What is Mainframe Computer?

Q. What is a Mainframe Computer?

A Mainframe-Computer is a big computer. IBM builds Mainframe Computers. Today, a Mainframe refers to IBM's zSeries computers. Big companies; Banks, Insurance Companies, Travel and Retail Sector, Telecom Companies employ Mainframes for processing their Business-Data. Today, thousands of people around the globe book flights, do Electronic Money Transfers, swipe their credit-cards for purchases. These transactions are processed on-the-fly in a snap by a Mainframe Computer.

Q. Why do big companies rely on Mainframes? Aren't Mainframes dead yet?

Today, all businesses trust Mainframe Computers to process their critical Business-Data. What distinguishes a Mainframe from other line-of-computers, its close cousins such as Micro and Mini-Computers? Available : Mainframe-Computers are always available, they are up and running all the time. They just don't fail. You'll be surprised to know, once a Mainframe Computer is started and powered on(IPL'ed), they can run for 5 to 10 years, at a stretch without failing. In other words, Mainframe Computers have very good up-times. The Mean Time Between Failures(MTBF) ranges from several months to even years. Reliable : IBM boasts that you can bet all your money on A Mainframe, when it comes to Reliability. Very often, you must have seen the horrific Blue-Screen of Death(BSOD) on Desktop Computers, and they crash! A Mainframe Computer reliably processes huge volumes of Business-Data, without crashing. Serviceable : Faults can be detected early on a Mainframe Computer. When some components fail, some of IBM's systems can automatically call the IBM Service center. Repairs can be done without disrupting the day-to-day operations. The RAS(Reliability-Accessibility-Serviceability) features of a Mainframe Computer give it an edge over many other computing systems.

Q. So, Mainframe Computers are good at everything?

Well, not quite. As the saying goes, "You can’t teach an old dog, new tricks". Mainframes are not good at number-crunching. Mainframes don't do scientific calculations. A Mainframe is not a Super-computer. You wouldn’t use a Mainframe Computer to calculate the value of Pi, up to 1000-decimal Places. Mainframe-Computers are not meant for speed. They aren’t fast, rather they can process humungous data reliably. You can't play Counter-Strike or Half-Life on a Mainframe. Mainframe Computers aren’t as User-Friendly as the Household–PC running Windows. You don’t have a rosy-rosy Desktop or a Mouse to point your cursor and click at. Instead, there’s a Keyboard and Dumb-Terminal(Monitor) at your disposal, or a PC running software that pretends like a dumb-terminal. In common parlance, people call these dumb-terminals as green-screens, because back then they had green characters on a black-background.

Q. How do you connect to a Mainframe Computer?

Mainframe Servers are generally housed at a big Data-Center. People around the world connect to the Mainframe-Computer, remotely over a Network, from their work-place or home, using the Keyboard and the Dumb-Terminal. Gee, you don’t have to sit physically near a Mainframe Box to do your work. This is how the Mainframe Screen looks like, when you first connect to it -

Q. What's the OS on a Mainframe Computer?

IBM zSeries Computers run the zO/S. Although, the official product-name is now zO/S, the former name MVS is still quoted on numerous occasions. MVS is said to be the OS, that makes the world move.

Q. How did MVS OS Evolve?

In April 1964, IBM announced OS/360, to support the newer architecture S/360 Line of machines. IBM actually released three different control-programs PCP(Primary Control Program), MFT(Multi-programming with a fixed number of tasks) and MVT(Multi-Programming with Variable Tasks). PCP could run just one task at a time, in 32 KB of memory. That’s right Kilo-Bytes, not Mega-bytes! With Multi-programming, the control could be assigned to another task, while first task was waiting for I/O. In 1972, IBM announced the System/370 Architecture which had Virtual Storage. IBM released OS/VS1 and OS/VS2, but these supported a single virtual storage address space. On the MVS(Multiple Virtual Storages), each program was assigned a different range of virtual address space. When MVS was developed, some Virtual Storage was still shared between different applications. The 16 MB of Virtual Address space supported by MVS was no longer enough. In 1979, IBM announced MVS/370 which supported cross-memory. It means Data and Control-Blocks could be moved to-and-fro between Private virtual address space and shared address space. This was called Virtual Storage Constraint Relief(VSCR). In 1981, IBM announced System 370/XA - Extended Architecture.

Q. What is a Batch Job?

If you are a college student, you would have probably written a computer application in C, and executed it on your desktop PC. You run the program interactively, you type in some input data, wait for the response(output), and then you type some more input, and this cycle continues. Such programs or systems are called as Online Systems or Transaction Processing Systems. This is because, such systems are always on, they give you immediate response, and you do things interactively. Running a program in Batch Mode, is opposite to running it interactively. In Batch Mode, you tell the Mainframe System - what input data to use, what program needs to be run(which set of instructions to perform on the data), where to store the output, right at the beginning, at the outset. This is called a JOB. This is typical of batch processing. Preparing credit-card statements or generating the payroll of a company, taking a backup of all records are typically JOBS which are run in batch mode. They involve a huge volume of data – so it’s going to take a long while till the final output is generated. You simply submit the JOB to the Mainframe Server, and then forget about it, continue doing something else. When the JOB will have finished, the mainframe will notify you. These systems are called Batch Processing Systems. Earlier, once when a user started a Batch Job, no one else could use the computer, once it began as a Batch Job. However, modern day operating systems like MVS allow you to run other programs, and do other things while a Batch Job runs.

Q. Look, on Windows PC, you interact with the computer using the Windows GUI – the Desktop Icons, the Menus, Buttons etc. How do you interact with an MVS Mainframe System?

MVS offers several ways to interact with it. The 3 main ways of interacting with MVS are TSO, ISPF and CICS. TSO stands for Time Sharing Option. TSO is the part of MVS that lets you use the MVS System interactively. Just as from the DOS-Prompt, you type in commands to create a directory, remove a directory, create a new file in editor, type in data, copy files, rename files delete files, the same way, one can type commands on the TSO. When IBM first introduced TSO in 1969, interactive computing was a hot new feature, which required a large portion of the memory. This is where the O comes from : O meaning option, TSO was optional : when all jobs were run as batch jobs. Since, it allows multiple users to share time on the system concurrently, it is Time Sharing. You said, All JOBs are run as Batch JOBs. Then, what about TSO? Well, under the hood, each TSO user session is run as a Batch Job. It can specify the procedure to be executed or the command when the user LOGON occurs. It also specifies the Terminal Monitoring Program to monitor TSO User Session. It specifies the data-sets to be allocated, when the user logs on. When you write a Batch Job means, you write its JCL(What Input Data Set, What Output DataSet, Which Program to run). Who writes the JCL for TSO? The System Programmer guys who maintain TSO.

Q. What is ISPF?

ISPF stands Interactive System Productivity Facility. ISPF is more popular than TSO, because its an easier way to get things done in MVS without knowing most of the TSO Commands. Rather than typing out commands, you go through a series of menu – just like the Nokia Mobile Handset, to do tasks, like creating a new file, deleting a file, editing a file, renaming, copying, writing and submitting JOBs.