RT-11 was run on PDP-11s as a real-time operating system. (RT for "Run Time" or "Real Time" OS). It did not do multitasking. It was small and could be generated by the user. The most common console was a printing terminal. DEC supported a VT-11 display device. The VT-11 was a vector graphics terminal. It had a graphic character generator for displaying text. It also had a light pen. A common program run on the system was "Lunar Lander." You adjusted the thrust and angle of the lander as it landed on the moon. If you landed in the right spot a little man would get out of the lander and plant a flag on the moon.
The RT-11 operating system could be stored on a 2.5mb removable hard disk platter. Or it could be booted from a 8 inch floppy disk. The system could support a real time clock, printing terminal, VT-11 vector graphic unit, 16 channel 100khz A/D converter with 2 channel D/A, a serial port 9600 baud, 16 bit bidirectional boards, etc. The file system used an 8.3 file name format. An assembly language and a FORTRAN compiler were available from DEC.
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. RSX-11 was a multiuser, multitasking operating system. RT-11 remained the operating system of choice for data acquisition systems where real time response was required.
RT-11 also ran on DEC-10s. It was run on DEC 11/34s to DEC 11/60s. It also ran on the PDT-11.
S&H Computing developed TSX-Plus OS, which is essentialy a multi-user, multi-processing extension of RT-11.