Chain Loading in BASIC Programs
In BASIC programs, chain loading is the purview of the CHAIN statement (or, in Commodore BASIC, the LOAD statement), which causes the current program to be terminated and the chained-to program to be loaded and invoked (with, on those dialects of BASIC that support it, an optional parameter specifying the line number from which execution is to commence, rather than the default of the first line of the new program). The common data area varies according to the particular dialect of BASIC that is in use. On BBC BASIC, for example, only a specific subset of all variables are preserved across a CHAIN. On other BASICs, the COMMON statement can be used in conjunction with CHAIN to specify which variables are to be preserved as common data across a chain operation.
Chain loading permits BASIC programs to execute more program code than could fit into available program and variable memory. Applications written in BASIC could thus be far larger than the size of working memory, via a set of cooperating programs that CHAIN back and forth amongst themselves as program flow moves within the overall application.
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