Editing RSX-11 (section)

===Main versions=== 
*'''RSX-11A''', '''C''' – small paper tape real time executives
*'''RSX-11B''' – small real time executive based on RSX-11C with support for disk [[input/output|I/O]].  To start up the system, first DOS-11 was booted, and then RSX-11B was started.  RSX-11B programs used DOS-11 macros to perform disk I/O.
*'''RSX-11D''' – a multiuser disk-based system, later evolved into IAS
*'''[[Interactive Application System|IAS]]''' – a timesharing-oriented variant of RSX-11D released at about the same time as the PDP-11/70. The first version of RSX to include DCL ([[Digital Command Language]]), which in IAS is known by its original name, PDS (Program Development System).<ref name=DEChist75/>
*'''RSX-11M''' – a multiuser version that was popular on all PDP-11s
*'''RSX-11S''' – a memory-resident version of RSX-11M used in embedded real-time applications. RSX-11S applications were developed under RSX-11M.
*'''RSX-11M-Plus''' – a much extended version of RSX-11M, originally designed to support the multi-processor PDP-11/74,<ref>{{cite web| url=https://groups.google.com/forum/#!msg/alt.folklore.computers/GTTkKHEJ1BU/pVJPmmOQLNsJ| title=Multiprocessing PDP-11s| author=Eric Postpischil| date=1990-01-22| work=alt.folklore.computers newsgroup| access-date=2015-08-25}}</ref> a computer that was never released, but RSX-11M-Plus was then used widely as a standard operating system on the PDP-11/70.
:RSX-11M-Plus also ran on PDP-11/44, PDP-11/84, PDP-11/94 (Unibus machines), as well as PDP-11/73, PDP-11/83, and PDP-11/93 (Qbus machines).  One of the advantages of RSX-11M-Plus over RSX-11M was that larger programs could be created. This was achieved by having the task builder (the linker) build the program to use the [[PDP-11 architecture#Memory expansion|separate instruction and data space]] feature of some PDP-11 models to put executable code and data into separate address spaces.  This also allowed programs to run faster, as it reduced the need for "overlays", in which you could overlay object modules at task build time, for very large programs.  Overlays were specified in a task build command file.