What is Mainframe Computer?

Q. What is a Mainframe Computer?

A. A Mainframe computer is used by large organisations for processing huge volumes of data e.g. census, consumer statistics, ERP and financial transaction processing. In the early days of computer, there was distinction between micro-computers(today’s desktop PCs), mini-computers(these have ceased to exist) and Mainframe computers. IBM introduced the IBM PC, as well as continues to develop faster and powerful mainframe servers today. Though the many other players entered the PC race, IBM still continues to dominate in the Mainframe Servers line of machines. A micro-computer has a single processor and only a few input-output devices. With the evolution VLSI technology, the entire CPU was the size of a small stamp and fabricated on a single chip. This miniaturization of the computer, gradually led to an increase in PC penetration. A mini-computer is a mid-range computer. You can think of them as stripped down versions of Mainframe Computers. One of the older mini-computers is IBM’s AS/400. However, many companies still use AS/400 machines, and hence this technology still continues to live on. A Mainframe server is a computer that does a huge amount of data-crunching. Whether, it is you swiping your card at POS terminal in Spencer’s retail store, or withdrawing money at the ATM, or booking railway tickets on IRCTC’s website, or doing net-banking at ICICI’s online website, there’s mainframe server which processes the data. One must understand that mainframe computers are not meant to be fast. Instead, they should process huge volumes of data, with reliability. The three important characteristics of Mainframe machine are Reliability, Accessibility and Serviceability(RAS). I shall discuss this at length, further ahead. Most Insurance companies, Banks and other financial institutions have their business data residing on Mainframe computers. Seventy percent of data in the world still lives on IBM Mainframes. (That’s why most companies still need manpower which knows such an old technology like Mainframes on board to support and enhance their computer applications)

Q. How did IBM Mainframe machines evolve?

Well, IBM first launched the System/360, then System/370, System 370/XA, Enterprise System Architecture(ESA)/370, System 390 and the latest machines in the Mainframe series is zSeries(pronounce Zee). A Mainframe Machine is also like an ordinary desktop PC, it also has an Operating System. The System/360 had the OS/360, the System/370 and its variants had the famous MVS(Multiple Virtual Storage) Operating System, the System/390 had OS/390 and the zSeries has the zOS.

Q. What does a Mainframe Computer look like? How do you operate it?

Let’s just try to see what’s life like, if you are Mainframe machine’s operator. A Desktop computer can fit into a trolley or a cabinet. A Mainframe Machine is generally big monster-size, would fill up a room. On a Desktop PC, you would sit near the physical computer and operate it using the keyboard and the mouse, and see the result on the monitor. Generally only 1 user uses the Desktop PC with Windows OS at a time. As far as mainframes are concerned, Mainframe machines are servers. You throw tasks to the Mainframe Machine, and the mainframe performs the processing, and stores the result. Many users throw requests to the Mainframe Machine concurrently, the Mainframe services these requests. Thus, as mainframe operator, generally you would not be present physically near the machine, you operate it remotely. You have a keyboard and monitor at you disposal(This is called a dumb terminal). Through the network, this terminal is connected to the Mainframe Server. There will thousands of terminals connected to a mainframe server, and many users can perform their tasks on the mainframe server concurrently. You might sit in the work-office of your company at New Delhi on a terminal and perform data-processing on the Mainframes Server located at Chennai.

Q. Hey, Windows is the most popular Operating System on a Desktop-PC. On the same lines, the popular Mainframes Operating System is MVS...

Bulls-eye! In fact, MVS is said to be the Operating System that makes the world move.

Q. What about Super-computers? Are mainframes faster than super-computers?

Don’t equate the word Mainframes to Speed. Mainframes have tremendous processing power or computational power, but of a different kind. Think of task, where we have 1,00,000 records of Input data, and we want to perform 3 calculations on each record, and then put the Output Records in a separate file. Thus, these are simple calculations, but on huge volumes of data. On the other hand, super-computers do complicated calculations like weather-forecasting etc. at very fast speeds. Reading and writing huge volumes of data, and performing simple calculations on them is the forte of a Mainframe Server running MVS. An Insurance company keeping track of its Insurers, a retail-store keeping record of its Inventory, or any large company keeping track of it employees and payroll would need a Mainframe with MVS.

Q. What’s that name again.. Multiple Virtual Storage? What crap?

Virtual Storage is a simple idea. Virtual storage simulates a large amount of main storage(Primary or main memory) by treating the auxiliary storage as an extension of the real storage. Thus, a program gets the feel, “Wow, I have an infinite amount of computer memory at my disposal.” Moreover, because it is a multi-user operating system, it allows many users to run the same programs and data at once. While many PC users make fun of the Mainframes running MVS, calling them dinosaurs, I personally believe, that the PC industry has taken a lot of lessons from the Mainframe line of machines. For example, MVS has built in features like recovery from faulty hardware like tape-drives or even faulty processors.

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.