RT-11
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RT-11 ('RT' for Real Time) was a real-time operating system for the DEC PDP-11 family of computers. It was a single-user operating system which did not support multitasking although RT-11 could be configured to support a single "foreground" task running concurrently with a single "background" task.
RT-11 was small and could be generated by the user, meaning a sufficiently skilled user with the source code could bootstrap their system from being without any OS into having a functional, configured RT-11 installation. The most common console was a printing terminal, but DEC also supported the VT-11 and VS-11 graphics display devices (vector graphics terminals with a graphic character generator for displaying text, and a light pen for graphical input).
A common program run on such a system was "Lunar Lander". Using the light pen, one would adjust the attitude of a lunar lander and the precentage thrust in an effort to have a controlled landing on the moon. If the landing was successful, a little man would exit the lander and plant a flag. If you landed in a certain spot on the moon's surface, the little man would exit the lander and order take-out!
There were quite a few other programs available including the KED visual editor, runoff, predecessor of troff and TeX, and a command shell, and a C compiler. The system was complete enough to handle most modern personal computing tasks, except for web browsing.
The RT-11 operating system could be stored on a 2.5MB removable hard disk platter, or quickly booted from an 8 inch floppy disk. The system supported a real time clock, printing terminal, VT-11 vector graphic unit, 16 channel 100 kHz A/D converter with 2 channel D/A, 9600 baud serial port, 16 bit bidirectional boards, etc. The file system supported names with six characters and a three-character extension ("6.3" vs. the MS-DOS 8.3 format) encoded in RAD50, which packed those 9 characters into only six bytes. Tools to develop and debug assembly-language programs were provided, but other languages including C, FORTRAN, PASCAL, and several versions of BASIC were available from DEC as 'layered products'. Versions of these and other programming languages were also available from other, third-party, sources.
One factor contributing to the simplicity (and speed) of RT-11 was the architecture of the file system; all files were contiguous, meaning that each file occupied consecutive blocks (the minimally addressable unit of disk storage, 512 bytes) on the disk. This meant that an entire file could be read (or written) very quickly. A side effect of this file system structure was that a disk might need to be periodically 'squeezed' to consolidate the unused portions.
Later, when DEC introduced the LSI computers, they were able to run RT-11 from an 8 inch floppy disk. Several years later, DEC introduced RSX-11, a multiuser, multitasking operating system, but RT-11 remained the operating system of choice for data acquisition systems where real time response was required.
RT-11 ran on all members of the PDP-11 family, from the PDP-11[/20] (the earliest released by DEC) to the PDP-11/93,94 (the last -11 released by DEC). It also ran on the Professional 300 Series and the PDT-11 ('PDT' for Programmed Data Terminal), both series of computers being based on a PDP-11 CPU, but using different bus and/or interrupt structures.
S&H Computing developed TSX-Plus, which was essentially a multi-user, multi-processing extension of RT-11.ru:RT-11