RT-11
INTRODUCTIONRT-11 (Real-time 11) was a low end, single user real-time operating system for the full line of Digital Equipment Corporation PDP-11 16-bit computers. RT-11 was first implemented in 1970. It was widely used for real-time computing systems, process control, and data acquisition across all PDP-11s. It was also used for low-cost general purpose computing.
SYSTEM FEATURES
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Multitasking
RT-11 systems did not support preemptive multitasking, but most versions could run multiple simultaneous applications. All variants of the monitors provided a background job. The FB, XM, and ZM monitors also provided a foreground job, and six system jobs if selected via the SYSGEN system generation program. These tasks had fixed priorities, with the background job lowest and the foreground job highest. It was possible to switch between jobs from the system console user interface, and SYSGEN could generate a monitor that provided a single background job (the SB, XB and ZB variants). The terms foreground and background are counterintuitive; the background job was typically the user's command-line interpreter; a foreground job might be doing something like non-interactive data collection.
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Source Code
RT-11 was written in assembly language. Heavy use of the conditional assembly and macro programming features of the MACRO-11 assembler allowed a significant degree of configurability and allowed programmers to specify high-level instructions otherwise unprovided for in machine code. 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. Developer's documentation included a kernel listing that included comments.
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Device Drivers
In RT-11, device drivers were loadable, except that prior to V4.0, the device driver for the system device (boot device) was built into the kernel at configuration time. Because RT-11 was commonly used for device control and data acquisition, it was common for developers to write or enhance device drivers. 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 (DECUS).
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Human Interface
Users generally operated RT-11 via a printing terminal or a video terminal, originally via a strap-selectable current-loop (for conventional teletypes) or via an RS-232 (later RS-422 as well) interface on one of the CPU cards; DEC also supported the VT11 and VS60 graphics display devices (vector graphics terminals with a graphic character generator for displaying text, and a light pen for graphical input). A third-party favorite was the Tektronix 4010 family.
The Keyboard Monitor (KMON) interpreted commands issued by the user and would invoke various utilities with Command String Interpreter (CSI) forms of the commands. RT-11 command language had many features (such as commands and device names) that can be found later in the PC DOS line of operating systems which heavily borrowed from RT-11. The CSI interpreter 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. All commands had a full form and a short one to which they could be contracted. For example, the RENAME command could be contracted to REN.
Batch files and the batch processor could be used to issue a series of commands with some rudimentary control flow. Batch files had the extension .BAT.
In later releases of RT-11, it was possible to invoke a series of commands using a .COM command file, but they would be executed in sequence with no flow control. Even later, it was possible to execute a series of commands with greater control through use of the Indirect Command File Processor (IND), which took .CMD control files as input.
Files with the extension .SAV were a sort of executable. They were known as "save files" because the RT-11 SAVE command could be used to save the contents of memory to a disk file which could be loaded and executed at a later time, allowing any session to be saved.
The SAVE command, along with GET, START, REENTER, EXAMINE and DEPOSIT were basic commands implemented in the KMON. Similar commands and utilities were later added to the DOS line of operating systems. These commands include DIR, COPY, RENAME, ASSIGN, CLS, DELETE, TYPE, HELP and others. The FORMAT command was used for physical disk formatting, although it was not capable of creating the file system; the INIT command was used for that. (analogue of DOS command FORMAT /Q). Most commands supported wildcards (e.g. *) in file names.
Physical device names were specified in the form 'dd{n}:' where 'dd' was a two-character alphabetic device name and the optional 'n' was the unit number (0–7). When the unit number was omitted, unit 0 was assumed. For example, TT: referred to the console terminal, LP: (or LP0:) referred to the parallel line printer, and DX0:, DY1:, DL4: referred to disk volumes (RX01 unit 0, RX02 unit 1, RL01 or RL02 unit 4, respectively). Logical device names consisted of 1–3 alphanumeric characters and were used in the place of a physical device name. This was accomplished using the ASSIGN command. For example, one might issue ASSIGN DL0 ABC which would cause all future references to 'ABC:' to map to 'DL0:'. Reserved logical name DK: referred to the current default device. If a device was not included in a file specification, DK: was assumed. Reserved logical name SY: referred to the system device (the device from which the system had been booted).
Later versions of RT-11 allowed specification of up to 64 units (0–77 octal) for certain devices, but the device name was still limited to three alphanumeric characters. This feature was enabled through a SYSGEN selection, and only applied to the DU and LD device handlers. In these two cases, the device name form became 'dnn:' where 'd' was 'D' for the DU device and 'L' for the LD device, and 'nn' was 00–77(octal).
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Software
RT-11 was distributed with many utilities. The utilities DIR, DUP, PIP and FORMAT were for managing disk volumes. TECO, EDIT, and the visual editors KED (for the DEC VT100) and K52 (for the DEC VT52) were used to create and edit source and data files. MACRO, LINK, and LIBR were for building executables. ODT, VDT and the SD device were used to debug programs. DEC's version of RUNOFF was for producing documents. Finally, VTCOM was used to connect with and use (or transfer files to and from) another computer system over the phone via a modem.
The system was complete enough to handle many modern personal computing tasks. Productivity software such as LEX-11, a word processing package, and a spreadsheet from Saturn Software, used under other PDP-11 operating systems, also ran on RT-11. Large amounts of free, user-contributed software for RT-11 were available from the DECUS including an implementation of C. Although the tools to develop and debug assembly-language programs were provided, 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 third-party sources. It was even possible to network RT-11 machines using DECNET, there was also support for TCP/IP and other protocols developed by third-party sources.
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Distributions and Minimal System Configuration
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. Distributions were available pre-installed or on punched tape, magnetic tape, cartridge tape, or floppy disk. A minimal but complete system supporting a single real-time user could run on a single floppy disk and in 8K 16-bit words (16KB) of RAM, including user programs. This was facilitated by support for swapping and overlaying. To realize operations on such small memory system, the keyboard command user interface would be swapped out during the execution of a user's program and then swapped into memory upon program termination. The system supported a real-time clock, printing terminal, VT11 vector graphic unit, 16 channel 100 kHz A/D converter with 2 channel D/A, 9600 baud serial port, 16 bit bidirectional boards, etc.
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File System
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). 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, as files were created and deleted on a volume over time, the unused disk blocks would eventually result in disk fragmentation which could become a constraint for creating large files; the remedy was to periodically “squeeze” a disk to consolidate (defragment) the unused blocks.
Each volume has only one directory which was pre-allocated at the beginning of the volume. The directory consists of an array of entries, one per file or unallocated space. Each directory entry is 8 (or more) 16-bit words, though a SYSGEN option allowed extra application-specific storage.
The file system is similar to that of OS/8; files must be stored contiguously on disk. If a new file does not fit in the contiguous space left by an old deleted file, the files on the disk must be moved together to create a larger area of free space. This feature is supported by the SQUEEZE command.
.SQU Device? DK0: RK0:/Squeeze; Are you sure? Y
To create a directory listing, use the DIR command; similar to that of DOS.
Batch scripts are named with the COM extension. As with DOS, they re executed with the ampersand in front of the file name. To execute a program from the command line interface (CLI), type RUN and the executable file name.
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Compatibility with Other DEC Operating Systems
Many RT-11 programs (those that did not need specialized peripherals or direct access to the hardware) could be directly executed using the RT-11 RTS (Run-time system) of the RSTS/E timesharing system or under RTEM (RT Emulator) on various releases of both RSX-11 and VMS.
The implementation of DCL for RT-11 increased its compatibility with the other DEC operating systems. Although each operating system had commands and options which were unique to that operating system, there were a number of commands and command options which were common.
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Hardware
RT-11 ran on all members of the DEC PDP-11 family, both Q-Bus and Unibus-based, from the PDP-11/05 (its first target, in 1970 (note that the PDP-11/05 was released 1972, so this probably was a PDP-11/20)), to the final PDP-11 implementations (PDP-11/93 and /94). In addition, it ran on the Professional Series (P/OS) 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 from Mentec), or as reverse-engineered clones in other countries (e.g. the DVK from the Soviet Union), RT-11 ran on these machines as well.
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Peripherals
Adding driver support for peripherals such as a CalComp plotter, typically involved copying files, and did not require a SYSGEN. The RT-11 device names are:
- DLn - RL01 disk cartridge
- DMn - RK06 disk cartridge
- DPn - RP02 or RP03 disk
- DSn - RJS03/4 disk
- DTn - TC11 DECtape
- DXn - RX01 diskette
- MMn - TU16 magtape
- MTn - TU10 or TS03 magtape
- RFn - RF11 fixed head disk drive
- RKn - RK05 disk cartridge
- DDn - TU58 DECtape II
- DKn - Logical device, user drive
- SYn - Logical device, disk n on the boot controller
The SHOW command will display device information on an RT-11 system.
.SHOW TT RK (Resident) RK0 = SY , DK BA PC NL LP MS MT DM DL DX DP 4 free slots
If you enter SHOW USERS on this single user system, it responds with "Nobody but you"; very clever!
DEC also sold RSX-11, a multiuser, multitasking operating system with real-time features, and RSTS/E (originally named RSTS-11) a multiuser time-sharing system. However, RT-11 remained the operating system of choice for data acquisition systems where real time response was required. The Unix operating system also became popular but lacked the real-time features and extremely small size of RT-11.
Derivative Operating Systems
- SHAREplus - HAMMOND software distributed a number of RT-11 compatible operating systems including STAReleven, an early multi-computer system and SHAREplus, a multi-process/multi-user implementation of RT-11 which borrowed some architectural concepts from the VAX/VMS operating system. RT-11 device drivers were required for operation. Transparent device access to other PDP-11s and VAX/VMS were supported with a network option. Limited RSX-11 application compatibility was also available. SHAREplus had its strongest user base in Europe.
- TSX-11 - developed by S&H Computing, was a multi-user, multi-processing implementation of RT-11.
The only thing it didn't do was handle the boot process, so any TSX-Plus machine was required to boot RT-11
first before running TSX-Plus as a user program. Once TSX-Plus was running, it would take over complete control
of the machine from RT-11. It provided true memory protection for users from other users, provided user accounts
and maintained account separation on disk volumes and implemented a superset of the RT-11 EMT programmed requests.
S&H wrote the original TSX because "Spending $25K on a computer that could only support one user?" bothered the founder Harry Sanders. The outcome was the initial four-user TSX in 1976. TSX-Plus, released in 1980, was the successor to TSX, originally released in 1976. The system was popular in the 1980s. RT-11 programs generally ran, unmodified, under TSX-Plus and most of the RT-11 utilities were used as-is under TSX-Plus. Device drivers generally required only slight modifications.
Depending on which PDP-11 model and the amount of memory, the system could support a up to 12 users (14-18 users on a 2Mb 11/73, depending on workload). The last version of TSX-Plus featured TCP/IP support.
RT-11 VERSIONS
Users could choose from four RT-11 variants with differing levels of support for multitasking:
- RT-11SJ (Single Job) allowed only one task. This was the initial distribution.
- RT-11FB (Foreground/Background) supported two tasks: a high-priority, non-interactive "Foreground" job, and a low-priority, interactive "Background" job.
- RT-11XM (eXtended Memory) a superset of FB, provided support for memory beyond 64kb, but required a minicomputer with memory management hardware; distributed from approx. 1975-on.
- RT-11ZM provided support for systems with Separate Instruction and Data space (such as on the Unibus-based 11/44, 45, 55, 70, 84, and 94 and the Q-Bus-based 11/53, 73, 83, and 93).
Specialized RT-11 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 gas chromatograph). Data collection ran in RT-11'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"!
- GT62 systems added a VS60 vector graphics peripheral (VT11-compatible) in a credenza cabinet.
- GAMMA-11 was a packaged RT-11 and PDP 11/34 system that was one of the first fully integrated Nuclear Medicine systems. It included fast analog/digital converters, 16 bit color graphical displays and an extensive software library for development of applications for the purpose of data collection, analysis and display from a nuclear medicine gamma camera.
Guides
Sources:
- Adapted from: Scholarly Community Encyclopedia, RT11. The text was also sourced from HandWiki: https://handwiki.org/wiki/Software:RT-11
- Adapted from: Wikipedia DEC RT-11
- Adapted from: Gunkies.org RT-11
Compiled on 08-25-2024 11:40:55