AS/400

AS/400 is a computer system from IBM. What does AS stands for. Well. That has been changing. Advanced Server, Advanced Series. What it is is a midrange computer family from IBM. It relies on five basic system concepts:

• Layered machine architecture. For example: Programs can only see down to Machine Interface, MI, which is defined in the operating system. Depending on the machine the functionalities can be implemented in hardware or in software. An old big machine can have the editor implemented on silicone, while others have it implemented as a normal program.
• Object-orientation. Everything in the system is an object. As always objects contains data and methods. There is no need for the user of an object to know how data is stored nor how the methods are implemented.
• Single-level storage. Objects are stored in memory - period. When you create an object, it is stored in memory. Primary memory? Secondary memory? It is stored in secondary memory (hard disk or DASD - Direct Access Storage Device). If it needs to be in RAM it is loaded automatically. The object only got one adress regardless if it is in RAM or on hard disk (or on which hard disk).
• Hierarchy if microprocessors. There is CPUs (or System Processor) and there is microprocessors. The microprocessors can handle I/O to disk, tape, serial lines, etc. They do that while the CPUs are working with something else. An AS/400 can have hundreds of processors.
• Operating System. The OS/400 is a single entity, intagrating all the software components (relational database, communications, networking, online education, context-sensitive online help, programming languages).

A program built for one AS/400 works on every AS/400. A program built for E02 with CISC processor works equally well on a 700 with RISC processor. So be it that the CISC uses 48 bits adressing and the RISC uses 64-bits adressing. The E02 was available in 1992 and the latest AS/400 700-series in 2000. (And before the E-series there was the D-series, C-series, B-series).