Add-ons By Other Companies
System Industries bought the only source license for RSTS to implement an enhancement called SIMACS (SImultaneous Machine ACceSs), which allowed their special disk controller to set a semaphore flag for disk access, allowing multiple WRITES to the same files on a RSTS System where the disk is shared by multiple PDP-11 RSTS systems. This feature was implemented in System Industries controllers that were attached to many DEC computers and designed by Dr. Albert Chu while he worked at System Industries.
This required many changes to the way access to disks was accomplished by the RSTS operating system. The FIPS (File Information Processing System) system, that handled i/o access, was single-threaded in RSTS. To allow a disk access to stall while another machine had active access to a block, required that the FIPS could timeout a request, go to the next request and 'come back' to the stalled one in a round robin fashion. The code to allow this was written by Philip Hunt while working at System Industries, in Milipitas, CA. He eventually worked for Digital Equipment in the New England area in the late 1980s.
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