Access Method Services-VSAM

Search Order for Locating
1.       Search all catalogs specified in STEPCAT
2.       If not found, search all catalogs specified in JOBCAT
3.       If not found, and the HLQ is an alias for a catalog, search the catalog. If the datatset is not found, fail the job.
4.       Otherwise search the master catalog.
For SMS managed datasets, JOBCAT and STEPCAT DD statements are not allowed and cause a job failure.
1.       The MCAT is searched, if found, verify if it is:
a.       A dataset name, then pick up the volume specification and check VTOC to locate the datatset.
b.       An alias, that is, the HLQ of the dataset name is equal to an alias entry pointing to an UCAT. In this case go to the referred UCAT.
2.       The UCAT is searched, if the dataset name is found, proceed as is on MCAT hit.

Tracks
The tracks are the thin concentric circular strips on a floppy medium or platter surface which actually contain the magnetic regions of data written to a disk drive. They form a circle and are (therefore) two-dimensional. At least one head is required to read a single track. All information stored on the hard disk is recorded in tracks.

Cylinders
A cylinder comprises the same track number but spans all such tracks across each platter surface that is able to store data (without regard to whether or not the track is "bad"). Thus, it is a three-dimensional object. Any track that comprises the same cylinder can be written to and read from while the actuator assembly remains stationary. One way drive makers have been able to increase drive speed is by increasing the number of platters that can be read at a given time.

Cylinders are vertically formed by tracks. In other words, track 12 on platter 0 plus track 12 on platter 1 etc. is cylinder 12. The number of cylinders of a disk drive exactly equals the number of tracks on a single surface in the drive.

Sectors
Tracks are subdivided into a number of sections. Each section is called a sector. Sectors are the smallest storage units on a hard drive. Typically a sector will hold 512 bytes of information. [1] Some vendors of hard drives and software developers are attempting to change the amount of data stored in a sector to 4,096 bytes.[2]

Volumes
"Logical drive" and "volume" should be considered synonymous, however "volume" and "partition" are not synonymous. 

IDCAMS


IDCAMS which is also known by the functionality it provides - Access Method Services - or, simply, AMS.
The basic JCL to run the utility is:


//IDCAMS   JOB 'JAY MOSELEY',CLASS=A,MSGLEVEL=(1,1),MSGCLASS=A
//IDCAMS   EXEC PGM=IDCAMS,REGION=4096K                       
//SYSPRINT DD  SYSOUT=A                                       
//SYSIN    DD  *                                              
               /* UTILITY COMMAND STATEMENTS */
/*                                                              
//                                                            
Some of the functions require additional DD statements when they are referenced by the AMS command statements, but SYSPRINT and SYSIN are the only absolute requirements. IDCAMS Utility can't be executed without 2 DD Statements .

Statement Syntax

The format of AMS commands is basically free form, and resembles REXX or PL/1.  The default statement margins are positions 2 through 72.  Any command statement may be continued from one line to the next by following the last parameter in a line with a hyphen (-).  A value may be continued by immediately following it with a plus sign (+).  A comment may be embedded in the command statements by enclosing the comment characters with /* and */.  Blank lines may be included at any point before, interspersed with, or following AMS commands.  Positional parameters are not optional and must precede all keyword parameters.  Keyword parameters can stand alone or they may have an associated set of values or a subparameter list enclosed in parentheses.  Parameters, subparamenters, and values are separated from one another by spaces, commas, or comment blocks. 

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