ISAM - Indexed-Sequential Access Method

ISAM, like SAM, is a record-oriented access method for disk files. Processing is based on a file composed or index and data blocks. Each data record contains a key, and these act as a sort criterion (for index and data blocks, see the “Introductory Guide to DMS” [1]).

There are two versions of the ISAM access method, capable of processing files with two different block formats (see chapter “Access Methods” in the “Introductory Guide to DMS” manual [1).

• NK-ISAM (Non-Key ISAM) processes files with the block format “DATA”: These do not contain a separate PAM key. The DMS management information is kept in a block control field within the PAM page.

• K-ISAM (Key ISAM) processes files with the block format “PAMKEY”: These are characterized by the fact that DMS management information for each PAM page is kept in a separate PAM key located outside the page.

By means of the BLKCTRL operand in the macros FILE and FCB, the user can select whether a K file or an NK file is to be processed:

• BLKCTRL=DATA/DATA2K/DATA4K declares an NK-ISAM file

• BLKCTRL=PAMKEY declares a K-ISAM file

As K-ISAM is becoming less and less relevant in practice, from BS2000 OSD/BC V11.0 onwards ISAM files are created as NK-ISAM on K disks by default as well.

If the user makes no explicit specification for BLKCTRL, the BLKCTRL system parameter controls in general whether the file is created on K disks with the BLKCTRL=PAMKEY or DATA/NO attribute.
If the file is to be created as ISAM file, the ISBLKCTL system parameter controls whether the file is created as an NK-ISAM file on K disks as well. The default setting of ISBLKCTL is c'NONKEY', i.e. an ISAM file is created as NK-ISAM file on K disks by default as well.

Macros for the ISAM access method

The macros for the ISAM access method can be split into function classes:

Administration:	Macros with administration functions that support file processing.
Access:	Macros that access the data on a file.

Macro call	Function class	Brief description
ADDPLNK	Administration	Assigns a pool link name to a user ISAM pool
CREAIX	Administration	Creates a secondary index for an ISAM file
DELAIX	Administration	Deletes secondary indices for an ISAM file
DELPOOL	Administration	Deletes a user ISAM pool
ELIM	Access	Erases a record from the file
GET	Access	Reads the following record in the file (sequential reading)
GETFL	Access	When using marked ISAM keys: reads the following record in the marking area (sequentially)
GETKY	Access	Reads the first record with the specified key
GETR	Access	Reads the previous record (sequential reading, in reverse)
INSRT	Access	Adds a record with a new ISAM key to the file
ISREQ	Access	Removes an ISAM lock
OSTAT	Access	Gives details of the number and type of synchronous file accesses
PUT	Access	Writes records sequentially to the end of file (including checking the key for valid sequence)
PUTX	Access	Replaces a record provided by GET, etc.
REMPLNK	Administration	Deletes the pool link name
RETRY	Access	Resets the ISAM pointer after execution of the EXLST-PGLOCK and repeats the last macro
SETL	Access	Positions the ISAM pointer for subsequent sequential processing at start or end of file or a particular record
SHOPLNK	Administration	Gives details of assignment of ISAM pools to pool link names
SHOPOOL	Administration	Gives details of attributes and allocation states of ISAM pools
STORE	Access	Adds a record with a new ISAM key to the file Overwrites a record with an existing ISAM key, of multiple assignment of ISAM keys is not permitted Adds a record with an existing ISAM key to the file as the last record with this key