Mainframes 360
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Saturday, November 17, 2012


  This post is partially complete and several sections may require editing and proof-reading. The contents on time sharing, TSO and ISPF is under updation.

Early Batch-oriented Operating Systems
In the early 1950s, many organizations bought electronic computers such as the IBM 701(known as the Defense Calculator) and IBM 704. There were only 19 IBM 701s manufactured in the world. A computer system was delivered as hardware and a set of manuals. These computers were single-user, non-shareable, one job at a time systems. The IBM 701 rented at an astonishing $23,750 a month. This is a picture of an IBM 701 computer room(Source :

How were these computers operated? Computer sessions were scheduled as a blocks of time, by programmers. The programmer entered the computer room and signed a First-In-First-Out list, much like you do at a crowded restaurant. The programmer then frequently checked the progress and estimate when he would reach the top. When his time was close, he would be ready with a deck of cards in his hand. When the previous person finished, the next programmer rushed in, checked the printer board , the punch board, the reader board, hung a magnetic tape, punched his time in a mechanical clock, set the console switches, loaded the card deck, prayed that the first card wouldn't jam, and pressed the load button. The typical 701 setup took about five minutes.

There was always unused time between computer sessions. There used to be backlogs of work that was to be programmed and tested, as well as a back-log of production trails. There was a shortage of computer time.

To push more jobs per shift through the installed computers, IBM customers like General Motors developed an operating system GMR NAA I/O(General Motors and North American Aviation) I/O System. It first ran in production in 1956. The Operating sytem completed a job in 3 distinct phases - an Input translation phase, the execution phase and an output translation phase. After one job was complete, the next job in quueue would automatically start. Programmers were thrown out of the computer rooms and professional operators were hired. This was the world's first operating system on a mainframe.

A Batch processing Operating System consisted of a component called the Batch Monitor or Supervisor, that was a permanent part of the computer memory. The memory map below shows the arrangement of the Operating system and the user job in computer memory.

Multi-programmed Operating Systems
A program is a sequence of instructions. There are declarations, assignments, arithmetic operations like ADD and SUBTRACT, I-O operations like READ and WRITE. Generally, performing Input/Output e.g. reading a card from a deck takes longer than other operations like adding two numbers up. The CPU spent a lot of time doing I/O. Programs took longer to complete. The new mainframe computer System/360(S/360) had an improved machine architecture. It allowed the CPU to free itself from the I-O operation. The CPU was capable of performing other instructions of a program p2, while the I-O operation of program p1 was in progress.

I-O Channels and Interrupt hardware
The new mainframe computer Systems/360 had an I-O subsytem. The I-O subsystem has one or more channels. A channel is a special I/O processor and has an independent data and address path between the I-O device and computer memory. When a program executes an I-O instruction, say a READ instruction on a device, the CPU transfers the details of the READ operation to the I-O subsystem. The CPU is not involved in the I-O operation beyond this point. The I-O subsystem and the CPU can function independently. The CPU, however, has no way of knowing when the I-O subsystem has completed its work, unless the I-O subsystem tells it. Thus, I-O subsystem sends a flash, a signal to the CPU. This is called an I-O Interrupt. The System/360 is fitted with special interrupt hardware. Interrupt hardware causes the CPU to execute the Supervisor, which realizes that the I/O operation is complete and processes the interrupt.

Channel Program and Channel Command Words(CCW)
A Channel Program contains instructions for the I-O channel to perform an I-O operation. The channel program is a list of instructions called Channel Command Words(CCW). The Channel Program is stored in Main Memory. The channel program is passed to the I-O Channel, at I/O time.

In order to write your own channel program, you should have profound knowledge of the characteristics of the I-O device. For example, to write a channel program for a disk-drive, you must know its physical structure, data-formats, access mechanisms etc. The channel programs for a disk-drive and a tape would be different. The reference manual for any I-O device(like a disk-drive or tape), details all the Channel Command Words(CCWs) you can use with the device. For example, with 3380 disk drives, the SEEK command is used to access a particular (Cylinder,Track) position. The SEEK head command is used to control a Read/Write Head within the current cylinder.

Programmers needed a device-independent way of doing Input-Output. Access-methods offer a device-independent way to do Input-Output. With access-methods, you don't need to write your own channel programs. When a user-program executes I-O operations like OPEN, READ, WRITE and CLOSE the access-method software builds a channel program in memory. After the list of Channel Command Words(CCWs) are ready, the access method directs the Operating System(OS) supervisor to execute the channel program by means of EXCP(Execute Channel Program) instruction.

The OS supervisor starts the Input-Output by issuing STARTIO <device-address> command. This causes the I-O Channel to fetch an address from the Channel Address Word(CAW). The Channel Address Word(CAW) contains the address of the first CCW in main storage. This is loaded into Instruction Counter(IC) of the I-O Channel. The STARTIO instruction is complete and the CPU is free to perform other tasks.

OS/360 and its variants
IBM launched its new mainframe computer System/360 in 1964. There are three variants of OS/360 operating system.

(i) PCP(Primary Control Program) in April, 1964 – This was meant for small users with less memory. It could run one job at a time.
(ii) MFT(Multi-programming with Fixed number of tasks) in April, 1966 – This was targeted towards mid-range users. Each job was processed in the fixed area of the main storage known as a partition. The number of partitions were decided at install-time. When a job waited for the completion of an event such as I/O, another job of lower priority was allowed to proceed.
(iii) MVT(Multi-programming with Variable number of tasks) in August 1967 – MVT was for the top-end. It could run a variable number of tasks.

The origins of Time-Sharing 
Time-Sharing reduces the delays in receiving results. A large number of jobs share the system resources concurrently. Actually, only one job is active at any moment in time. One job is assigned the main storage region and short period of processing time called a time-slice. The other jobs are saved on secondary storage, while the job in the main storage is being run.

Once the time-slice is over or the job enters the WAIT state for terminal I/O, the main storage image of the job is copied to auxiliary storage. The Operating System dispatcher brings another job into the same region of main storage and is given a time-slice.

The credit for the genesis of time-sharing goes to Bob Bemer. He wrote of, how computers of the future were going to be costly and more powerful, and hence they must be shared by many users, in his article How to Consider A Computer, published under the Data Control section of the Automatic Control Magazine, in March, 1957.

The Compatible Time Sharing System(CTSS) was the first Time Sharing operating system, built at MIT in 1961.

IBM had developed CP/40 and CP/67 time-sharing systems as a research initiative. Instead, IBM introduced TSO(Time Sharing Option) as an optional feature in OS/360 MVT, which is now a standard component of the zO/S.

TSO(Time Sharing Option)
The TSO/E software presents a command-line interface to the zOS. When you log on to TSO, a READY prompt is displayed on the terminal.


You may enter commands like ALLOC, EDIT, DELETE, RENAME, SUBMIT, CANCEL to perform day-to-day tasks such as creating, editing and deleting files, submitting jobs to the system etc.The TSO software processes the commands, you type at the READY prompt. TSO is a command processor. For example, I have typed LISTCAT command to list the attributes of the file SYSADM.DEMO.SRCLIB(We’ll discuss about files at length, later).

DATASET-OWNER-----(NULL) CREATION--------2011.143
RELEASE----------------2 EXPIRATION------0000.000
VOLSER------------OS39M1 DEVTYPE------X'3010200F' FSEQN---------

But, how does TSO accomplish time-sharing? Here is an analogy. When still images are flashed before your eyes in rapid succession at about 24 frames per second, the naked eye perceives it as a motion picture.

A TSO session is really a job, which runs the TSO software program. As an example, if two users A and B are logged-on, there would be two TSO jobs running. Time is sliced between the two TSO jobs. Assume a time-slice is 2 seconds. Under the hood, the CPU runs the TSO job A for 2 secs., then switches to TSO Job B, it runs for 2 secs., and back to TSO job A for 2 seconds and so forth. In reality, the CPU rapidly switches back and forth between the two TSO tasks, giving an illusion that both the tasks are running at the same time. 

On a real mainframe system, there are hundreds of jobs competing for the CPU, the TSO jobs of the active users, other batch job(s) and STARTed tasks.

ISPF(Interactive System Productivity Facility)
Personal computers having x86 architecture support operating systems like the MS Windows. The MS Windows OS has a command-line interface called the DOS-Prompt. On the DOS-prompt, you enter commands like MOVE, REN, COPY, COPYCON, EDIT etc. to do routine things such as creating a file/folder, editing a file, move or copy files. MS Windows also offers a Graphical User Interface(GUI) called the Desktop(Shell).

On clicking the Start button, different menus appear. A logically-related set of functions are grouped under one menu. For example, application programs are grouped under Programs menu. Utilities such as the Notepad, Paint, Calculator are below the Accessories> Menu. Network settings, Printer settings are found under the Settings Menu. To run an application like Microsoft Word, you navigate to Start >> Programs >> Microsoft Office >> Microsoft Word.

TSO Software on the mainframe system can be likened to the DOS-prompt. To do things like creating files, edit files, and running programs you would enter commands at the TSO prompt. As an example, I wanted to create a new file SYSADM.MY.TEST.FILE, so I keyed in the whole ALLOC command, with all its parameters in full glory.



ISPF is equivalent to a Graphical User Interface(GUI) on the Mainframe. ISPF has window-like panels. This is what an ISPF window looks like. It is used to search a file on the mainframe computer.

   Menu  RefList  RefMode  Utilities  Help                                      
Data Set List Utility
Option ===>

blank Display data set list P Print data set list
V Display VTOC information PV Print VTOC information

Enter one or both of the parameters below:
Dsname Level . . . SYSADM.DEMO.REXXLIB
Volume serial . .

Data set list options
Initial View . . . 1 1. Volume Enter "/" to select option
2. Space / Confirm Data Set Delete
3. Attrib / Confirm Member Delete
4. Total / Include Additional Qualifiers

When the data set list is displayed, enter either:
"/" on the data set list command field for the command prompt pop-up,
an ISPF line command, the name of a TSO command, CLIST, or REXX exec, or
"=" to execute the previous command.

ISPF has a menu-driven interface. A picture of the ISPF Home screen is shown below. As you can see, it has menus, like 1 - View, 2 - Edit, 3 - Utilities, 4 - Foreground, 5 - Batch and so forth. To select menu, just key in the appropriate option.

   Menu  Utilities  Compilers  Options  Status  Help                            
ISPF Primary Option Menu
Option ===>

0 Settings Terminal and user parameters User ID . : SYSADM
1 View Display source data or listings Time. . . : 14:42
2 Edit Create or change source data Terminal. : 3278
3 Utilities Perform utility functions Screen. . : 1
4 Foreground Interactive language processing Language. : ENGLISH
5 Batch Submit job for language processing Appl ID . : ISR
6 Command Enter TSO or Workstation commands TSO logon : ISPFPROC
7 Dialog Test Perform dialog testing TSO prefix: SYSADM
9 IBM Products IBM program development products System ID : P390
10 SCLM SW Configuration Library Manager MVS acct. : ACCT#
11 Workplace ISPF Object/Action Workplace Release . : ISPF 5.2
M More Additional IBM Products

Enter X to Terminate using log/list defaults

To allocate a new file, I shall navigate to 3 - Utilities Menu. I type the Option 3 on the screen and press enter.

   Menu  Utilities  Compilers  Options  Status  Help                            
ISPF Primary Option Menu
Option ===>

0 Settings Terminal and user parameters User ID . : SYSADM
1 View Display source data or listings Time. . . : 14:42
2 Edit Create or change source data Terminal. : 3278
3 Utilities Perform utility functions Screen. . : 1
4 Foreground Interactive language processing Language. : ENGLISH
5 Batch Submit job for language processing Appl ID . : ISR
6 Command Enter TSO or Workstation commands TSO logon : ISPFPROC
7 Dialog Test Perform dialog testing TSO prefix: SYSADM
9 IBM Products IBM program development products System ID : P390
10 SCLM SW Configuration Library Manager MVS acct. : ACCT#
11 Workplace ISPF Object/Action Workplace Release . : ISPF 5.2
M More Additional IBM Products

Enter X to Terminate using log/list defaults

On the Utilities menu, there are several options, 1 – Library, 2 – Dataset, 3 – Move/Copy and so forth. Select menu option 2 to allocate a new dataset.

   Menu  Help                                                                   
Utility Selection Panel
Option ===>

Library Compress or print data set. Print index listing. Print,
rename, delete, browse, edit or view members
2 Data Set Allocate, rename, delete, catalog, uncatalog, or display
information of an entire data set
3 Move/Copy Move, or copy members or data sets
4 Dslist Print or display (to process) list of data set names.
Print or display VTOC information
5 Reset Reset statistics for members of ISPF library
6 Hardcopy Initiate hardcopy output
7 Transfer Download ISPF Client/Server or Transfer data set
8 Outlist Display, delete, or print held job output
9 Commands Create/change an application command table
11 Format Format definition for formatted data Edit/Browse
12 SuperC Compare data sets (Standard Dialog)
13 SuperCE Compare data sets Extended (Extended Dialog)
14 Search-For Search data sets for strings of data (Standard Dialog)
15 Search-ForE Search data sets for strings of data Extended (Extended Dialog)

I shall just key in the name of the file SYSADM.MY.TEST.FILE and press enter. A new file would be created.

   Menu  RefList  Utilities  Help                                               
Data Set Utility
Option ===>

Allocate new data set C Catalog data set
R Rename entire data set U Uncatalog data set
D Delete entire data set S Short data set information
blank Data set information V VSAM Utilities

ISPF Library:
Project . . Enter "/" to select option
Group . . . /
Confirm Data Set Delete
Type . . . .

Other Partitioned, Sequential or VSAM Data Set:
Data Set Name . . .
Volume Serial . . . (If not cataloged, required for option "C")

Data Set Password . . (If password protected)

ISPF makes life easy. ISPF windows and menus, lift the burden of having to remember TSO commands such as ALLOC, EDIT, MOVE, COPY and their complete syntax.


Tuesday, November 13, 2012

What are mainframe computers?

What are mainframe computers?

A mainframe computer is a big computer. IBM builds mainframe computers. Today, a mainframe refers to IBM's System Z servers. Many large corporations like banks, insurance companies, travel and retail sector, telecom companies process data on a mainframe. Today, thousands of people around the globe book flights, do wire transfers, make credit-card purchases. These transactions are processed in a snap by a mainframe.

Mainframe RAS features

Reliability is the ability of computer hardware or software to consistently perform according to its specifications. Mathematically, reliability measure is the mean time to failure(MTTF). Availability is the ratio of the uptime of a system to the total time. Serviceability is an expression of the ease with which a system can be maintained or repaired.

The reliability, availability and serviceability features are in the System Z's DNA. System Z is designed to provide an availability of 99.999% (five 9s). The "Z" in System Z's brand-name stands for zero down-time. System Z servers have reported an MTTF of 40 years. Organizations run their mission-critical applications on mainframe to provide 24/7 availability.

System Z processors use an instruction retry mechanism. Every instruction that executes successfully on the System Z processor is chekpointed, the processor state is saved. If an error is detected, a recovery sequence commences. The processor is fenced off. The processor state is refreshed from the last checkpoint and the instruction is retried. Like having spare tyres on a truck, System Z offers chip redundancy. If one processor fails, System Z can dynamically switch to a spare without losing a beat! Thousands of error checkers(parity, residue, illegal state, hardware redundancy compares and so on) are embedded on the chip.

Are Mainframe computers good at everything?

Well, not quite. Mainframes are not good at number-crunching or 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 games like Counter-Strike or Half-Life on a Mainframe.

Mainframe computers don’t have a beautiful user-interface like the PC at your home. You wouldn’t find a desktop wallpaper or icons on a mainframe computer.

Mainframe Hardware
Fig. A walkthrough the zEnterprise EC 12
A Mainframe computer has processing units(PUs), memory, I/O channels, control-units and peripheral devices. A processing unit(PU) is the brain of the mainframe computer, that executes instructions. A mainframe computer has many processors. The total of all processor units(PUs) are inside a cage(frame) called the Central Processing Complex(CPC).

There are specialized PUs capable of performing specific tasks. The main processors are called the Central processor(CPs). There are PUs for encryption and decryption(CPACF), Linux workloads(IFL), the co-ordination of the system efforts(ICF), those that assist the CP for any workload on the I-O subsystem(SAP), spares that come in handy when a CP fails, others for executing Java code(zAAP) and providing accelerated DB2 performance(zIIP).

Buy a z10 Mainframe server, and you’d get 12 central processors, 12 IFLs, 12 ICFs, 6 zAAPs and 6 zIIPs(Source: IBM).

The CPC cage also contains main storage(RAM). A z10 mainframe can provide upto 384 GB of RAM memory.

A channel is an independent data and control path between I/O devices and the memory. Peripheral devices like disk-drives, tapes, printers, card-readers, card-punch are connected to a mainframe computer using channels.

Because peripheral devices are relatively slower than the CPU, the CPU could waste time waiting for data from a peripheral device. A I-O Control unit is a self-contained processor, and has logic to work with a particular type of peripheral device. Channels connect to control units, which in turn manage the peripheral device.

A personal computer offers different ports to connect peripherals. It has USB ports, SCSI bus. The Mac has high-speed firewire port. Likewise, on the mainframe, channels can be OSA, ESCON or FICON. OSA Express channels are used to connect to standard networking technologies such as LAN(Ethernet), Token Ring. ESCON and FICON channels have fiber-optic cables.

Mainframe Peripherals
Just as you use a keyboard, mouse and a CRT display to operate a Personal Computer, in the early days you operated a mainframe computer by a terminal. A terminal had a display and a keyboard. A very popular terminal manufactured by IBM in the 70’s was the 3278 Terminal. Have a look at the photograph below.

Terminals connected to a mainframe computer remotely, over a network. They used the IBM 3270 protocol for communication. IBM no longer manufactures terminals, instead you use a PC running a software that mimics the terminal. Terminal emulators such as the IBM Personal Communications are quite popular.

Storage devices such as Magnetic Disk drives are used as secondary memory. IBM uses the term DASD(Direct Access Storage Device) pronounced as dazz-dee for hard-disks. Disk drives support random access to data. IBM 3380 and 3390 DASD models are widely in use. The picture below shows a 3380 DASD assembly(Source:IBM Archives).


Tape drives are also used for storage. A Tape drive allows sequential access of data. Very large files can be stored on tape-drives. Tape drives are often used for a backup of data. IBM continues to innovate the tape drive technology. IBM 3480 tapes were very popular in the last century. The picture below shows two 3480 tapes(in the front) and two 3420 tapes(the tape reels at the back). Tapes now come in a square cartridge form, instead of a tape reel.

The IBM 3480 (two drives in front) and IBM 3420 (two drives in back) Tape Drives

Punched Cards(Hollerith Cards) although obsolete, have been used over a century for recording information. A punched card was a stiff paper, that represented information by the presence or absence of holes. Punched cards were used by Joseph Jacquard in the early 19th century in textile looms. By using a particular punched card, you could select a weaving pattern.

Herman Hollerith proposed the use of punched cards to store information about US nationals during the 1890 census. Jacquard cards were read-only. Hollerith envisioned that punched cards could be used as a read/write technology. A key-punching machine could be used to punch data such as age, sex or marital status of the citizen. In the picture below, you see the operators preparing data for the census.(Source: Computer Science Labs).


By the early 20th century, punched cards began to be used everywhere in the United States. Your gas bill would arrive every month with a punched card, that stored the customer name, the address, the bill amount and the due-date. In the 20th century, punched cards were used as legal documents such as US government checks.

Hollerith founded a company, The Tabulating Machine Company(1896), which after a few buyouts, became known as International Business Machines(IBM)(1911). IBM manufactured the standard 80 column punched card. A deck of cards were fed through a hopper into a card-reader. Output data could be punched onto cards using a card-punch.

80 column Punched Card

The photograph above(Source: IBM Archives) shows a standard 80-column IBM card. A legacy of 80-column card is, even today terminals connected to a mainframe server have a display of 24 rows x 80 columns. The columns 73-80 contained sequence numbers and were used for sorting the punched cards.

Mainframe computer industry and the people involved
To oversee the running of a mainframe computer, you need to hire professionals called System Operators(Sysops).  Mainframe sysops have good knowledge of the mainframe architecture and the operating system. The operating system on the z10 mainframe computer is the zOS(pronounced as zee-OS). A Mainframe operator starts(IPL – Initial Program Load) or shuts down the mainframe and various regions(applications). He must monitor workload on the system, and reply to any messages. He can issue commands to vary various peripheral devices online or offline.

Just as MS-DOS or Linux offer a command-line interface, the zOS too has a command-line, at which the system operator issues MVS commands to start or shut-down applications(e.g. START DB2), to monitor the workload(DISPLAY A,L) etc. Console is another term for the command-line interface of zOS. The photograph below shows how the console looks.

     *13.24.28          *IWM048E WLM RUNNING IN GOAL MODE WITH THE DEFAULT      
*13.28.36 *IFB081I LOGREC DATA SET IS FULL,13.28.36,

- 13.30.18 STC00024 +DFHWB1008 CICS CICS Web environment initialization is
- complete.
- 13.30.19 STC00024 +DFHSI1517 CICS Control is being given to CICS.
- 13.30.19 STC00024 +DFHEJ0102 CICS Enterprise Java domain initialization
- has ended.
13.33.26 STC00037 $HASP373 FTPD STARTED
- 13.33.31 STC00034 +EZY2702I Server-FTP: Initialization completed at
- 19:33:31 on 11/13/12.
- 13.33.39 STC00037 IEF404I FTPD - ENDED - TIME=13.33.39
- 13.59.47 STC00025 IEF404I BPXAS - ENDED - TIME=13.59.47
14.14.24 TSU00038 $HASP373 SYSADM STARTED
- 02.24.00 STC00024 +DFHIC0801 CICS CICS time altered from 24.00.000 to
- 02.23.590 - date 11/14/12 - relative day 001
00- 00.00.02 STC00024 +DFHIC0801 CICS CICS time altered from 24.00.000 to
- 00.00.016 - date 11/15/12 - relative day 002

IEE612I CN=01 DEVNUM=0700 SYS=P390


Application programmers design and build application software that runs on Mainframe computers for companies. First, the customers(end-users) specify the business requirements. Business analysts gather these business requirements and with the experience they have acquired, translate it into functional and technical requirements, and give a briefing to the application developers. The application developers then create technical designs, write code in High-level languages such as Cobol, C/C++, PL/1, and test it.

System programmers perform hardware-software upgrades, capacity planning and trains other sysops. He is a watch-dog. He installs the operating system. He applies patches(patches are called PTFs on mainframe). He also maintains other system software or products running on the mainframe.

Connecting to a Mainframe computer
Typically at a customer site, mainframe servers are generally housed in a large area of building space. This is called the data center or the raised floor. 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, or a PC running a software that pretends to be a 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 -

   Menu  Utilities  Compilers  Options  Status  Help                            
ISPF Primary Option Menu
Option ===>

0 Settings Terminal and user parameters User ID . : SYSADM
1 View Display source data or listings Time. . . : 15:09
2 Edit Create or change source data Terminal. : 3278
3 Utilities Perform utility functions Screen. . : 1
4 Foreground Interactive language processing Language. : ENGLISH
5 Batch Submit job for language processing Appl ID . : ISR
6 Command Enter TSO or Workstation commands TSO logon : ISPFPROC
7 Dialog Test Perform dialog testing TSO prefix: SYSADM
9 IBM Products IBM program development products System ID : P390
10 SCLM SW Configuration Library Manager MVS acct. : ACCT#
11 Workplace ISPF Object/Action Workplace Release . : ISPF 5.2
M More Additional IBM Products

Enter X to Terminate using log/list defaults

Tuesday, November 6, 2012

Pseudo-conversational Programs

Pseudo-conversational coding technique 
CICS allows many users to log on and use CICS concurrently. Computer main storage is finite. If numerous users are running transactions, that demand considerable main storage, a Short-On-Storage(SOS) condition could occur, causing delays.

Pseudo-conversational programming is a coding technique, that results in the program being loaded into memory when required and released during the operator “think time”. Resources are not held.

Shared Code, Re-entrancy
CICS employs several task-management techniques to efficiently utilize computer memory. One such technique is shared-code. If two passengers are to book their railway tickets at a time, there will be two instances of the BOOK transaction viz, task X and task Y. On CICS, task X and task Y will share the same program-code, and merely have their own separate copies of working storage areas in memory. This significantly reduces the computer memory used.

Shared-code may be shared, after a user is completely finished with it(serially reusable), or may be shared, even if the first user is not finished(reentrant).

You see, CICS allows multi-tasking. Two or more tasks X and Y can run simultaneously. Behind the scenes, time is sliced into equal slots, say 2 seconds. Task X is run for 2 seconds, then it is interrupted, and task Y is run for 2 seconds, it is interrupted, and again task X is run for 2 seconds and so forth. CICS switches back-and-forth between the two tasks in a round-robin fashion.


This is possible only, if CICS applications are re-entrant. On the CICS environment, IBM uses the term quasi-reentrant. If a mid-execution stop can occur anywhere in the program, the program code is said to be completely reentrant. If execution can stop only at I/Os or system calls, the program code is said to be quasi-reentrant. Making CICS programs reentrant is generally done automatically, when you supply the RENT option while compiling CICS programs.

To many people who are thrown to work at a mainframe computer on their first job, they feel lost. Mainframe people seem to speak a completely different language and that doesn't make life easy. What's more, the books and manuals are incredibly hard to comprehend.

"What on earth is a Mainframe?" is an absolute beginner's guide to mainframe computers. We'll introduce you to the hardware and peripherals. We'll talk about the operating system, the software installed on a mainframe. We'll also talk about the different people who work on a mainframe. In a nutshell, we'll de-mystify the mainframe.

Readers based in India, can buy the e-book for Rs. 50 only or the print book. International readers based in the US and other countries can click here to purchase the e-book.