Overview of nonprivileged access methods

The selection of an access method depends on the requirements for each specific problem to be solved and on the special features of the individual access methods. These access methods are dealt with in detail in corresponding sections A comparative overview of the features of the nonprivileged access methods is provided below.

Selection based on volume

Volumes	ISAM	SAM	BTAM	UPAM	FASTPAM	DIV	EAM
Public disk
K disk	x	x	-	x	x	x	x
NK2 disk	x	x	-	x	x	x	x
NK4 disk	x	x	-	x	x	x
Private disks
NK2 disk	x	x	-	x	x	x	x
NK4 disk	x	x	-	x	x	x	x
Tape		x	x	x

Table 32: Access method selection based on volume

Selection based on access mode

Access method	ISAM	SAM	BTAM	UPAM	FASTPAM	DIV	EAM
Block-oriented	-	-	x	x	x	x	x
Record-oriented	x	x	-	-	-	-	-

Table 33: Access method selection based on access mode

Selection based on record type

Record type	ISAM	SAM
Variable	x	x
Fixed	x	x
Undefined	-	x

Table 34: Access method selection based on record type

Selection based on data structure

Definition of data structure	ISAM	SAM	BTAM	UPAM	FASTPAM	DIV	EAM
by DMS	x	x	-	-	-	-	-
by the user	-	-	x	x	x	x	-

Table 35: Access method selection based on data structure

Selection based on lifespan of file

Lifespan of the file	ISAM	SAM	BTAM	UPAM	FASTPAM	DIV	EAM
Permanent file	x	x	x	x	x	x
Temporary file	-	-	-	-	-	-	x

Table 36: Access method selection based on lifespan of file

Selection based on multi-user mode

Multi-user mode	ISAM	SAM	BTAM	UPAM	FASTPAM	DIV	EAM
Processing	x	-	-	x	x	x	-

Table 37: Access method selection based multi-user mode

Advantages and uses of individual access methods

ISAM – Indexed-Sequential Access Method

ISAM is a record-oriented access method for disk files which allows records to be accessed via predefined (primary and secondary) keys. Files can also be processed sequentially with ISAM.

SAM – Sequential Access Method

SAM is an access method that supports the sequential reading and writing of logical records.

UPAM – User Primary Access Method

UPAM is a block-oriented access method in BS2000 for random access to disk files. Read or write access to any block of a file is possible at any time. The basis for file processing is a standard block; record structures are not supported.

BTAM – Basic Tape Access Method

The access method BTAM is used to save and retrieve block-oriented data in a sequentially organized tape file. BTAM can also be used to process tape files which were not created in a BS2000 system, provided such files comply with the hardware conventions for recording data on magnetic tapes.

FASTPAM – Fast Primary Access Method

FASTPAM (FAST Primary Access Method) is a block-access method for NK4 disk files. It is characterized by the following features:

• reduced path lengths for I/O execution,

• a clear and efficient interface, and

• support for I/Os in data spaces.

DIV – Data-In-Virtual

DIV is an access method that differs from the access methods ISAM, SAM and UPAM in that it does not require files to be structured in records or blocks and works without I/O buffers and operations such as GET, PUT, etc.

Parts of a file or an entire file are mapped in a virtual address space by the DIV function MAP. Then data from the file can be accessed using CPU instructions (MVC, CLI,...).

EAM – Evanescent Access Method

EAM is the access method for EAM files in BS2000. EAM files are not cataloged. As a result of this, no disk access is necessary when an EAM file is opened. EAM files are automatically deleted on completion of a task.