RT-11
It was widely used for real-time computing systems, process control, and data acquisition across all PDP-11s.
RT-11 distributions included the source code of the operating system and its device drivers with all the comments removed and a program named "SYSGEN" which would build the operating system and drivers according to a user-specified configuration.
DEC encouraged such driver development by making their hardware subsystems (from bus structure to code) open, documenting the internals of the operating system, encouraging third-party hardware and software vendors, and by fostering the development of the Digital Equipment Computer Users Society.
RT-11 systems did not support preemptive multitasking, but most versions could run multiple simultaneous applications.
The CSI form expected input and output filenames and options ('switches' on RT-11) in a precise order and syntax.
The command-line switches were separated by a slash (/) rather than the dash (-) used in Unix-like operating systems.
These commands include DIR, COPY, RENAME, ASSIGN, CLS, DELETE, TYPE, HELP and others.
This feature was enabled through a SYSGEN selection, and only applied to the DU and LD device handlers.
The utilities DIR, DUP, PIP and FORMAT were for managing disk volumes.
Finally, VTCOM was used to connect with and use (or transfer files to and from) another computer system over the phone via a modem.
[6] Large amounts of free, user-contributed software for RT-11 were available from the Digital Equipment Computer Users Society (DECUS) including an implementation of C. Although the tools to develop and debug assembly-language programs were provided, other languages including C, Fortran, Pascal,[7] and several versions of BASIC were available from DEC as "layered products" at extra cost.
The RT-11 operating system could be booted from, and perform useful work on, a machine consisting of two 8-inch 250KB floppy disks and 56KB of memory, and could support 8 terminals.
Other boot options include the RK05 2.5MB removable hard disk platter, or magnetic tape.
RT-11 implemented a simple and fast file system employing six-character filenames with three-character extensions (6.3) encoded in RADIX-50, which packed those nine characters into only three 16-bit words (six bytes).
Each directory entry is 8 (or more) 16-bit words, though a sysgen option allowed extra application-specific storage.
The Unix operating system also became popular, but lacked the real-time features and extremely small size of RT-11.
In addition, it ran on the Professional Series and the PDT-11 "Programmed Data Terminal" systems, also from DEC.
Since the PDP-11 architecture was implemented in replacement products by other companies (E.g., the M100 and family from Mentec), or as reverse-engineered clones in other countries (E.g., the DVK from the Soviet Union), RT-11 runs on these machines as well.
Adding driver support for peripherals such as a CalComp plotter,[11] typically involved copying files, and did not require a SYSGEN.
Users could choose from variants with differing levels of support for multitasking, memory size, and separate I & D (Instruction and Data) address space: Several specialized PDP-11 systems were sold based on RT-11: Several clones of RT-11 were made in the USSR:
