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The IBM AS/400 (Application System/400) is a family of midrange computers from IBM announced in June 1988 and released in August 1988. It was the successor to the System/36 and System/38 platforms, and ran the OS/400 operating system. 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, increasing to 250,000 systems by 1994, and about 500,000 shipped by 1997.A key concept in the AS/400 platform is Technology Independent Machine Interface (TIMI), a platform-independent instruction set architecture (ISA) that is compiled along with the native machine language instructions. The platform has used this capability to change the underlying processor architecture without breaking application compatibility. 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. 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. Due to the use of TIMI, applications for the original CISC-based programs continued to run on the new systems without modification. The RS64 was replaced with POWER4 processors in 2001, which was followed by POWER5 and POWER6 in later upgrades.
The AS/400 went through multiple re-branding exercises, finally becoming the System i in 2006. In 2008, IBM consolidated the separate System i and System p product lines (which had mostly identical hardware by that point) into a single product line named IBM Power Systems. The name "AS/400" is sometimes used informally to refer to the IBM i operating system running on modern Power Systems hardware.

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