AS/400

The Application System/400 (also known as AS/400), now System i (also known as iSeries), is a type of computer produced by IBM. It was first produced in 1988. It was then renamed to the eServer iSeries in 2000 as part of IBM's e-Server branding initiative. more...

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Now with the global move of the server and storage brands to the System brand with the Systems Agenda, the family has been renamed to System i in 2006, with the POWER5-based members of the series being called the System i5.

Features

The AS/400 is an object-based system with an integrated DB2 database that was designed to implement E. F. Codd's relational database model, which is based on Codd's 12 rules, in the operating system and hardware.

Instruction set

The AS/400 and its successors survive because their instruction set (called TIMI for "Technology Independent Machine Interface" by IBM) allows the operating system and application programs to take advantage of advances in hardware and software without recompilation. TIMI is a virtual instruction set; it is not the instruction set of the underlying CPU. All user-mode programs are stored as TIMI instructions, which means that it is not possible for them to use the instruction set of the underlying CPU, thus ensuring hardware independence. This is conceptually somewhat similar to the virtual machine architecture of programming environments such as Smalltalk, Java and .NET. The key difference is that it is embedded so deeply into the AS/400's design as to make all applications and even the bulk of its operating systems binary-compatible across different processor families.

Note that, unlike some other virtual-machine architectures in which the virtual instructions are interpreted at runtime, TIMI instructions are never interpreted. They constitute an intermediate compile time step and are translated into the processor's instruction set as the final compilation step. The TIMI instructions are stored within the final program object, in addition to the executable machine instructions. This is how a program compiled on one processor family (e.g. the original CISC AS/400 processors) can be moved to a new processor (e.g. PowerPC) without re-compilation. The program is saved from the old machine and restored onto the new machine, whereupon the operating system discards the old machine instructions and re-translates the TIMI instructions into machine instructions for the new processor.

The AS/400's instruction set defines all pointers as 128-bit. This is one of the surviving features of the System/38, which used the 128-bit address space to enable all devices, including disk and tape storage, to be directly addressable.

The S/38 used 48 bit addressing. This was expanded to 64 bits with the original release of the AS/400.

I/O architecture

An AS/400 system is actually an intelligent network of computers: database I/O is handled not by direct constant intervention of the CPU, but instead by processors dedicated to database and "channel" I/O. Likewise, the interactive terminal sessions are offloaded to a workstation processor. In this way, the apparently low-power/low clock-speed CPU is really not a limiting factor in the overall performance of the system - and performance can be modified to meet requirements: no terminal I/O processors and more disk subsystem for a dedicated web server, for example.

Read more at Wikipedia.org