Pos 355 Introduction to Operation Systems

A mainframe is a state-of-the-art computer for mission critical tasks. In the "ancient" mid-1960s, all computers were mainframes, since the term referred to the main CPU cabinet. Today, it refers to a class of ultra-reliable medium and large-scale servers designed for enterprise-class and carrier-class operations.

IBM owns the mainframe business. Although many tried to compete by offering compatible mainframes that ran the same IBM applications, only Amdahl (Fujitsu) has remained as a competitor in this arena Unisys, Sun and others make mainframe-class machines, but run under proprietary or Unix-based operating systems, not IBM operating systems.

Why do mainframes cost a million dollars or more when the raw gigahertz (GHz) rating of their CPUs is no higher than a PC costing 1,000 times less. Quite often in fact, the ratings are lower. Here are the reasons.

Mainframes support symmetric multiprocessing (SMP) with as many as 32 central processors in one system. They are highly scalable. CPUs can be added to a system, and systems can be added in clusters. Built with multiple ports into high-speed caches and main memory, a mainframe can address as much as 256GB of RAM. They connect to high-speed disk subsystems that can hold many terabytes of data.

A mainframe provides enormous throughput by offloading its input/output processing to a peripheral channel, which is a computer itself. Mainframes can support hundreds of channels, up to 1,024 in some models. Additional computers may act as I/O traffic cops between the CPU and the channels and handle exceptions (channel busy, channel failure, etc.).

All these subsystems handle the transaction overhead, freeing the CPU to do real "data processing" such as computing balances in customer records and subtracting amounts from inventories, the purpose of the computer in the first place.

Mainframe operating systems are generally rock solid. IBM, for example, has more than 40 years of experience developing them for the same hardware family, which of course, has evolved highly over the years. As for hardware reliability, a lot of circuitry in a mainframe is designed to detect and correct errors. Every subsystem is continuously monitored for potential failure, in some cases even triggering a list of parts to be replaced at the next scheduled maintenance. As a result, mainframes are incredibly reliable with mean time between failure (MTBF) up to 20 years!

Once upon a time, mainframes meant "complicated" and required the most programming and operations expertise. No longer. Networks of PC clients and servers are just as complex, if not more so. Large enterprises have their hands full supporting thousands of PCs along with Windows, Unix and Linux and maybe some NetWare for good measure. With more than two trillion dollars worth of mainframe applications in place, mainframes may hang around for quite a while.

Client/Server is a scalable architecture, whereby each computer or process on the network is either a client or a server. Server software generally, but not always, runs on powerful computers dedicated for exclusive use to running the business application. Client software on the other hand generally runs on common PCs or workstations. Clients get all or most of their information and rely on the application server for