Editing RT-11 (section)

==Features==

===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 (operating system)|kernel]] listing that included comments.

===Device drivers===
In RT-11, [[device driver]]s<ref>{{cite news |newspaper=[[Computerworld]]
 |title=DEC RP02/RP03 device drivers included |date=July 20, 1981 |page=58}}</ref><ref>{{cite news |newspaper=[[Computerworld]]
|title=3Com software drivers |date=October 26, 1981 |page=51}}</ref> 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 [[DECUS|Digital Equipment Computer Users Society]].

===Multitasking===
RT-11 systems did not support [[Computer multitasking|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 [[Scheduling (computing)|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).<ref name="RT11.HC82Decem" />  The terms ''foreground'' and ''background'' are counterintuitive; the background job was typically the user's [[Command-line interface|command-line interpreter]]; a foreground job might be doing something like non-interactive data collection.

===Human interface===
[[File:RT-11 help.jpg|thumb|The end of the {{mono|HELP}} command output from [[:en:RT-11|RT-11SJ]] displayed on a [[:en:VT100|VT100]].]]
Users generally operated RT-11 via a [[Teleprinter|printing terminal]] or a [[Computer terminal|video terminal]], originally via a strap-selectable current-loop (for conventional teletypes) or via an [[RS-232]] (later [[EIA-422|RS-422]] as well) interface on one of the CPU cards; DEC also supported the VT11 and VS60 graphics display devices ([[vector graphics]] [[Computer display|terminal]]s 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 [[DOS]] line of operating systems which heavily borrowed from RT-11. The CSI form expected input and output [[filename]]s and [[Command-line argument|options]] ('switches' on RT-11) in a precise order and syntax. The command-line switches were separated by a slash (<code>/</code>) rather than the dash (<code>-</code>) 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 (command)|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 [[filename extension|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 great 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 (command)|START]], REENTER, EXAMINE and DEPOSIT were basic commands implemented in the KMON. Some commands and utilities were later borrowed in the [[DOS]] line of operating systems. These commands include [[dir (command)|DIR]], [[copy (command)|COPY]], [[ren (command)|RENAME]], [[assign (command)|ASSIGN]], [[cls (command)|CLS]], [[del (command)|DELETE]], [[TYPE (DOS command)|TYPE]], [[help (command)|HELP]] and others. The [[format (command)|FORMAT]] command was used for physical disk formatting, although it was not capable of creating file system, for which purpose the INIT command was used (analogue of DOS command FORMAT /Q). Most commands supported using [[Wildcard character|wildcards]] 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 <code>ASSIGN</code> command. For example, one might issue <code>ASSIGN DL0 ABC</code> 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).

===Software===
RT-11 was distributed with utilities for performing many actions. The utilities ''[[dir (command)|DIR]]'', ''DUP'', ''[[Peripheral Interchange Program|PIP]]'' and ''[[format (command)|FORMAT]]'' were for managing disk volumes. [[TECO (text editor)|TECO]], ''EDIT'', and the [[visual editor]]s ''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. [[Octal Debugging Technique|ODT]], ''VDT'' and the SD device were used to debug programs. DEC's version of [[Runoff (program)|Runoff]]<ref>{{cite news |newspaper=[[Computerworld]]
 |title=Runoff polishes text on RSTS/E, RT-11 |date=September 25, 1978 |page=35}}</ref> 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.<ref>{{cite news
 |newspaper=[[Computerworld]] |title=LEX-11 on all DEC operating systems
 |date=April 20, 1981 |page=59}}</ref> Large amounts of free, user-contributed software for RT-11 were available from the [[DECUS|Digital Equipment Computer Users Society (DECUS)]] including an implementation of [[C (programming language)|C]]. Although the tools to develop and debug assembly-language programs were provided, other languages including C, [[Fortran]], [[Pascal (programming language)|Pascal]],<ref>{{cite news |newspaper=[[Computerworld]]
|title=PASCAL on RT-11 |date=March 10, 1980 |page=102}}</ref> 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. It is even possible to network RT-11 machines using [[DECNET]], the [[Internet protocol suite|Internet]] and protocols developed by other, third-party sources.

===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 disk]]s 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, [[TU58|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 operation 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&nbsp;kHz A/D converter with 2 channel D/A, 9600 baud serial port, 16 bit bidirectional boards, etc.

===File system===
RT-11 implemented a simple and fast [[file system]] employing six-character filenames with three-character extensions (6.3) encoded in [[DEC RADIX 50|RADIX-50]], which packed those nine characters into only three 16-bit words (six bytes). This resulted in allowable filename characters being 'A'-'Z', 'a'-'z' forced to upper case, '0'-'9', '$', '%', '*'. Filenames could be prefixed by a device name to specify a device different to the default DK: device, also encoded in RADIX-50.

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 likely [[File system fragmentation#Free space fragmentation|not remain contiguous]], which could become the limiting factor in creating large files; the remedy was to periodically “squeeze” (or "squish") a disk to consolidate the unused portions.<ref>{{cite web |website=BitSavers
 |url=http://bitsavers.trailing-edge.com/pdf/dec/pdp11/rt11/v5.6_Aug91/AA-5284H-TC_RT-11_System_Message_Manual_Aug91.pdf
 |title=RT-11 System Message Manual
 |quote=Compress the volume by using the monitor SQUEEZE command}}</ref><ref>{{cite web
|url=https://gordonbell.azurewebsites.net/Digital/RT-11%20System%20Users%20Guide%201977.pdf
|title=RT-11 System Users Guide 1977 |quote=The SQUEEZE command consolidates in a single area all unused ...}}</ref>

Each volume has only one directory which was preallocated 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.<ref>{{cite web
 |url=http://cryptosmith.com/2013/10/19/digitals-rt-11-file-system
 |title=Digital's RT-11 File System |date=20 October 2013 |access-date=January 1, 2015}}</ref>

===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 [[OpenVMS|VMS]].

The implementation of [[DIGITAL Command Language|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.