Documentation of the Video Graphics System Message Handler (VMH), that provides interprocess communication and message switching among Video Graphics terminal users and multiaccess and personal programs running in the connected computers--which, at RAND, includes the entire ARPA Computing Network. VMH enables terminals and programs to communicate selectively, either one-to-one or multiplexed, across the boundaries set by computer system architecture. Message privacy is preserved. Written in OS/360 Assembler Language, VMH is a set of interrelated high-priority routines available to programs via macro instructions and to terminals via communication control commands. The message path scheme is separate from the message handling, which (1) permits users to be kept up to date about message path activity, and (2) makes the control system modifiable by simply adding new control requests and status values. VMH has operated almost continuously since early 1971, supporting 32 terminals. Tested throughput averages 600-850 messages/sec on a 360/65.
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