IBM Mainframe Tutorial

A mainframe (also known as "big iron")

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A mainframe (also known as "big iron") is a high-performance computer used for large-scale computing purposes that require greater availability and security than a smaller-scale machine can offer. Historically, mainframes have been associated with centralized rather than distributed computing, although that distinction is blurring as smaller computers become more powerful and mainframes become more multi-purpose. Today, IBM emphasizes that their mainframes can be used to serve distributed users and smaller servers in a computing network.

The mainframe is sometimes referred to as a "dinosaur" not only because of its size but because of reports, going back many years, that it's becoming extinct. In 1991 Stewart Alsop, the editor of InfoWorld, predicted that the last mainframe would be retired by 1996. However, in February 2008 IBM released a new mainframe, the z10. Steve Lohr wrote about the mainframe as "the classic survivor technology" in The New York Times ("Why old technologies are still kicking"):

I.B.M. overhauled the insides of the mainframe, using low-cost microprocessors as the computing engine. The company invested and updated the mainframe software, so that banks, corporations and government agencies could still rely on the mainframe as the rock-solid reliable and secure computer for vital transactions and data, while allowing it to take on new chores like running Web-based programs.

The original mainframes were housed in room-sized metal frames, which is probably where the name derives from. In the past, a typical mainframe might have occupied 2,000 - 10,000 square feet. Newer mainframes are about the same size as a large refrigerator.

What is Mainframe?

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Mainframe is a High performance Multi User computer system which is the most scalable, available, reliable and secured machine in the world capable of performing some Million Instructions per second (upto 569,632 MIPS) with the following characteristics:

1) Reliable single-thread performance, which is essential for reasonable operations against a database.

2) Maximum I/O connectivity, which means mainframes excel at providing for huge disk farms.

3) Maximum I/O bandwidth, so connections between drives and processors have few choke-points.

4) Reliability--mainframes often allow for "graceful degradation" and service while the system is running.

Applications of mainframe various from computing the Random number series to census, industry and consumer statistics, ERP and financial transaction processing..

SPECIAL-NAMES

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This optional paragraph may be used to associate a name with a variety of items, such as system functions, alphabets, symbolic characters, classes, special currency signs or decimal point character. For example:

SPECIAL-NAMES. CLASS HEX-CHARACTER IS "0" THRU "9" "A" THRU "F"

INPUT-OUTPUT SECTION

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This section is primarily used to code the FILE-CONTROL paragraph.

FILE-CONTROL

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FILE-CONTROL

This paragraph is used to associate the files to be used in the program with specific I/O devices. For example:

FILE-CONTROL.

SELECT PRINTER-FILE

ASSIGN TO SYS015-UR-1403-S.

In the example, PRINTER-FILE is the name of the file to be used throughout the program. SYS015-UR-1403-S is a system name, which will be used to link the file to something in the operating system. The method of such connection varies from system to system

DATA DIVISION

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DATA DIVISION

The DATA DIVISION is where all of the data used by a program is defined. It is divided into five sections: the FILE, WORKING-STORAGE, LINKAGE, REPORT, and COMMUNICATION SECTIONs, although most programs only include the FILE and WORKING-STORAGE SECTIONs. The purpose of each of the sections follows

FILE SECTION

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FILE SECTION

The FILE SECTION is used to define the files that will be used in the program. There will be on file description (FD) for each file followed by a record description for each different record format associated with the file. For example:

FD	FILE-NAME.
01	RECORD-NAME.
	05	FIELD-NAME-1	PIC X(5).
	05	FIELD-NAME-2	PIC 99V99.
	05	FIELD-NAME-3	PIC X(20).

In the example, the file is named FILE-NAME-1. It is the same file name that was specified in the SELECT statement for the file. The 01 following the FD is called a record description entry. Multiple 01 entries can be included if there is more than one applicable record format for the file. See Data Items for further information on the record description and its subordinate items.

WORKING-STORAGE SECTION

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WORKING-STORAGE SECTION

The WORKING-STORAGE SECTION is used to define any data that will be used in the program that is not part of a file. These may include, but are not limited to accumulators, lookup tables, print line formats, save fields, etc. The structure of the items is as described in the Data Items section of this tutorial.

LINKAGE SECTION

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LINKAGE SECTION

The LINKAGE SECTION is used in a subprogram to define data that will be passed as arguments to the routine. The structure of the items is as described in the Data Items section of this tutorial. Each 01 level item in the LINKAGE SECTION should be included in the PROCEDURE DIVISION USING heading. For example:

PROCEDURE DIVISION USING	PARAMETER-1
	PARAMETER-2
	PARAMETER-3.

In this example, the subprogram would have 3 passed parameters called PARAMETER-1, PARAMETER-2, and PARAMETER-3. Each of them would be defined as an 01 level item in the LINKAGE SECTION. The order that the items are defined in the LINKAGE SECTION is unimportant.

REPORT SECTION

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REPORT SECTION

The REPORT SECTION can be used to produce standard control break driven reports. It is not commonly used

COMMUNICATION SECTION

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COMMUNICATION SECTION

The COMMUNICATION SECTION is used for communicating between two programs running simultaneously on a computer which supports such operations. It is not commonly used.

PROCEDURE DIVISION

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The PROCEDURE DIVISION consists of a series of procedures called paragraphs, each designed to perform a specific function. A paragraph consists of a paragraph name coded in Area A and a series of procedural statements designed to perform a desired function coded in Area B. Procedures frequently execute other procedures as indicated in the following example:

0000-MAIN-DRIVER.

PERFORM 1000-INITIALIZATION-ROUTINE.

PERFORM 2000-MAIN-PROCESSING

UNTIL END-OF-FILE.

PERFORM 3000-FINALIZE-ROUTINE.

STOP RUN.

Notice the use of punctuation in the preceding paragraph. Each statement should be a sentence, that is, it should end with a period unless it is part of a conditional sentence. Sentences which are continued across multiple lines should be indented on the succeeding lines. Phrases which modify the behavior of a statement such as UNTIL in the PERFORM statement should be coded on a new line so that they stand out. For more tips on making your code easy to follow, see Style.

REMARKS

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This optional paragraph was removed from the standard in 1985, but many COBOL compilers still support its use. Code the paragraph name followed by any desired comments, such as the primary purpose of the program. For example:

REMARKS. THIS IS THE MAIN SCREEN HANDLER FOR EMPLOYEE FILE MAINTENANCE.

The SET Statement

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Syntax

Format 1

SET

{

index-name-1
identifier-1

}

... TO

{

index-name-2
identifier-2
integer-1

}

Format 2

SET {index-name-3} ...

{

UP
DOWN

}

{

identifier-3
integer-2

}

Format 3

SET

{

{mnemonic-name-1}

... TO

{

ON
OFF

}}

...

Format 4

SET {condition-name-1} ... TO TRUE...

Description

The SET statement is used to set values for indices and conditions. Each of the four formats works differently and is described below in the correspondingly numbered item.

The first format of the SET statement is used to assign values to indices or assign the value of an index to another variable. The variable(s) between SET and TO is set to the value of the data item or literal following TO.

The second format of the SET statement is used to increase or decrease the value of an index. The variable(s) between SET and UP or DOWN is INCREASED (for UP) or DECREASED (for DOWN) by the value of the data item or literal following BY.

The third format of the SET statement is used to set the value of an external switch. The mnemonic(s) between SET and TO is set to ON or OFF according to the ON or OFF option supplied.

The fourth format of the SET statement is used to set the condition-name conditions to true. The condition(s) between SET and TO is set to TRUE.

Tips

Regular MOVE and mathematical statements do not work on indices. Use the SET statement to manipulate an index.

For format 4, if more than one value is associated with the condition-name being true, the first value listed is the one that will be used.

The SET Statement

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The SET Statement

Syntax

Format 1

SET

{

index-name-1
identifier-1

}

... TO

{

index-name-2
identifier-2
integer-1

}

Format 2

SET {index-name-3} ...

{

UP
DOWN

}

{

identifier-3
integer-2

}

Format 3

SET

{

{mnemonic-name-1}

... TO

{

ON
OFF

}}

...

Format 4

SET {condition-name-1} ... TO TRUE...

Description

The SET statement is used to set values for indices and conditions. Each of the four formats works differently and is described below in the correspondingly numbered item.

The first format of the SET statement is used to assign values to indices or assign the value of an index to another variable. The variable(s) between SET and TO is set to the value of the data item or literal following TO.

The second format of the SET statement is used to increase or decrease the value of an index. The variable(s) between SET and UP or DOWN is INCREASED (for UP) or DECREASED (for DOWN) by the value of the data item or literal following BY.

The third format of the SET statement is used to set the value of an external switch. The mnemonic(s) between SET and TO is set to ON or OFF according to the ON or OFF option supplied.

The fourth format of the SET statement is used to set the condition-name conditions to true. The condition(s) between SET and TO is set to TRUE.

Tips

Regular MOVE and mathematical statements do not work on indices. Use the SET statement to manipulate an index.

For format 4, if more than one value is associated with the condition-name being true, the first value listed is the one that will be used.

ENVIRONMENT DIVISION

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ENVIRONMENT DIVISION
" ALPHABET
" CLASS
" CONFIGURATION SECTION
" CURRENCY
" DECIMAL-POINT
" FILE-CONTROL
" INPUT-OUTPUT SECTION
" OBJECT-COMPUTER
" SELECT
" SOURCE-COMPUTER
" SPECIAL-NAMES and SYMBOLIC.

DATA DIVISION

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DATA DIVISION
" CD (Communication Description)
" COMMUNICATION SECTION
" Condition Name
" Data Description
" FD (File Description)
" FILE SECTION
" LINKAGE SECTION
" Record Description
" RD (Report Description)
" Reference Modification
" SCREEN SECTION
" SD (Sort Description)
" Subscripting
" Table Handling
" WORKING-STORAGE SECTION

PROCEDURE DIVISION

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PROCEDURE DIVISION

" ACCEPT
" ADD
" ALTER
" CALL
" CANCEL
" CLOSE
" COMPUTE
" CONTINUE
" DELETE
" DISPLAY
" DIVIDE
" ENTER
" EVALUATE
" EXIT
" GENERATE
" GO
" GOBACK
" IF
" INITIALIZE
" INITIATE
" INSPECT
" MERGE
" MOVE
" MULTIPLY
" OPEN
" PERFORM
" READ
" RECEIVE
" RELEASE
" RETURN
" REWRITE
" SEARCH
" SEND
" SET
" SORT
" START
" STOP
" STRING
" SUBTRACT
" TERMINATE
" UNSTRING
" USE
" WRITE

IDENTIFICATION DIVISION

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IDENTIFICATION DIVISION

" AUTHOR
" DATE-COMPILED
" DATE-WRITTEN
" INSTALLATION
" PROGRAM-ID
" REMARKS
" SECURITY

The DATE-COMPILED Paragraph

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The DATE-COMPILED Paragraph
Syntax
Format
DATE-COMPILED. comment-entry.

Description
The DATE-COMPILED paragraph is used to document the last time that the program was compiled. DATE-COMPILED is in Area A followed by a period. The actual date will be supplied by the compiler at compile time, so no additional entry is necessary.
Tips
1. This paragraph has been marked for removal from the COBOL language, because the information is usually provided in the heading on the listing produced by the compiler.
2. You can include this paragraph in a program to be compiled on a compiler from which the paragraph has been removed by including *'s in column 7.

The PROGRAM-ID Paragraph

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The PROGRAM-ID Paragraph
Syntax
Format
PROGRAM-ID. program-name.

Description
The PROGRAM-ID paragraph is used to document the name of the COBOL program. The program name must be a valid identifier. PROGRAM-ID is in Area A followed by a period and the program name is in Area B followed by a period.
Tips
1. This is the only paragraph that is required in the IDENTIFICATION DIVISION.
2. Although the program name must follow the rules for identifiers, the name provided is frequently used by the operating system, so there may be additional restrictions on the name depending on the computer system involved.

PROCEDURE DIVISION

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The ADD Statement
Syntax
Format 1
ADD { identifier-1
literal-1 } ... TO {identifier-2 [ROUNDED]} ...
[ON SIZE ERROR imperative-statement-1]
[NOT ON SIZE ERROR imperative-statement-2]
[END-ADD]
Format 2
ADD { identifier-1
literal-1 } ... TO { identifier-2
literal-2 } GIVING {identifier-3 [ROUNDED]}...
[ON SIZE ERROR imperative-statement-1]
[NOT ON SIZE ERROR imperative-statement-2]
[END-ADD]
Format 3
ADD { CORRESPONDING
CORR } identifier-1 TO identifier-2 [ROUNDED]
[ON SIZE ERROR imperative-statement-1]
[NOT ON SIZE ERROR imperative-statement-2]
[END-ADD]

Description
The ADD statement is used to add numeric items together. Each of the three formats works slightly differently and is described in the correspondingly numbered area.
1. The first format is used to add field(s) or value(s) to another field.
The field(s) or value(s) listed between ADD and TO are summed and added to the value of the field(s) following the TO, where the answer is stored.
For example, in ADD A TO B, the value in A is added to the value in B and the result is stored in B.
The value in A is unchanged.
Furthermore, in ADD C D TO E F, the sum of the values in C and D is added to the value in E, storing the answer in E and the sum of C and D is also added to F storing the value in F. The values in C and D are unchanged.
2. The second format is used to add field(s) or value(s) to one another, storing the answer in a different field.
The field(s) or value(s) listed between ADD and GIVING are added and stored in the field following the field(s) following the GIVING.
For example, in ADD A TO B GIVING C, the value in A is added to the value in B and the result is stored in C.
The values in A and B are unchanged.
3. The third format is used to add subordinate field(s) of one group to subordinate field(s) in another, storing the answer in those fields.
Those subordinate field(s) of the group item identifier-1 are added to and stored in those with exactly the same name in the group item identifier-2.
The names of the subordinate items must be spelled exactly the same way in both groups to qualify to participate in the addition.
Tips
1. The fields to be added must have numeric pictures, that is, they can only have the characters 9, S, and V in their PIC clauses.
2. Receiving fields may be either numeric or numeric edited fields.
3. Use the SIZE ERROR clause to detect field overflow on the receiving field.
4. In all formats, the mathematically correct results are computed, but if the receiving field is too short in either the integer or decimal portion,the result will be truncated, the integer on the left and the decimal on the right. Including the ROUNDED phrase will result the answer field being rounded instead of truncating. Rounding is always done in the least significant portion of the answer.

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The ALTER Statement
Syntax
Format
ALTER {procedure-name-1 TO [PROCEED TO] procedure-name-2} ...
Description
The ALTER statement is used to modify the branching behavior of a GO TO statement.
In order to use the ALTER statement you must have a paragraph in the PROCEDURE DIVISION that consists of exactly one GO TO statement. The GO TO must be the only statement in the paragraph. Procedure-name-1 is the name that paragraph and procedure-name-2 is the name of any other paragraph in the PROCEDURE DIVISION. If the GO TO in procedure-name is subsequently executed, control will pass to procedure-name-2 instead of the name originally coded in the GO TO statement.

Tips
1. This statement was eliminated from the COBOL language and it was only included in this manual in case you encounter a previously coded program that uses this statement.
2. Use of this statement is considered to be a very bad idea.
3. If in spite of tips 1 and 2, you still elect to code an ALTER statement, please don't address your problems to the Ask The Expert section of this website.

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The PERFORM Statement
Syntax
Format 1
PERFORM [procedure-name-1 [{ THROUGH
THRU } procedure-name-2 ] ]
[imperative-statement-1 END-PERFORM]
Format 2
PERFORM [procedure-name-1 [{ THROUGH
THRU } procedure-name-2 ] ]
{ identifier-1
integer-1 } TIMES [imperative-statement-1 END-PERFORM]
Format 3
PERFORM [procedure-name-1 [{ THROUGH
THRU } procedure-name-2 ] ]
[ WITH TEST { BEFORE
AFTER }] UNTIL condition-1
[imperative-statement-1 END-PERFORM]
Format 4
PERFORM [procedure-name-1 [{ THROUGH
THRU } procedure-name-2 ] ]
[ WITH TEST { BEFORE
AFTER }]
VARYING { identifier-2
index-name-1 } FROM { identifier-3
index-name-2
literal-1 } BY { identifier-4
literal-2 }
UNTIL condition-2
[AFTER { identifier-4
index-name-3 } FROM { identifier-5
index-name-4
literal-3 } BY { identifier-5
literal-2 }
UNTIL condition-3] ...
[imperative-statement-1 END-PERFORM]

Description
The PERFORM statement is used to execute code. There are two different modes of operation.
The first mode is used to execute code in another area of the program and is invoked by mentioning a procedure or paragraph name. In this mode, the code in the other paragraph(s) is executed and control is returned to the statement following the PERFORM statement.
The second mode is called an in-line PERFORM, because the code to be executed is actually coded in place inside of the PERFORM statement. In the second mode, the code which is executed is that which lies between the PERFORM and the END-PERFORM.
Each of the four formats works slightly differently and is described in the correspondingly numbered area.
1. The first format is used to execute the referenced code one time.
2. The second format is used to execute the referenced code multiple times. The actual number of times is indicated by identifier-1 or integer-1.
3. The third format is also used to execute the referenced multiple times. The actual number of times is dependent upon condition-1. The code will be repeatedly executed until condition-1 is true. The condition will be tested before the code is actually tested unless the WITH TEST AFTER phrase is included.
4. The fourth format is used to execute the referenced multiple times with the actual number of times being dependent upon a condition like the third format. The primary difference between format 3 and format 4 is the VARYING phrase, which is used to count in a variable during the repetitions. The VARYING variable is set to the FROM value before the referenced code is executed. Before each subsequent pass, the VARYING variable is incremented by the amount of the BY value. The optional AFTER phrase may be included to vary the value of an additional variable. If this option is used, AFTER variable will be incremented through its entire cycle for each time the VARYING variable is cycled.

Tips
1. Use the PERFORM statement to access code which needs to be executed from more than one location.
2. Use the PERFORM statement to structure your code. Make a main routine that PERFORMs other routines which in turn PERFORM other routines which themselves PERFORM other routines until the level of complexity of the PERFORMed routines is easily comprehended.
3. Use the WITH TEST AFTER option to force the code to be PERFORMed at least once.
4. Use the VARYING ... AFTER option to pass through all the entries in multiple dimensional tables.
5. If the number of lines in the routine is relatively small, use an in-line PERFORM instead of PERFORMing a separate paragraph. The next programmer will appreciate not having to look all through the program to find the one or two lines of code in the paragraph you PERFORMed