AS/400

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The AS/400 is an IBM minicomputer for small business and departmental users, released in 1988 and still in production under the name iSeries. The AS/400 is an object oriented system with an integrated database that was designed to implement E.F.Codd's relational database model in the operating system and hardware . All software necessary to run this computer is included and integrated.

The iSeries/AS400 is the follow-on to the System/38 database machine initially released in 1985. AS/400 (known as Silverlake) was released in 1989, and the product line was refreshed and named iSeries in 2000.

Features include an extremely fast DBMS, a menu-driven interface, multi-user support, terminals (IBM 5250) and printers, security, communications and an extensive library-based operating system, OS/400. The big selling point is that applications can run without modification on any model in the product line, from single-user development machines to 8- or 16-way multiprocessor clusters.

The machine was originally based on a custom IBM CISC CPU, but was later migrated to a PowerPC based RISC CPU family eventually known as RS64. The latest models are based on the POWER5 (announced 4 May 2004) processor.

The machine survives because the Hardware abstraction layer of its microcode (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. This means that a program written and compiled on a S/38 can be run as a native 64 bit program. The HAL allows a system that costs $9000 to run the exact same operating system and software as a $2 million system.

The AS/400's TIMI enforces the use of a virtual instruction set for all user mode programs - it is not possible for such programs to utilise the instruction set of the underlying CPU - thus ensuring hardware independence. Therefore the AS/400 can be regarded as the precursor (and possibly inspiration) of modern virtual machine based environments such as SmallTalk, Java and .NET. The AS/400's instruction set defines all pointers as 128-bit to allow seamless transition from older to new processor generations.

Another feature that was recently introduced to the AS/400, LPAR, came from IBM's mainframe line of computers. LPAR (Logical PARtitioning) facilitates running multiple instances of operating systems simulataneously on one AS/400 unit. A system setup with LPAR can even run different operating systems on different partitions while ensuring that one OS cannot run over the memory or resources of another. Each LPAR is given a portion of system resources (memory, hard disk space, and CPU time) via a system of weights that determines where unused resources are allocated at any given time. Some of the OSes supported (and commonly used) under the LPAR scheme are OS/400, AIX, Linux, and Windows NT.

Additionally, tasks such as handling record locks and queuing updates are managed automatically by the system, making high-speed, multiuser applications easy to create, maintain, and extremely reliable.

Programming languages available for this machine include RPG, assembly language, C, C++, Java, COBOL, SQL, BASIC, PHP, and REXX. Several CASE tools are available: Synon, AS/SET, and Lansa.

Commands in the Command Language (CL) are promptable and most provide cursor sensitive help to make entering parameters easier. A command consists of a three letter verb and a subject part. This is intended to make it easy to understand CL code. Examples:

CRTUSRPRF - Create user profile
DLTLIB - Delete lib
CPYF - Copy file

It was designed as the successor of the IBM System/38 and the IBM System/36. The programmers that worked on OS/400, the operating system of the AS/400, did not have a UNIX background. Dr. Frank Soltis, the head architect, says that this is the main difference between this and any other operating system.

Despite the lack of "UNIX-Y" background, the AS/400 has over the years, picked up the programming/runtime model found on UNIX, Linux, and PC systems. Traditional AS/400 programming is a "one-stop shop", where a programmer writes computer code, compiles the code, and then executes the code. There is no link step that is found in other environments.

However, in 1995, that changed. IBM re-christened the one-stop shop programming style "OPM" (for Original Programming Model) and introduced a new language paradigm called "ILE" (for Integrated Language Environment). ILE had significant enhancements over OPM, including the ability to create modules (similar to .obj or .lib files), and then bind (link) the modules together into a single executable. The executable could be created as a program or a service program (service programs are similar to .lib or .dll files).

The real power of the ILE environment is in the "integrated" aspect, however. Modules in ILE-compliant languages (RPG, COBOL, C, C++, and CL) could be created and bound together. For the first time, AS/400 programmers could exploit the strengths of each ILE-compliant language and create programs. Also, with the introduction of service programs, standard routines could be externalized more easily, and increase modularity. To ensure proper migration to the ILE environment, OPM RPG and COBOL programs could be migrated to ILE easily.

AS/400 articles

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