TSX-Plus is a multi-user operating system for the PDP-11/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 operating system (TSX-Plus installs on top of RT-11). The system is highly configurable and tunable.
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 kibibytes of memory, out of a maximum possible 4 mebibytes 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.
The software included a WP package named Lex-11 and a spreadsheet from Saturn Software. The machine slowed considerably if more than 8 students wanted to use the word-processing package at the same time. There was also a decision-table language called "D" from the NCC in Manchester which worked very well on TSX Plus.
The system was popular in the 1980s, although had a major number of clones. The latest version of TSX-Plus has TCP/IP support. The system is still in use and supported.