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AS/400 System Concepts and Architecture

Why are System Concepts and Architecture important to a business person?

Business leaders do not start by choosing a computer system. They start by choosing an application that fits their business needs. The AS/400 has tens of thousands of good business applications worldwide. Because of that, very often the computer system is considered first.

Why should the AS/400 architecture matter to a business person? This section helps you understand why.

If you compared the identical application running on an AS/400 to a non-AS/400 system, you would continue to choose AS/400. Why? Because, although the two systems can appear to be equivalent today, the accelerating rate of change of both hardware and software technologies necessitates that the system you select has been designed with the future in mind. The AS/400 accommodates inevitable, rapid, and dramatic technology changes with minimum relative effort. Ask any system manufacturer: What is there about your system that is future-oriented? and What has your record been in the past few years, as technologies have changed? We believe the IBM AS/400 will be the number one choice.

Paradoxically, the characteristic of the most advanced design and technology is that you do not notice it ... you are not meant to do so. It accommodates rapidly-changing hardware and software technologies in stride--permitting you to fully exploit the latest technologies.

System Concepts

The AS/400 is designed and built as a total system, fully integrating all the hardware and software components that a business demands. As a general-purpose business and network system, it is optimized for the required environment with these unique benefits:

A concise and expanded explanation of AS/400's architecture is contained in a renowned book " Inside the AS/400 (ISBN: 1-882419-66-9) ", written by the AS/400's Chief Architect, Dr. Frank G. Soltis. The book is now in its second edition, published by Duke Communications International.

AS/400 Advanced Application Architecture

Technology-Independent Machine Interface

The AS/400 is atypical in that it is defined by software, not by hardware. In other words, when a program presents instructions to the machine interface for execution, it "thinks" that the interface is the AS/400 hardware. But it is not! The instructions, presented to that interface, have to pass through a layer of microcode before they can be understood by the hardware itself. This design insulates application programs and their users from changing hardware characteristics through this comprehensive layer of microcode. When a different hardware technology is deployed, IBM rewrites sections of the microcode to absorb the fluctuations in hardware characteristics. As a result, the interface presented to the customer remains the same.

This interface is known as the Technology-Independent Machine Interface (or TIMI). The microcode layer is known as the System Licensed Internal Code (or SLIC). The brilliance of this design was dramatically illustrated when the AS/400 changed its processor technology from CISC (Complex Instruction Set Computing) processors to 64-bit RISC (Reduced Instruction Set Computing) processors in 1995. With any other system, the move from CISC to RISC would involve recompiling (and possibly some rewriting) of programs. Even then, the programs would run in 32-bit mode on the newer 64-bit hardware. This is not so with the AS/400, because of TIMI. Customers were able to save programs off their CISC AS/400s, restore them on their new RISC AS/400s, and the programs would run. Not only did they run, but they were fully 64-bit programs.

As soon as they made this transition, customers had 64-bit application programs that ran on a 64-bit operating system containing a 64-bit relational database that fully exploited the 64-bit RISC hardware.

TIMI and SLIC have just taken 64-bit RISC processor technology in their stride. These same architectural features will be exploited to fully accommodate post-RISC technologies, which may have 96-bit or 128-bit processors.

Many of the frequently-executed routines that, on an ordinary system reside in the operating system, have been moved to the SLIC. Because the SLIC is closer to the silicon, routines placed there run faster than routines placed "higher" in the machine. There is an important performance gain. Examples of some basic supervisory. Therefore, resource management functions that are in SLIC are validity and authorization checks.

Operating System OS/400

One of the single most dramatic things about AS/400 is that its operating system, OS/400, is a single entity. This section describes the meaning of this concept.

Once you buy an AS/400, you do not have to continue shopping for system software components before it is ready to run your business. All of those software components, for relational database, comprehensive security, communications with a broad range of diverse systems, including Internet capabilities, and many more, are already there.

They are all fully integrated into OS/400 (AS/400 operating system). By "fully integrated", we mean fully tested, too. All those components, prerequisites for running business applications in the 1990s, work together and are fully tested together, so that OS/400 operates as a single entity.

An ordinary machine does not have this approach to its operating system design. An ordinary operating system, which does the basic system housekeeping, needs to have a range of software products added to it before the environment is ready to support modern business applications. Examples of this are: software for the relational database, support for various communications environments, software for security, support for an interactive environment, for multimedia, for availability, and recover ability, and so on. On an ordinary machine, these software modules are provided by third parties. A customer has to make sure someone has integrated all these modules and performed the tests necessary to make sure that they all function together. When one of the software components has a new release, a customer needs to make sure that component is replaced, plus any other software modules that it depends on. The modules need to be at compatible release levels. Also, should a software malfunction occur, how do you establish precisely which modules are causing it? Can you be certain that multiple third-party software vendors will agree with your diagnosis when you blame their software?

There are none of these problems with the OS/400. To achieve the functionality that is standard in OS/400, a customer would integrate typically between 10 and 25 different modules of software. OS/400 is installed with all these capabilities as standard. When software is updated, a new release of OS/400 is made available. Customers do not have to install individual system software components, nor do they have to check that new releases can co-exist.

Hierarchy of Microprocessors

The simplified figure on the previous page shows that, as well as its main system processor, AS/400 has a range of other processors, each dedicated to a particular I/O (Input/Output) device type. A single large AS/400 configuration can have well over 200 processors.

The main system processor (which itself can be comprised of 12 separate processors) may encounter a request for data to be read from or written to any I/O device. That request for data is delegated to the particular microprocessor dedicated to that I/O device. Meanwhile, the main system processor continues executing another application program. Nanoseconds (10-9 second) is the unit of time used to measure main storage access times. I/O operations are measured in milliseconds (10-3 second).

This design provides the AS/400 with its outstanding performance in the commercial, transaction-based, environment. The AS/400 is designed for business computing, and one of the main characteristics of that environment is that it is I/O-intensive, rather than compute-intensive.

In addition to the benefit of outstanding performance in the business environment, this design gives AS/400 an elegant method of integrating diverse environments into a single, harmonious customer solution. The microprocessors that look after a particular I/O device are accommodated on I/O cards that fit into slots on the AS/400's system bus. One of these cards may be the Integrated Netfinity Server This is a PC on a card, and enables the AS/400 to run, Windows NT server, for example. The AS/400's Internet firewall capability also exploits the Integrated Netfinity Server .

Single-Level Storage

Just as application programs on an AS/400 are unaware of underlying hardware characteristics because of the TIMI. They are also unaware of the characteristics of any storage devices on the AS/400, because of single-level storage.

As with TIMI, the concept of single-level storage means that the knowledge of the underlying characteristics of hardware devices (in this case, the hardware storage devices-- main storage and disk storage) reside in the SLIC. All of the storage is automatically managed by the system. Programs work with objects (see next section), and objects are accessed by name, never by address. No user intervention is ever needed to take full advantage of any storage technologies.

The AS/400's address size is vast. The AS/400 system can address the number of bytes that 64 bits allows it to address. The value 264 is equal to 18,446,744,073,709,551,616. Therefore, the AS/400 system can address 18,446,744,073,709,551,616 bytes, or 18.4 quintillion bytes. To put this into more meaningful terms, it is twice the number of millimeters in a light-year. Light travels approximately 6,000,000,000,000 miles in one year.

Single-level storage enables another extremely important AS/400 benefit--object persistence. Objects are discussed in the next section of this introduction. Object persistence means that the object continues to exist in the memory system forever. An ordinary machine requires that information be stored in a separate file system if the information is to be shared or if it is to be retained for a long time. Persistence of objects is extremely important for future support of object-oriented databases. Objects need to continue to exist even after their creator goes away. The AS/400 is uniquely positioned to exploit this characteristic of object persistence, where ordinary systems use a less-elegant mechanism that requires them to store their persistent objects in a separate file system, with all the attendant performance implications.

Logical Partitioning (LPAR)

As the performance of an enterprise class server grows, so does the requirement to divide that performance to run multiple workloads independently. Partitioning enterprise servers has become commonplace in the mainframe market since its introduction the mid-1980s. Typically, separate partitions are used for test releases of applications or to service multiple business units or companies from a single server.

The AS/400's implementation is an adaptation of the S/390 logical partitions concept with flexible and granular allocation of system resources. The AS/400 logical partition implementation introduces both the flexibility to allocate interactive performance, and high- speed internal communications between partitions.

Logical partitions (LPAR) enable multiple independent OS/400 instances or partitions in an n-way symmetric multiprocessing AS/400e 7xx, 6xx and Sxx. Each partition requires its own processor(s), memory, disk(s), system console, with a CD-ROM and tape drive that can be allocated to each LPAR. With LPAR, you can address multiple system requirements in a single machine to achieve server consolidation, business unit consolidation, mixed production/test environments, integrated clusters, and more.

All V4R4 systems have a primary partition with all resources initially allocated to it. Creating and managing secondary partitions is performed from the primary partition. Movement of processors, memory, and interactive performance between partitions is achieved with only an IPL of the affected partitions. Movement of IOP resources is achieved without an IPL.

OS/400 is licensed once for the entire system by its normal processor group, regardless of the number of partitions. License management across partitions is not supported. OS/400 V4R4 must be installed on each partition. Previous releases are not supported on any logical partition.

The following Web site is available for installation support and technical guidance. When planning logical partitions for an AS/400 system, rely on this Web site for information, direction and management tips: http://www.as400.ibm.com/lpar/

It is important to understand that a failure in the primary partition affects all of the logical partitions that are created. For example, if the primary partition is powered down, all of the secondary partitions are also powered down.


An object is a container. Everything the system uses--user and system data structures--is packaged in one of these containers. The objects are encapsulated, which means that you cannot see inside. Inseparable from an object is the list of valid ways in which that object can be used. All objects are structured with a common object header, and a type-dependent functional portion. Therefore, on the AS/400, instructions can only work on what they are supposed to work on. You cannot have data treated as executable code (so that the processor tries, for example, to execute someone's shoe-size), or executable code treated as data, (by having something written into the middle of it). Certain instructions apply to all objects, while other instructions work only on specific types of objects. It is not possible to misuse an object, unlike the situation that exists other systems without an object-based approach.

There are two important consequences of an object-based design. The first is that a system built around an object model supports machine independence. This means that technology changes can be made in the environment without affecting application programs. The second consequence is that an object-based design delivers a high level of system integrity.


The AS/400e has the most brilliant architecture found on any business computing system. There are many examples of where the AS/400's architecture has delivered on its promise of making the most advanced technology readily and continuously available to its customers. For example, the AS/400 has enabled its customers to:

The AS/400's future-oriented architecture has enabled us to take rapidly-changing hardware and software technologies in our stride. This same, flexible architecture will continue to serve us well in enabling our customers to continue to deploy the very latest technologies while causing the minimum possible disruption to their work.

by c4 2002-09-07