RSX-11 is a discontinued family of multi-user real-time operating systems for PDP-11 computers created by Digital Equipment Corporation.

In widespread use through the late 1970s and early 1980s, RSX-11 was influential in the development of later operating systems such as VMS and Windows NT.

As the original Real-Time System Executive[1][2] name suggests, RSX was designed (and commonly used) for real time use, with process control a major use.

The homonymic relation between DEC, DEX and deques (used as the primary linkage mechanism in the kernel) appealed to my sense of whimsy.

But in a short time I was asked to submit the choice to the corporate legal department for a trademark search and registration.

Bob sat back in silence as I kept looking at each acronym, seeing how it flowed off my tongue, what impression it gave me, and most importantly, the overall feeling about it.

"The porting effort first produced small paper tape based real-time executives (RSX-11A, RSX-11C) which later gained limited support for disks (RSX-11B).

While RSX-11D was being completed, Digital set out to adapt it for a small memory footprint, giving birth to RSX-11M, first released in 1973.

[12] Principles first tried in RSX-11M appear also in later designs led by Cutler, DEC's VMS and MICA and Microsoft's Windows NT.

[13][14][15] Under the direction of Ron McLean a derivative of RSX-11M, called RSX-20F, was developed to run on the PDP-11/40 front-end processor for the KL10 PDP-10 CPU.

[16] Meanwhile, RSX-11D saw further developments: under the direction of Garth Wolfendale (project leader 1972–1976) the system was redesigned and saw its first commercial release.

However a copy of the kernel source is present in every RSX distribution, because it was used during the system generation process.

Hobbyists can run RSX-11M (version 4.3 or earlier) and RSX-11M Plus (version 3.0 or earlier) on the SIMH emulator thanks to a free license granted in May 1998 by Mentec Inc.[27] Legal ownership of RSX-11A, RSX-11B, RSX-11C, RSX-11D, and IAS never changed hands; therefore it passed to Compaq when it acquired Digital in 1998[28] and then to Hewlett-Packard in 2002.

[29] In late 2015 Hewlett-Packard split into two separate companies (HP Inc. and Hewlett Packard Enterprise),[30] so the current owner cannot be firmly established.

In 1968,[34] the Soviet Government decided that manufacturing copies of IBM mainframes[35] and DEC minicomputers,[36][37] in cooperation with other COMECON countries,[34][38] was more practical than pursuing original designs.

RSX-11 provided features to ensure better than a maximum necessary response time to peripheral device input (i.e. real-time processing), its original intended use.

Correspondingly, a jumbled light pattern (reflecting memory fetches) is a visible indication that the computer is under load (and the idle task is not being executed).