System Industries developed a capability for having more than one DEC CPU, but not at the same time, have write access to a shared disk. They implemented 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; the disk is shared by multiple DEC systems. SIMACS existed on
VAX and
PDP-11 RSTS systems. ==Advertising==