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CHAIN Statement


To transfer control to the specified program and pass (chain) variables to it from the current program.


CHAIN[MERGE] filename[,[line][,[ALL][,DELETE range]]]


MERGE overlays the current program with the called program.


The called program must be an ASCII file (previously saved with the a option) if it is to be merged (see the MERGE command).

filename is the name of the program that is called to be chained to. The .BAS extension is assumed unless another is specified.

line is a line number or an expression that corresponds to a line number in the called program. It is the starting point for execution of the called program. For example the following begins execution of PROG1.BAS at line 1000:

10 CHAIN "PROG1", 1000

If line is omitted, execution begins at the first line.

line is not affected by a RENUM command. However, the line numbers in the specified range are affected by a RENUM command.

ALL specifies that every variable in the current program is chained to the called program. For example:

20 CHAIN "PROG1", 1000, ALL

If the ALL option is omitted, the current program must contain a COMMON statement to list the variables that are passed.

CHAIN executes a RESTORE before it runs the program that it is to be chained to. The READ statement then gets the first item in the DATA statement. Reading will not resume where it left off in the program that is being chained.

After an overlay is executed and used for a specific purpose, it is usually desirable to delete it so that a new overlay may be brought in. To do this, use the DELETE command.

The CHAIN statement with the MERGE command leaves the files open and preserves the current option base setting.

If the MERGE command is omitted, the OPTION BASE setting is preserved, and CHAIN preserves no variable types or user-defined functions for use by the chained program. That is, any DEFINT, DEFSNG, DEFDBL, DEFSTR, or DEF FN statement containing shared variables must be restated in the chained program.

When using the merge command, place user-defined functions before any CHAIN MERGE statements in the program. Otherwise, they will be undefined after the merge is complete.