Transaction I/O - Introduction; Defining Display Screens; Uses Of Data Definition Statements; The Copy Library And Data Definition Statements - IBM 5280 Programmer's Manual

Chapter 1. Transaction I/O - Introduction
Transaction I/O is an IBM extension to COBOL that gives flexibility in
writing data to and reading data from a display screen. The purpose of this
chapter is to introduce transaction I/O and provide some rules that must be
followed in writing transaction I/O applications. Subjects covered include:
• Statements used to define display screens and what they can do in
formatting the screen and editing data entered by an operator
• How the statements are compiled with the COBOL source program
• Steps to follow in writing an interactive application
• A summary of the 5280 keyboard and how some of the keys interact
with your COBOL program
Defining Display Screens
The statements which you code to define display screens are called Data
Definition Statements (DDS). The Data Definition Statements for a screen
consist of one record description statement followed by one or more field
description statements.
Uses of Data Definition Statements
Here are some examples of what you can do with the Data Definition
Statements:
• Define the layout of the screen: the position of the fields that are to
appear on the screen, the data, if any, that is to appear in the fields, and
how the characters appear on the screen (highlight, blink, reverse image,
underline, nondisplay, and column separators).
• Cause the data to be edited as the operator enters it. For example, you
can define a field so that only alphabetic characters can be entered. If
the operator tries to enter a numeric, the keyboard will lock and an error
code will be displayed.
• Set the shift key on a character-by-character basis, so that it shifts
either to the lower symbol or to the upper symbol automatically as the
cursor enters each position in the field
The Copy Library and Data Definition Statements
The coded Data Definition Statements must be library text in a COBOL
library. At compilation, the COBOL compiler brings them into your program
and converts them into COBOL source statements.
The COBOL library is allocated by the facilities of the host system. The rules
for defining the library for each of the host compilers are given in Chapter 7.
Chapter 1. Transaction I/O - Introduction 1- 1