In COBOL-85, it is allowed to code nested programs, i.e. one program may contain another program(s). The another program(s) themselves may contain yet another program(s)
Below image depicts the concept of Nested programs
With introduction of nested program concept, following are few statements also introduced:-
END PROGRAM statement: All contained and containing programs are terminated using END PROGRAM statement. END PROGRAM statement is described in detail here.
IS GLOBAL clause: If you want to access working storage data item of contained program in containing prgrams then declare them with ‘IS GLOBAL’ clause. "IS GLOBAL clause" is described in detail here.
IS EXTERNAL clause: If you want to access working storage data item of containing programs in contained prgram then declare them with ‘IS EXTERNAL’ clause. "IS EXTERNAL clause" is described in detail here.
IS {COMMON/INITIAL} clause: With introduction of nesting of programs, the syntax of PROGRAM-ID paragraph has been extended by appending "IS {COMMON/INITIAL} clause". "IS {COMMON/INITIAL} clause" is described in detail here.
Each of above statements/clauses explained in detail in upcoming articles
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