[12] When IBM's engineers attempted to port the operating systems and software of their existing platforms, they discovered that it would be impossible without making extensive changes to the Iliad processor for each individual operating system – changes which the Iliad's architects were unwilling to make.

[12][15] Silverlake's goal was to deliver a replacement for the System/36 and System/38 in as short of a timeframe as possible, as the Fort Knox project had stalled new product development at Rochester, leaving IBM without a competitive midrange system.

In 1990, IBM Rochester began work to replace the AS/400's original System/38-derived 48-bit CISC processors with a 96-bit architecture known as C-RISC (Commercial RISC).

In 1991, at the request of IBM president Jack Kuehler, a team under the leadership of Frank Soltis delivered a proposal to adapt the 64-bit PowerPC architecture to support the needs of the AS/400 platform.

[22] IBM initially attempted to create a single PowerPC implementation for both AS/400 and high-end RS/6000 systems known as Belatrix.

The AS/400 family line was rebranded several times in the 1990s and 2000s as IBM introduced newer generations of hardware and operating system.

Although announced in 1988, the AS/400 remains IBM's most recent major architectural shift that was developed wholly internally[citation needed].

(At the time, many of Microsoft's business and financial systems ran on the AS/400 platform, rumored as ended around 1999 with the introduction of Windows 2000.

[31] The high-level instruction set (called TIMI for "Technology Independent Machine Interface" by IBM), allows application programs to take advantage of advances in hardware and software without recompilation.

This was the original design feature of the System/38 (S/38) in the mid 1970s planning for future use of faster processors, memory and an expanded address space.

Many of the advanced features associated with the AS/400 are implemented in the operating system as opposed to the underlying hardware, which changed significantly throughout the life of the AS/400 platform.

Unlike the "everything is a file" principle of Unix and its derivatives, on IBM i everything is an object (with built-in persistence and garbage collection).

SSP guests were supported using emulation from OS/400 V3R6 through V4R4 using the Advanced 36 Machine facility of the operating system, a feature distinct from the System/36 Environment compatibility layer which requires System/36 software to be recompiled.