EAM - Evanescent Access Method

EAM is used for processing task-specific files in the SYSEAM area. EAM is a block-oriented access method and is particularly suitable for the rapid processing of job-dependent work files.

Macro for the EAM access method

All EAM functions are controlled by the EAM macro. The EAM macro covers the following functions:

Macro	Function
EAM	sets up and opens a new file opens an existing file reads (blockwise, sequential or direct) writes (blockwise, sequential or direct) checks and waits for I/O termination closes a file deletes a file

The desired operation is selected by specifying a hexadecimal operation code in the MFCB, and initiated by the EAM macro. The effect is determined by the MFCB fields which EAM additionally evaluates after analyzing the operation code (see table in chapter "MFCB (Mini File Control Block)", section "Structure of the MFCB").

The EAM macro controls all EAM accesses. EAM has the following characteristics:

• EAM files are not cataloged. As a result of this, no disk access is necessary when an EAM file is opened.

• Each EAM file is automatically deleted when the job which opened it is terminated (temporary file).

• Communication between EAM and the user takes place only via the EAM control block (MFCB = Mini File Control Block). No facilities exist for modifying this control block at OPEN time.

• EAM works exclusively with public volumes (pubsets). No distinction is made between disks with and without PAM keys (K and NK disks).

• The space requirements for the EAM routines and the runtimes for read and write accesses are less than for the standard access methods for cataloged files.

• An EAM file can be processed only by the job which created and opened it. One job may open and process several EAM files.

• EAM is block-oriented and is based on blocks of 2048 bytes each (= a PAM page). For chained I/O, up to 16 sequential blocks may be transferred at one time.

• EAM files may not exceed 32 GB.

• If a program is restarted using RESTART-PROGRAM, all existing EAM files belonging to the job are erased.

Note

Where the EAM file is created depends on whether or not shared pubsets are used and on the definitions set by systems support. For more information, see the “Introduction to System Administration” [7].