Ports — A Method for Dynamic Interprogram Communication and Job Control

Describes Ports, a unified method for communication between a computer program and terminals, files, peripheral devices, other programs, and supervisory software. In ISPL (Incremental System Programming Language, described in R-563), each job has a Port named MONITOR that handles resource allocation: creating and deleting files, assigning file space, core space, processor time. This design permits a hierarchical system of monitors, each controlling the jobs running under it. By routing output to a user terminal, Ports enables online debugging and simulation of rewritten files of programs. The Port concept improves modularity in three ways: (1) Each connection need not be specified by the programmer but can be decided at execution. (2) Linkage between programs is co-routine rather than subroutine, which simplifies programming, retains context, and removes the need for hierarchical organization. (3) With different connections via Ports, the same system can be used in many ways, e.g., online or off, in simulation mode, audit-trailed, or data breakpointed. (See also R-562, R-603, R-622, RM-5611.)