TSX-Plus
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TSX-Plus is a multi-user operating system for the LSI-11 series of computers. It was developed by S&H Computer Systems, Inc. and is based on DEC's RT-11 single-user real-time OS (that is, TSX-Plus installs on top of RT-11). The system is highly configurable and tunable.
The system is interesting because, due to the constraints of the memory management system in the PDP-11/LSI-11, the entire operating system core must occupy no more than 40 Kbytes of memory, out of a maximum possible 4 Mbytes of physical memory that can actually be installed in those machines (mandated by the 22-bit address space). Despite these limitations, TSX-Plus is actually able to service up to about 30 simultaneous users while still maintaining decent response times.