Writing a CICS Map, is just like designing a Web-Page in HTML. CICS Maps are coded using BMS Instructions. BMS stands for Basic Mapping Support. The three important BMS Instructions are DFHMSD, DFHMDI and DFHMDF.
The DFHMSD instruction is used to define a New Mapset. A Mapset is a list or collection of Maps. You put related Maps or CICS GUI Screens together into1 Mapset. Generally, when you have a big system, with hundreds of CICS Screens, you group all the screens related to same functionality under one common Umbrella, under one Mapset. A Mapset is a means to gift-wrap many CICS Pages together.
The DFHMDI Instruction is used to define a New Map(within a Mapset). A Map represents a single CICS GUI Screen. You code the DFHMDI Instruction to tell, what is the size or dimensions of the CICS Page, just like your PC-Screen has a resolution of 1024x768. The DFHMDI Instruction of the Map, also tells what’s the line the Cursor should be initially positioned, when the Screen is displayed.
Every CICS GUI Screen has many Fields – Title field, constant-fields, data-entry Fields. To create a New Field on a Map, you code the DFHMDF Instruction. The DFHMDF Instruction defines the placement of the Field(where does it appear on the Screen), is it a Display-Only constant-field or a Data-Entry Field, what is the Length of the Field, does it have an initial-value and much more.