Memory Management Techniques
Memory management techniques refer to how programs allocate, use, and release memory during the program execution. This involves defining data structures in the DATA DIVISION and using storage areas in WORKING-STORAGE. Proper memory management ensures efficient program execution, minimizes wasted resources, and prevents memory-related errors.
Below are the list of memory management techniques available in COBOL -
- REDEFINES
- RENAMES
- SYNCHRONIZED Clause
- Computational Items (USAGE Clause)
REDEFINES
REDEFINES defines a new variable for the existing variable, which means two variables share the same memory area. It is a way to declare multiple variables for a single memory area in different ways based on the requirement.
level-number target-variable REDEFINES source-variable [PIC ...].- level-number - The level number at which the variable is defined. It should be the same for both source and target variables.
- target-variable - Name of the new variable that is going to share the storage of source-variable.
- source-variable - Name of the variable whose storage is being redefined by target-variable.
- PIC .. - Optional. Additional clauses that describe the data, like PIC, VALUE, etc.
Example - Redefining a variable with same length.
----+----1----+----2----+----3----+----4----+----5----+
...
WORKING-STORAGE SECTION.
01 WS-VAR.
05 WS-VAR1 PIC X(20).
05 WS-REQ-VAR1 REDEFINES WS-VAR PIC X(20).
...
PROCEDURE DIVISION.
MOVE "MAINFRAME TECHNOLOGY" TO WS-VAR1.
DISPLAY "WS-VAR1: " WS-VAR1.
DISPLAY "WS-REQ-VAR1: " WS-REQ-VAR1.
DISPLAY " ".
MOVE "MAINFRAME APPLICATION SYSTEM" TO WS-REQ-VAR1.
DISPLAY "WS-VAR1: " WS-VAR1.
DISPLAY "WS-REQ-VAR1: " WS-REQ-VAR1.
...JCL to execute the above COBOL program −
//MATEPKRJ JOB MSGLEVEL=(1,1),NOTIFY=&SYSUID //* //STEP01 EXEC PGM=REDEFINE //STEPLIB DD DSN=MATEPK.COBOL.LOADLIB,DISP=SHR //SYSOUT DD SYSOUT=*
When the program compiled and executed, it gives the following result −
WS-VAR1: MAINFRAME TECHNOLOGY WS-REQ-VAR1: MAINFRAME TECHNOLOGY WS-VAR1: MAINFRAME APPLICATIO WS-REQ-VAR1: MAINFRAME APPLICATIO
RENAMES
RENAMES clause regroups the existing group of items and assigns a new name. It creates another logical group by regrouping some or all elementary variables of a group. Special purpose level number 66 is used to code the RENAMES clause.
01 WS-VAR-GRP1.
05 WS-VAR-A PIC ...
.
05 WS-VAR-N PIC ...
05 WS-VAR-O PIC ...
.
05 WS-VAR-Z PIC ...
66 WS-VAR-GRP2 RENAMES VAR-A THRU VAR-N.- WS-VAR-GRP1 - Specifies source group variable.
- WS-VAR-A, ..., WS-VAR-N - Specifies starting and ending elementary variables to be renamed.
- WS-VAR-GRP2 - Specifies target variable.
Example - Variable declaration using 66-level number and the variable usage in PROCEDURE DIVISION.
----+----1----+----2----+----3----+----4----+----5----+
...
WORKING-STORAGE SECTION.
01 WS-VAR.
02 WS-GRP-ITEM1.
05 WS-VAR1 PIC X(10) VALUE "MAINFRAMES".
05 FILLER PIC X(01).
05 WS-VAR2 PIC X(08) VALUE "ARE VAST".
05 FILLER PIC X(01).
05 WS-VAR3 PIC X(01) VALUE "&".
05 FILLER PIC X(01).
05 WS-VAR4 PIC X(10) VALUE "LEGENDARY".
05 FILLER PIC X(01).
05 WS-VAR5 PIC X(10) VALUE "SYSTEMS".
* Renaming WS-GRP-ITEM1
66 WS-GRP-ITEM2 RENAMES WS-VAR1 THROUGH WS-VAR2.
...
PROCEDURE DIVISION.
DISPLAY "GROUP ITEM1: " WS-GRP-ITEM1.
DISPLAY "GROUP ITEM2: " WS-GRP-ITEM2.
...JCL to execute the above COBOL program −
//MATEPKRJ JOB MSGLEVEL=(1,1),NOTIFY=&SYSUID //* //STEP01 EXEC PGM=RENAME //STEPLIB DD DSN=MATEPK.COBOL.LOADLIB,DISP=SHR //SYSOUT DD SYSOUT=*
When the program compiled and executed, it gives the following result −
GROUP ITEM1: MAINFRAMES ARE VAST & LEGENDARY SYSTEMS GROUP ITEM2: MAINFRAMES ARE VAST
Explaining Example -
In the above example, WS-GRP-ITEM1 is a group item with multiple variables. WS-GRP-ITEM2 is defined as the renaming of WS-GRP-ITEM1 from WS-VAR1 THROUGH WS-VAR2. So, WS-GRP-ITEM2 displays the data from WS-VAR1 to WS-VAR2.
SYNCHRONIZED Clause
Due to the variable allocation at the word boundaries, some bytes are unused between the boundary start and the previous allocation ending. These unused bytes are called Slack bytes. It allocates the variables at their respective natural memory boundaries (immediately after the previous allocation ends) and removes slack bytes.
data-item PIC data-type(length) [SYNCHRONIZED [LEFT|RIGHT]]- data-item - Name of the data item being defined.
- LEFT - Optional. It specifies left alignment and used for alphanumeric items.
- RIGHT - Optional. It specifies right alignment and used for numeric items. If neither coded, RIGHT is the default.
Example - Explaining the concept with student record.
01 STUDENT.
05 STUDENT-NO PIC 9(02).
05 STUDENT-NAME PIC X(12).
05 STUDENT-GRADE PIC 9(02).
05 STUDENT-CLASS PIC X(03).The compiler inserts the slack bytes after STUDENT-NO to start the next item STUDENT-NAME from the boundary. Similary, for the STUDENT-GRADER to start the STUDENT-CLASS from the boundary. The declaration becomes after slack bytes is -
01 STUDENT.
05 STUDENT-NO PIC 9(02).
[05 SLACK-BYTES PIC XX. INSERTED BY COMPILER]
05 STUDENT-NAME PIC X(12).
05 STUDENT-GRADE PIC 9(02).
[05 SLACK-BYTES PIC XX. INSERTED BY COMPILER]
05 STUDENT-CLASS PIC X(03).Total 4 slack bytes inserted if we are not used SYNC clause. To avoid the slack bytes, the layout should be declared with SYNC clause like below -
01 STUDENT.
05 STUDENT-NO PIC 9(02).
05 STUDENT-NAME PIC X(12) SYNC.
05 STUDENT-GRADE PIC 9(02).
05 STUDENT-CLASS PIC X(03) SYNC.The below diagram represents the same memory allocation along SYNC clause and there will be no slack bytes anymore -
Computational Items (USAGE Clause)
Computational items refer to memory storage forms that are mainly used for arithmetic calculations. These items are typically defined with the USAGE clause to specify their internal representation.
The list of USAGE modes in COBOL are -
- DISPLAY
- COMP | COMPUTATION
- COMP-1 | COMPUTATION-1
- COMP-2 | COMPUTATION-2
- COMP-3 | COMPUTATION-3
- COMP-4 | COMPUTATION-4
- COMP-5 | COMPUTATION-5
DISPLAY Computation
DISPLAY computation uses the character form. In character form, one character equals one byte (8 bits) of storage. If no usage clause is used, then DISPLAY usage will be applied by default.
It applies to the Numeric, Alphabetic and Alphanumeric data types. For example -
01 WS-VAR.
05 WS-VAR1 PIC 9(06) USAGE DISPLAY.
05 WS-VAR2 PIC 9(06).BINARY | COMP | COMPUTATION
COMP (BINARY) stores signed decimal numbers in pure binary format and applicable to numeric data items. This format is often used for arithmetic operations as it provides efficient storage and fast computation. The below table represents the storage occupied based on the number of digits in the PICTURE clause –
| Digits in PICTURE clause | Storage occupied |
|---|---|
| 1 through 4 | 2 bytes (halfword) |
| 5 through 9 | 4 bytes (full word) |
| 10 through 18 | 8 bytes (doubleword) |
01 WS-VAR.
05 WS-VAR1 PIC 9(5) USAGE IS COMP.
05 WS-VAR2 PIC 9(8) COMP.Both WS-VAR1 and WS-VAR2 occupies 4 bytes of memory.
COMP-1 | COMPUTATION-1
COMP-1 stores the numbers as single-precision (32 bit) floating-point numbers and applicable to numeric data items. It has no PICTURE clause, which is 4 bytes long (FULL WORD). COMP-1 data is stored in the format of mantissa and exponent.
| Digits in PICTURE clause | Storage occupied |
|---|---|
| 1 through 16 | 4 bytes (FULL WORD) |
01 WS-VAR.
05 WS-PI USAGE IS COMP-1.
05 WS-RADIUS USAGE IS COMP-1.COMP-2 | COMPUTATION-2
COMP-2 stores the numbers as internal double-precision (64 bit) floating-point numbers and applicable to numeric data items. It has no PICTURE clause, and it is 8 bytes long (DOUBLE WORD). COMP-2 data is stored in the format of mantissa and exponent.
| Digits in PICTURE clause | Storage occupied |
|---|---|
| 1 through 32 | 8 bytes (DOUBLE WORD) |
01 WS-VAR.
05 WS-PI USAGE IS COMP-2.
05 WS-RADIUS USAGE IS COMP-2.COMP-3 | COMPUTATION-3
COMP-3 (or Packed Decimal or Packed Numeric) stores the decimal numbers in a compact binary-coded decimal (BCD) format and applicable to numeric data items. COMP-3 variable contains any of the digits 0 through 9, a sign. It can have a value not exceeding 18 decimal digits.
The formula for memory calculation of the COMP-3 with n digits (variable length + 1 byte for SIGN if exists) in the declaration is -
- No. of bytes = Round ((n + 1)/2) - Where n is an odd number.
- No. of bytes = Round (n/2) - Where n is an even number.
01 WS-VAR.
05 WS-WIDTH PIC S9(02) USAGE IS COMP-3.
05 WS-AREA PIC S9(06) COMP-3.WS-AREA variable is declared as COMP-3, with a signed byte plus 6 digits. A total of 7 bytes and the (n+1)/2 formula will apply as n is an odd number. So, a total of 8/2 = 4 bytes allocated for WS-AREA.
COMP-4 | COMPUTATION-4 | COMP-5 | COMPUTATION-5 -
COMPUTATION-4 or COMP-4 or COMPUTATION-5 or COMP-5 is the equivalent of COMP or BINARY. The data items are represented in storage as binary data.
