Because software and not hardware is now the dominant cost in computing, users of mainframes will not trade them in for computers with modern design features incompatible with their software. This underlies the demand for the Job Control Language (JCL) program which provides the means of communication between an application, the operating system, and the hardware. In this fourth edition, JCL features that are no longer supported by IBM or are known to cause problems in distributed environments are separated out in a chapter on obsolete technology in legacy systems. Further updates for this edition include: changes in architecture, hardware, devices, and terminology; the addition of more tutorial aimed at readers coming from a PC background instead of a mainframe background; an explanation of JCL's role in batch programming and production jobs; new sections on programming in multi-platform systems and on operating system facilities; and examples that reflect concerns like Y2K and security.