To run a program off the AS/400 family, the program must use files.
Programs perform read and write operations on these files. In general,
a file (or file object) is comprised of a description that contains a unique
set of characteristics. These characteristics describe how the data
associated with the file is organized. For example, files can be
structured to contain separate records and fields.
The information required in a file description is determined by the file
type. Different file types have different capabilities. The file
description contains the valid operations of a particular file, and determines
how a program can use the data. When creating a file using DDS, the
file description is based on DDS specifications.
Files that reside on an AS/400 business computing system include:
- Database files
- The database file contains the data and the descriptions of the
data. This description is used to control the flow of data between the
program and the database file. For instance, how input data is to be
presented to a program from internal storage and how output data is to be
presented to internal storage from a program. There are two kinds of
database files: physical files and logical files
- Device files
- A device file does not have actual data associated with it. It
specifies how a device can be used. The actual device is externally
attached hardware, such as a display station, printer, tape or diskette unit,
or a communication file. The device description defines the device to
the system, while the device file formats output data from the program for
presentation to the device, and formats input data from the device for
presentation to the program. The different types of device files
described using DDS are: printer files, display files, and Intersystem Communications
Function (ICF) files
- Spooled files
- You can specify a diskette or printer file to be spooled. When you
specify a file to be spooled, the data is stored as an file object until the
program or device is available to process it. Spooling can usually
shorten the run-time of the job and increase the number of jobs that can be
run because spooling allows the job the continue independently of the speed or
availability of the device.
- Distributed Data Management (DDM) files
- DDM files describe database files stored on a remore system. This
type of file allows an application program to use data stored on another
system in the network. Programs using DDM files run as though the
remote database files were local database files.
- Save files
- Save files can be used to store saved data on a disk to perform
input/output operations from a high-level language program, or to receive
objects sent through the network. This type of file does not require
diskettes or tapes. It is stored in auxiliary storage.
(C) Copyright IBM Corporation 1992, 2005. All Rights Reserved.