I’m frequently asked the question, "When is an AS/400 not an AS/400?" With all the changes that have occurred to the AS/400 in the past few years, it’s easy to argue that today’s AS/400 hasn’t merely evolved but has transformed into something altogether different. At what point does it cease being an AS/400?

My answer to this question is always the same. The AS/400 isn’t defined by any particular hardware or software configuration. Take away the PowerPC processor technology, and it’s still an AS/400. Take away RPG (a thought most AS/400 customers find revolting!) and 5250 support (another difficult concept for many customers), and you still have an AS/400. In fact, the AS/400 is defined by five fundamental architecture principles, and everything else can change except these sacred principles:

• technology independence
• object-based design
• hardware integration
• software integration
• single-level storage

Most customers are familiar with these principles, but they may not think of them as the AS/400’s defining characteristics. Indeed, many users take most of these concepts for granted. However, the importance of these five architectural principles cannot be overstated, for they represent everything that AS/400 customers value in a business computer.

Technology Independence

The first and perhaps most important principle is technology independence. Unlike other computer systems, the AS/400 isn’t defined by hardware. This means that a program doesn’t "speak" directly to the hardware; it "speaks" to a technology-independent machine interface (TIMI, or simply the machine interface).

Between this interface and the actual hardware are about four million lines of operating system software called the System Licensed Internal Code (SLIC).

This software layer insulates application programs from the underlying hardware characteristics. An AS/400 program has no knowledge of the underlying hardware; this knowledge remains entirely within the SLIC. This means that when the processor technology changes, IBM can rewrite the SLIC components that are aware of those technology changes and thus preserve the integrity of the machine interface. Due to this technology- independent design, customer application programs are also unaware of technology changes and can exploit new technology without being disrupted by it.

The importance of this architectural principle was dramatically illustrated when AS/400 processor technology moved from 48-bit CISC to 64-bit RISC. Many customers needed merely to save their programs off their CISC machines and restore them on their new RISC machines to run them as fully 64-bit programs. No other system can do that; conventional architectures require at least a recompilation of legacy programs and usually some rewriting before you can run them in a different system architecture. But the AS/400 lets customers run original 48-bit applications as 64-bit applications — on a 64-bit operating system that contains a 64-bit relational database.

Object-based Design

The AS/400 is a fully object-based system. This means that everything within the system — programs, data files, message queues — is an object. Each object has two inseparable parts: a descriptive part, which defines the valid ways of using that data; and a data part, which serves as the functional aspect of the object.

If an object is defined as a program, its descriptive part states that the data part will be treated as executable, read-only, compiled code. The only operations allowed on this object are those that make sense for a program. For example, you can write into the middle of a data file, but you can’t write into the middle of compiled code; the system won’t let it happen. Thus, AS/400 objects’ two-part design insures data integrity for all objects in the system.

Object-based design has important security implications. For example, one mechanism by which computer viruses enter systems is by masquerading as data. Once inside, the virus tries to become executable code and wreak havoc. Such a change of characteristics isn’t possible on the AS/400 — if the system permits a package to enter the system as data, it must retain the characteristics of data forever. It cannot change its mind and become executable code.

As a key part of the AS/400’s fundamental design, objects are one of many reasons that the AS/400 enjoys an almost legendary reputation for rock-solid security and integrity.

Hardware Integration

Whereas the engineering/scientific computing environment is compute intensive (meaning users perform complex operations on a relatively small amount of data), the general business computing environment is information intensive (meaning users perform simple operations on a large amount of data). Because the AS/400 is optimized for the general business environment, it contains hardware design characteristics that enable it to deliver outstanding performance in an information-intensive environment.

In a typical business transaction, an application program is loaded into main storage, and the main processor then begins to execute it. When the main processor comes across a request for data to be read from disk, for example, it delegates that request to the input/output processor (IOP) that’s dedicated to the disk device. Then the main processor diverts its attention to another application program - the task that it’s dedicated to doing — and returns to the original program only when the data it requested earlier is available in main storage.

On a large AS/400, you can have over 200 IOPs connected to high-speed buses, creating an extremely powerful server. Such a server design is excellent for information-intensive applications.

Software Integration

Certain software items are fundamental to all businesses. In addition to the basic operating system drivers that handle the various I/O devices, businesses always need software for standard computing functions, such as security, communications, Web-serving, and backup and recovery. With a traditional system, customers often must purchase additional software components to add to their base operating system. These customers must also ensure that the release levels of additional modules are compatible with the release levels of all the other items they plan to integrate.

With the AS/400, however, all necessary business software components are fully integrated into the standard operating system. IBM tests all these components in the context of the other components, so the entire operating system works as one entity. Furthermore, when IBM makes changes to OS/400, it gives customers a new release of the entire operating system; thus, there are never any release conflicts between individual components of OS/400 because IBM ships a complete, fully tested operating system to customers with each release.

Two benefits of this highly integrated operating system are immediately apparent to customers: fast deployment of new business solutions and a remarkably low total cost of ownership.

Single-level Storage

The AS/400’s massive 64-bit address space can address 18 quintillion bytes of data! Architecturally, the AS/400 is designed to be capable of even more than this — up to 128- bit addressing.

Mapped into this 64-bit space is the "real" storage: disk drives and main memory. But customers need not be aware of any of the storage technologies that underlie the huge address space because the AS/400 manages them automatically. As far as customers are concerned, all programs and data simply reside in this massive space. Users don’t need to worry about where a program resides; they need only reference it by name.

Similarly, customers don’t need to worry about making extensions to files that are full. The AS/400 handles this automatically, too. And when customers add more storage devices to the machine, they don’t need to redistribute data across them; the system recognizes the new available storage and uses it. Most AS/400 installations don’t even have a traditional database administrator because they don’t need one. The system does much of this type of work on its own.

Processing business applications in a multi-application, multi-user environment involves frequent switching between different tasks. Because of its single-level storage, the AS/400 accomplishes this function much more efficiently than conventional systems. Switching to a new task in the AS/400 is as simple as performing a branch instruction to the location where the new task resides. There’s no need (as there is in Unix and Windows systems) to re-create a separate address space before the execution of a new task can begin. Designed for the frequent task-switching that characterizes business environments, the AS/400’s single-level storage not only simplifies storage management, but it also delivers exceptional performance.

Putting It All Together

The AS/400 architecture is designed to be extremely flexible to accommodate the hardware and software technologies of the new millennium. By preparing users for the future while equipping them with the business applications they need to succeed today, the AS/400 can truly be described as "beyond technology." (To learn about the new CD Beyond Technology: AS/400, the Ultimate Business Machine, see the sidebar, "Beyond Technology,"above.)

Frank G. Soltis of IBM Rochester created the technology-independent architecture of the AS/400. He is the AS/400 chief scientist for IBM and a professor of computer engineering at the University of Minnesota.