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RT-11 ('RT' for Real Time) was a small, single-user real-time operating system for the DEC PDP-11 family of computers.
Users could choose from three variants with differing levels of support for multitasking:
- RT-11SJ (Single Job) allowed only one task.
- RT-11FB (Foreground/Background) supported two tasks: a high-priority, non-interactive "Foreground" job, and a low-priority, interactive "Background" job.
- RT-11VM (Virtual Memory) supported up to 6 "system" jobs in addition to the Foreground and Background jobs, but required a CPU with memory management hardware.
RT-11 was written in assembly language with heavy use of the conditional assembly and macro programming features of the MACRO-11 assembler. Every distribution included the source code of the operating system and its device drivers (with all the comments removed), and a utility named "SYSGEN" which would build the operating system and drivers according to a user-specified configuration. (The Linux kernel uses a similar approach; see menuconfig.) RT-11 device drivers were loadable, not built into the kernel. It was not uncommon for end-users to write or enhance device drivers.
Users generally operated RT-11 via a printing terminal or a video 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).
RT-11 came with several programs available including the KED visual editor (a cut-down version of the EDT text editor), TECO, runoff (predecessor of troff and TeX), a command shell (later versions supported DCL) and a batch processor. The system was complete enough to handle most modern personal computing tasks, except for web browsing. Large amounts of free, user-contributed software for RT-11 were available from DECUS, the DIGITAL Equipment Users Society, including a C compiler and run-time library.
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" at extra cost. 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. DEC also sold 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* (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.
Specialized versions
Several specialized PDP-11 systems were sold based on RT-11:
- LAB-11 provided an LPS-11 analog peripheral for the collection of laboratory data
- PEAK-11 provided further customization for use with gas chromatographs (analyzing the peaks produced by the GC); data collection ran in RT11's foreground process while the user's data analysis programs ran in the background.
- GT4x systems added a VT11 vector graphics peripheral. Several very popular demo programs were provided with these systems including Lunar Lander and a version of Spacewar!.
Compatibility with other DEC operating systems
Many RT11 programs (those that didn't depend upon specialized peripherals or direct access to the hardware) could be directly executed under the RT11 Run-time system of the RSTS/E timesharing system.
"RT-11"