Table of Contents
DEC RT-11 Survival Guide
What is RT-11?
RT-11 (Real-Time 11) is a disk-based, real-time operating system developed by Digital Equipment Corporation (DEC) for its popular line of PDP-11 minicomputers. First released in 1970, RT-11 was designed to be lightweight, fast, and highly responsive, making it ideal for real-time applications such as industrial control, scientific data acquisition, and laboratory automation. Unlike DEC's more complex timesharing system (RSTS/E) or the later UNIX-like RSX-11, RT-11 focused on simplicity and low overhead, often running comfortably on machines with as little as 28 KB of memory.
RT-11 gained widespread popularity throughout the 1970s and early 1980s due to its efficiency and ease of use. It supported both foreground and background job processing, allowing a high-priority real-time task to run concurrently with an interactive user session. The system's command-line interface, based on the Keyboard Monitor (KMON), was known for being straightforward and responsive even on floppy-disk-based systems. Many early microcomputer operating systems, including CP/M and even the first versions of MS-DOS, borrowed concepts from RT-11 — most notably its simplified file naming convention (6+3 characters) and command structure. Although DEC phased out RT-11 in the late 1980s in favor of RSX-11 and later VMS, the operating system maintains a loyal following among PDP-11 enthusiasts, collectors, and retrocomputing hobbyists, and it continues to run on emulators like SIMH today.
RT-11 Command Reference
RT-11 commands are interpreted by the Keyboard Monitor (KMON). Commands can be abbreviated to the first four unique characters. Wildcard `*` is supported.
Basic RT-11 Commands Table
| Command | Function | Basic Example |
|---|---|---|
| DIRECTORY (or DIR) | Lists the files on a device or directory | .DIR DX1: |
| COPY | Copies files from one device or location to another | .COPY DX0:. DY1:. |
| TYPE | Displays the contents of a text file on the terminal | .TYPE STARTS.COM |
| RUN | Loads and starts an executable program (usually .SAV files) | .RUN DUNGEO |
| DELETE | Removes one or more files from a disk | .DELETE TEMP.DAT |
| RENAME | Changes the name of an existing file | .RENAME OLDNAME.NEW NEWNAME.NEW |
| INITIALIZE | Prepares (formats) a disk volume for use with RT-11, creating a new directory | .INITIALIZE DY1: |
| SHOW | Displays information about the system configuration, resident devices, and jobs | .SHOW |
| SQUEEZE | Rearranges files on a disk to reclaim “free” space left by deleted files (like a defragmenter) | .SQUEEZE DK0: |
| HELP | Displays information about a specific command or general system usage | .HELP COPY |
| @ | Executes a command file (batch processing) | @STARTF.COM |
| SET | Controls various system options and device characteristics | .SET TERMINAL WIDTH:80 |
| SETUP | Configures hardware characteristics for specific devices | .SETUP TERMINAL LIGHT |
| DATE | Displays or sets the current system date | .DATE 25-DEC-1985 |
| TIME | Displays or sets the current system time | .TIME 14:30 |
RT-11 File Specification Format Table
| Component | Format | Description | Example |
|---|---|---|---|
| Device | ddn: | Physical or logical drive identifier | DX0:, RK1:, DL0: |
| Filename | 1-6 characters | User-assigned file name | PROG1, START |
| Type | 1-3 characters | File type identifier | .SAV, .MAC, .TXT, .SYS |
| Full example | ddn:filename.type | Complete file specification | DX1:MYPROG.SAV |
Common Device Names Table
| Device Code | Description |
|---|---|
| DX | Floppy disk |
| RK | RK05 cartridge disk |
| DL | RL01/RL02 disk |
| TT | Terminal |
| LP | Line printer |
| SY | System device (usually the boot device) |
Command Line Control Keys Table
| Key | Function |
|---|---|
| CTRL/C | Aborts the currently executing command or program |
| CTRL/O | Suppresses terminal output from a command (toggles on/off) |
| CTRL/S | Temporarily suspends output to the terminal |
| CTRL/Q | Resumes terminal output after a CTRL/S |
| CTRL/U | Cancels the current line you are typing |
| CTRL/B | Switches control to the background job |
| CTRL/F | Switches control to the foreground job |
Complete Combined Page Example
RT-11 Command Reference
RT-11 commands are interpreted by the Keyboard Monitor (KMON). Commands can be abbreviated to the first four unique characters. The wildcard * is supported.
Basic Commands
| Command | Function | Basic Example |
|---|---|---|
| DIRECTORY (DIR) | Lists files on a device or directory | .DIR DX1: |
| COPY | Copies files | .COPY DX0:. DY1:. |
| TYPE | Displays text file contents | .TYPE STARTS.COM |
| RUN | Loads and executes a program | .RUN DUNGEO |
| DELETE | Removes files | .DELETE TEMP.DAT |
| RENAME | Renames a file | .RENAME OLD.NEW NEW.NEW |
| INITIALIZE | Formats a disk volume | .INITIALIZE DY1: |
| SHOW | Shows system configuration | .SHOW |
| SQUEEZE | Defragments a disk | .SQUEEZE DK0: |
| HELP | Displays help | .HELP COPY |
| @ | Runs a command file | @STARTF.COM |
| SET | Sets system options | .SET TERMINAL WIDTH:80 |
| SETUP | Configures hardware | .SETUP TERMINAL LIGHT |
| DATE | Sets/shows date | .DATE 25-DEC-1985 |
| TIME | Sets/shows time | .TIME 14:30 |
File Specification
| Component | Format | Example |
|---|---|---|
| Device | ddn: | DX0: |
| Filename | 1-6 chars | PROG1 |
| Type | 1-3 chars | .SAV |
| Full | ddn:filename.type | DX1:MYPROG.SAV |
Common Device Names
| Code | Device |
|---|---|
| DX | Floppy disk |
| RK | RK05 cartridge disk |
| DL | RL01/RL02 disk |
| TT | Terminal |
| LP | Line printer |
| SY | System device |
Control Keys
| Key | Function |
|---|---|
| CTRL/C | Abort command/program |
| CTRL/O | Toggle output suppression |
| CTRL/S | Pause output |
| CTRL/Q | Resume output |
| CTRL/U | Cancel current line |
| CTRL/B | Switch to background job |
| CTRL/F | Switch to foreground job |
BASIC-11
| Command | Function | Example |
|---|---|---|
| FREE | Displays the amount of RAM memory currently available for program statements and variables [citation:1]. | FREE |
| LIST | Lists the current program lines on the terminal. | LIST 100-200 |
| LOAD | Loads a specified .BAS file from disk into memory. | LOAD “PROG1” |
| NEW | Clears the current program from memory. | NEW |
| OLD | Equivalent to LOAD (used in some DEC BASIC versions). | OLD “PROG1” |
| RUN | Executes the program currently in memory. | RUN |
| SAVE | Saves the current program to a disk file. | SAVE “PROG1” |
| SYSTEM | Exits BASIC and returns to the RT-11 Monitor prompt (`.`). | SYSTEM |
| Statement | Function | Example |
| DATA | Holds numeric and string constants used by READ statements. | 100 DATA 10, 20, “HELLO” |
| DIM | Allocates space for arrays. | 20 DIM A(10), B$(5) |
| END | Terminates program execution. | 999 END |
| FOR…TO…STEP | Creates a loop that executes a block of code repeatedly. | 30 FOR I=1 TO 100 STEP 2 |
| GOSUB | Branches to a subroutine. | 150 GOSUB 1000 |
| GOTO | Unconditionally branches to a specified line number. | 50 GOTO 200 |
| IF…THEN…ELSE | Conditionally executes code. | 60 IF X=10 THEN PRINT “YES” ELSE PRINT “NO” |
| INPUT | Accepts input from the terminal. | 70 INPUT “ENTER VALUE”; A |
| LET | Assigns a value to a variable (usually optional). | 80 LET X = 5 (or 80 X=5) |
| NEXT | Marks the end of a FOR loop. | 40 NEXT I |
| ON…GOTO / GOSUB | Computed branch. | 90 ON I GOTO 100,200,300 |
| READ | Assigns a value from a DATA statement to a variable. | 110 READ A, B, C$ |
| REM | Inserts a comment or remark. | 10 REM THIS IS A COMMENT |
| RESTORE | Resets the DATA pointer, allowing READ statements to start over. | 120 RESTORE |
| RETURN | Returns from a GOSUB subroutine. | 2000 RETURN |
| SLEEP | Puts the processor into low-power “Stop Mode” (hardware specific) [citation:1]. | 710 SLEEP |
| STOP | Halts program execution and returns to the command prompt. | 95 STOP |
| Function | Description | Example |
| ABS(X) | Returns the absolute value of X. | ABS(-10) returns 10 |
| CHR$(N) | Returns the ASCII character corresponding to the code N. | CHR$(65) returns “A” |
| COS(X) | Returns the cosine of angle X (in radians). | COS(0) returns 1 |
| INT(X) | Returns the integer part of X (floor function). | INT(3.7) returns 3 |
| LEFT$(A$,N) | Returns the leftmost N characters of string A$. | LEFT$(“HELLO”,2) returns “HE” |
| LEN(A$) | Returns the number of characters in string A$. | LEN(“TEST”) returns 4 |
| MID$(A$,P,N) | Returns N characters from string A$ starting at position P. | MID$(“HELLO”,2,2) returns “EL” |
| ONTIME(GOSUB) | Causes an interrupt to occur after a specified system time interval [citation:1]. | 100 ONTIME 10 GOSUB 5000 |
| POS | Returns the current print head position. | PRINT POS |
| RIGHT$(A$,N) | Returns the rightmost N characters of string A$. | RIGHT$(“HELLO”,2) returns “LO” |
| RND | Returns a random number between 0 and 1. | X = RND |
| SIN(X) | Returns the sine of angle X (in radians). | SIN(1.5708) returns 1 |
| SQR(X) | Returns the square root of X. | SQR(25) returns 5 |
| STR$(X) | Converts a numeric value X into a string. | STR$(123) returns “123” |
| TIME | Returns the current system time counter [citation:1]. | PRINT TIME |
| VAL(A$) | Converts the string A$ into a numeric value. | VAL(“123”) returns 123 |
### 💡 Tips for Using This Documentation
1. Line Numbers Only: The BASIC-11 on RT-11 is a traditional line-numbered BASIC. You cannot execute statements like `PRINT “HELLO”` directly unless you type it as `PRINT “HELLO”` in immediate mode or include it in a numbered line.
2. Check Syntax with “SYN”: While older BASIC-11 versions don't have a built-in HELP, they often support the `SYN` (Syntax check) command. Typing `SYN` followed by a line number or range will check syntax without executing the code [citation:2].
3. String Handling: Most standard string functions (`LEFT$`, `RIGHT$`, `MID$`, `LEN`) are available. String variables are denoted by a `$` suffix (e.g., `A$`, `NAME$`).