Dynamic Loading - History

History

Dynamic loading was a common technique for IBM/360 Operating systems (1960s to, the - still extant - Z/Architecture), particularly for I/O subroutines, and for COBOL and PL/1 runtime libraries. As far as the application programmer is concerned, the loading is largely transparent, since it is mostly handled by the operating system (or its I/O subsystem). The main advantages are:

  • Fixes (patches) to the subsystems fixed all programs at once, without the need to relink them
  • Libraries could be protected from unauthorized modification

IBM's strategic transaction processing system, CICS (1970s onwards) uses dynamic loading extensively both for its kernel and for normal application program loading. Corrections to application programs could be made offline and new copies of changed programs loaded dynamically without needing to restart CICS (that can, and frequently does, run 24/7).

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