Explain BMS Maps and Screen Handling in IBM Mainframes

24 / 60

CICSintermediate

BMS Maps and Screen Handling

Understand how Basic Mapping Support (BMS) separates screen layout from program logic for 3270 terminal interfaces.

BMS separates what a screen looks like from the program that fills it in. The layout is defined once, assembled, and used by any program.

BMS is like separating a printed form's template (where each blank line and label sits on the page) from the handwriting that fills it in — you can redesign the form's layout entirely without changing a single instruction about what data goes in the 'name' blank versus the 'address' blank.

Key Concepts

That separation predates the MVC pattern by decades and is the same idea.

A map is defined with macros.

    CUSTMAP  DFHMSD TYPE=&SYSPARM,MODE=INOUT,LANG=COBOL,TIOAPFX=YES
    CUSTM1   DFHMDI SIZE=(24,80)
    CUSTID   DFHMDF POS=(5,20),LENGTH=9,ATTRB=(UNPROT,IC)
    CUSTNAME DFHMDF POS=(7,20),LENGTH=30,ATTRB=(PROT)
             DFHMSD TYPE=FINAL

POS is row and column, LENGTH is the field width, and ATTRB says whether the user may type into it, whether it is bright, whether it accepts only digits.

POSLENGTHATTRB

Assembling produces two things, and knowing both is the point:

    physical map   a load module CICS uses to paint the screen
    symbolic map   a COBOL copybook your program moves data through

The copybook generates several subfields per screen field, with suffix letters:

    CUSTIDL   PIC S9(4) COMP    length -- how many characters were entered
    CUSTIDA   PIC X            attribute -- set it to change colour or
                                protection at runtime
    CUSTIDI   PIC X(9)         input -- what the user typed
    CUSTIDO   PIC X(9)         output -- what you send

So the program never touches screen coordinates. It moves values into CUSTNAMEO and reads CUSTIDI.

CUSTNAMEOCUSTIDI

Sending and receiving.

    EXEC CICS SEND MAP('CUSTM1') MAPSET('CUSTMAP') ERASE END-EXEC.
    EXEC CICS RECEIVE MAP('CUSTM1') MAPSET('CUSTMAP') END-EXEC.

The detail interviewers ask about: only modified fields come back. A 3270 terminal transmits the fields the user changed, not the whole screen.

    user typed into it       CUSTIDL = 9, CUSTIDI holds the value
    user left it alone       CUSTIDL = 0, CUSTIDI is low-values

So checking the length subfield before using the value is standard defensive practice. Reading CUSTIDI without checking CUSTIDL gives you low-values and treats "left blank" as a real entry.

CUSTIDICUSTIDL

Setting attributes at runtime is how errors are shown: move a bright-red attribute into CUSTIDA and resend the map, and that field is highlighted with the rest of the screen unchanged. That is the standard validation pattern in a BMS application.

CUSTIDA

What the interviewer is probing.1. "What is a BMS map?" Probing: the basics. Stalls: "A screen." Moves up: a screen layout defined separately from the program, generating both a physical map for display and a symbolic map the program uses as a data structure.

2. "What is the attribute byte for?" Probing: the detail. Stalls: "Colours." Moves up: controlling each field — protected or unprotected, bright or dark, and the modified data tag that decides whether the field is transmitted.

3. "What does the modified data tag change?" Probing: the subtle part. Stalls: "Nothing." Moves up: only modified fields are sent back, so a field the user did not touch arrives as low values — reading it as data is a classic bug.

4. "How is a map changed without recompiling the program?" Probing: the practical question. Stalls: "You cannot." Moves up: cosmetic changes to the physical map alone can be reassembled, but changing field names or lengths changes the symbolic map and needs a recompile.