This emulator began as a personal project. Years ago I was a programmer for RSTS/E on a PDP‑11/45, and I vividly remembered the console’s idle loop light pattern — but not quite how it looked. With real hardware long gone, I decided to recreate it in software.
My initial plan was to emulate the PDP‑11/45, but the expanded memory of the PDP‑11/70 (nearly 4 MB!) proved irresistible. The result was a
PDP‑11/70 emulator, later joined by a
PDP‑11/45 version and even a
PDP‑10.
The core objective was met: I can now see the RSTS/E console light pattern again. Interestingly, later releases (such as v9.6) changed the pattern, and other operating systems have their own distinctive styles. RSX and BSD 2.11 show lively variations, while Unix V5 and Ultrix keep the lights almost static, reflecting heavy use of WAIT mode.
Collecting the operating systems themselves was a challenge. One highlight was tracking down RSTS/E V06C, which has its own
story.
Many of these systems include source code, offering fascinating insights into their design. Their influence on later computing is profound — Unix in particular feels familiar to modern administrators. I sometimes wonder if encountering Ultrix earlier might have steered my career differently.
The emulator boots with a custom PDP‑11 program that drives its own light patterns. Loaded at address 120000, it includes a LIGHTS command that cycles through virtual addresses with WAIT instructions, displaying the contents of R0 and the current instruction address. You can also use BOOT to start guest operating systems, or ODT to launch the Octal Debugging Tool.
Original PDP‑11s had no BIOS. Booting meant toggling in a tiny bootstrap loader from the front panel — unless you were lucky enough to have residual code in magnetic core memory. Below is an example bootstrap for the RK05 disk, where the value at address 1002 selects the unit (rk0, rk1, rk2, etc.).
To restart the emulator’s initial boot code (if not overwritten), use:
HALT, 120000, LOAD ADDRESS, ENABLE, START
Curious how it all works? The source is available on
GitHub. The main JavaScript files are pdp11.js, fpp.js, and iopage.js.
Happy emulating!
Paul Nankervis
Guest Operating Systems
Disk
OS
Notes
RK0
Unix V5
Boot using: unix, then login as root
RK1
RT‑11 v4.0
The smallest OS here
RK2
RSTS V06C‑03
Login as 1,2 (password SYSTEM) or 11,70 (password PDP)
RK3
XXDP
Diagnostic OS and utilities
RK4
RT‑11 3B
Distribution for RT‑11 Version 3B
TM0
RSTS 4B‑17
Boot ROLLIN from TM0, restore DK0, then reboot. Default login 1,2 (SYSTEM) or 11,70 (PDP).
Example boot of Unix V5
Boot> boot rk0
@unix
login: root
# cat /etc/passwd
root::0:1::/:
daemon::1:1::/bin:
bin::3:1::/bin:
# who
root tty8 Mar 21 12:09
# chdir /usr/sys/ken
# pwd
../usr/sys/ken
# ls -al
total 121
drwxr-xr-x 2 bin 352 Nov 26 18:13 .
drwxr-xr-x 5 bin 384 Nov 26 18:13 ..
-rw-r--r-- 1 bin 3855 Nov 26 18:13 alloc.c
-rw-r--r-- 1 bin 2159 Nov 26 18:13 clock.c
-rw-r--r-- 1 bin 2662 Nov 26 18:13 fio.c
-rw-r--r-- 1 bin 2941 Nov 26 18:13 iget.c
-rw-r--r-- 1 bin 2674 Nov 26 18:13 main.c
...
# ls -al /usr/games
total 64
drwxr-xr-x 2 bin 144 Nov 26 18:13 .
drwxr-xr-x 14 bin 224 Nov 26 18:13 ..
-rwxr-xr-x 1 bin 1562 Nov 26 18:13 bj
-rwxr-xr-x 1 bin 16268 Nov 26 18:13 chess
-rwxr-xr-x 1 bin 2468 Nov 26 18:13 cubic
-rwxr-xr-x 1 bin 624 Nov 26 18:13 moo
-rwxr-xr-x 1 bin 2192 Nov 26 18:13 ttt
-rw-rw-rw- 1 bin 294 Nov 26 18:13 ttt.k
-rwxr-xr-x 1 bin 5386 Nov 26 18:13 wump
# ls -al /bin
total 339
drwxr-xr-x 2 bin 944 Nov 26 18:13 .
drwxr-xr-x 9 bin 160 Jan 29 16:14 ..
-rwxr-xr-x 1 bin 1514 Nov 26 18:13 ar
-rwxr-xr-x 1 bin 7308 Nov 26 18:13 as
-rwxr-xr-x 1 bin 6042 Nov 26 18:13 bas
-rwxr-xr-x 1 bin 152 Nov 26 18:13 cat
-rwxr-xr-x 1 bin 5668 Nov 26 18:13 cc
....
# ls -al /usr/source/s2
total 281
drwxr-xr-x 2 bin 848 Jan 2 19:36 .
drwxr-xr-x 7 bin 112 Nov 26 18:13 ..
-rw-r--r-- 1 bin 4233 Nov 26 18:13 mail.c
-rw-r--r-- 1 bin 588 Nov 26 18:13 mesg.s
-rw-r--r-- 1 bin 1088 Nov 26 18:13 mkdir.s
-rw-r--r-- 1 bin 6922 Nov 26 18:13 mkfs.c
-rw-r--r-- 1 bin 563 Nov 26 18:13 mknod.c
...
# chdir /
# cal 10 1981
Oct 1981
S M Tu W Th F S
1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31
# basprint sqr(2)
1.414213562
10 print "Calculating e"
20 p = 20
30 k = 1
40 e = 0
50 for x=1 p
60 e = e + 1/k
70 k = k * x
80 next x
90 print "e=" e " (" exp(1) ")"
run
Calculating e
e=2.718281828 (2.718281828)
^D
# cat > hello.c
main(argc, argv)
char **argv;
{
printf("Hello world\n");
}^D
# cc hello.c
# a.out
Hello world
# cat > pi.f
c PI via Nilakantha series: 3 + 4/(2*3*4) - 4/(4*5*6) + 4/(6*7*8) ...
double precision pi, n, d
pi = 3.0
n = 4.0
d = 2.0
do 25 i=1,3000
pi = pi + n / (d * (d + 1.0) * (d + 2.0))
d = d + 2.0
n = -n
25 continue
write(6,70) pi
70 format('PI is approximately ', d2.16)
end^D
# fc pi.f
# a.out
PI is approximately 0.3141592653580503d+01
#
Example boot of RT11 v4.0
Boot> boot rk1
RT-11SJ V04.00C
.D 56=5015
.TYPE V4USER.TXT
Welcome to RT-11 Version 4. RT-11 V04 provides new hardware support
and some major enhancements over Version 3B.
Please use the HELP command; it describes the new options in many
of the utilities.
If you are using a terminal that requires fill characters,
modify location 56 with a Deposit command before proceeding with
system installation. LA36 DECwriter II and VT52 DECscope terminals
do NOT require such modification.
.D 56=0
.DIR
SWAP .SYS 25 01-Feb-82 RT11BL.SYS 65 01-Feb-82
RT11SJ.SYS 67 01-Feb-82 RT11FB.SYS 80 01-Feb-82
...
DISMT2.COM 8 01-Feb-82 DUNGEO.COM 1 07-Jun-86
DSAVE .DAT 10 07-Jun-86 ADVENT.DOC 4 20-Jul-85
DUNGEO.SAV 216 07-Jun-86 DINDX .DAT 20 07-Jun-86
DTEXT .DAT 383 07-Jun-86 ADVENT.SAV 93 20-Jul-85
ADVENT.VAR 22 20-Jul-85 ADVTXT.TXT 125 20-Jul-85
ODT11 .MAC 61 31-Oct-74 ANIMAL.SAV 22 02-Oct-85
BLKJAK.SAV 20 02-Nov-85 FORTH .MAC 146
RTLEM .MAC 363 06-Sep-73 BOOT .MAC 26 24-Mar-75
118 Files, 3392 Blocks
1370 Free blocks
.R DUNGEO
Welcome to Dungeon. This version created 10-AUG-78.
You are in an open field west of a big white house with a boarded
front door.
There is a small mailbox here.
>OPEN MAILBOX
Opening the mailbox reveals:
A leaflet.
>READ LEAFLET
Taken.
Welcome to Dungeon!
Dungeon is a game of adventure, danger, and low cunning. In it
you will explore some of the most amazing territory ever seen by mortal
man. Hardened adventurers have run screaming from the terrors contained
within.
In Dungeon, the intrepid explorer delves into the forgotten secrets
of a lost labyrinth deep in the bowels of the earth, searching for
vast treasures long hidden from prying eyes, treasures guarded by
fearsome monsters and diabolical traps!
No DECsystem should be without one!
Dungeon was created at the Programming Technology Division of the MIT
Laboratory for Computer Science by Tim Anderson, Marc Blank, Bruce
Daniels, and Dave Lebling. It was inspired by the Adventure game of
Crowther and Woods, and the Dungeons and Dragons game of Gygax
and Arneson. The original version was written in MDL (alias MUDDLE).
The current version was translated from MDL into FORTRAN IV by
a somewhat paranoid DEC engineer who prefers to remain anonymous.
On-line information may be obtained with the commands HELP and INFO.
>QUIT
Your score would be 0 [total of 560 points], in 1 move.
This gives you the rank of beginner.
Do you wish to leave the game?
Y
.MAC RTLEM
ERRORS DETECTED: 0
.LINK RTLEM
.RUN RTLEM
... attempt to land the lunar module using the mouse as a light pen - goodluck!
Example boot of RSTS V06C-03
Boot> boot rk2
RSTS V06C-03 Vixen (DK2)
Option: <LF>
You currently have: JOB MAX = 31, SWAP MAX = 28K.
You currently have crash dump disabled.
DD-MMM-YY? 31-OCT-76
12:00 PM? 9:03
INIT V06C-03 RSTS V06C-03 Vixen
Command File Name?
DETACHING...
I11/70
Password: PDP
Ready
DIR
Name .Ext Size Prot Date SY:[11,70]
ACEY .BAS 5 < 60> 31-Oct-76
TREK .BAS 16 < 60> 31-Oct-76
TREK .DOC 9 < 60> 31-Oct-76
ANIMAL.BAS 5 < 60> 31-Oct-76
STRTRK.BAS 27 < 60> 31-Mar-81
STRTR1.BAS 9 < 60> 31-Mar-81
ADVENT.DOC 4 < 60> 20-Jul-85
ADVENT.SAV 93 <124> 20-Jul-85
ADVENT.VAR 22 < 60> 20-Jul-85
ADVTXT.TXT 125 < 60> 20-Jul-85
SYSMAC.SML 42 < 60> 13-Mar-77
HELLO .MAC 1 < 60> 13-Mar-77
PRIME .BAS 1 < 60> 24-Mar-76
PI .BAS 1 < 60> 24-Mar-76
BOOT .MAC 26 < 60> 01-Jan-76
Total of 386 blocks in 15 files in SY:[11,70]
Ready
RUN PI
3.14159265358979
Ready
RUN ADVENT
WELCOME TO ADVENTURE!! WOULD YOU LIKE INSTRUCTIONS?
NO
YOU ARE STANDING AT THE END OF A ROAD BEFORE A SMALL BRICK BUILDING.
AROUND YOU IS A FOREST. A SMALL STREAM FLOWS OUT OF THE BUILDING AND
DOWN A GULLY.
E
YOU ARE INSIDE A BUILDING, A WELL HOUSE FOR A LARGE SPRING.
THERE ARE SOME KEYS ON THE GROUND HERE.
THERE IS A SHINY BRASS LAMP NEARBY.
THERE IS FOOD HERE.
THERE IS A BOTTLE OF WATER HERE.
GET FOOD
OK
QUIT
DO YOU REALLY WANT TO QUIT NOW?
Y
OK
YOU SCORED 32 OUT OF A POSSIBLE 350, USING 3 TURNS.
YOU ARE OBVIOUSLY A RANK AMATEUR. BETTER LUCK NEXT TIME.
TO ACHIEVE THE NEXT HIGHER RATING, YOU NEED 4 MORE POINTS.
Ready
RUN ACEY
ACEY DUCEY CARD GAME
CREATIVE COMPUTING MORRISTOWN, NEW JERSEY
ACEY-DUCEY IS PLAYED IN THE FOLLOWING MANNER
THE DEALER (COMPUTER) DEALS TWO CARDS FACE UP
YOU HAVE AN OPTION TO BET OR NOT BET DEPENDING
ON WHETHER OR NOT YOU FEEL THE CARD WILL HAVE
A VALUE BETWEEN THE FIRST TWO.
IF YOU DO NOT WANT TO BET, INPUT A 0
YOU NOW HAVE 100 DOLLARS.
HERE ARE YOUR NEXT TWO CARDS:
4
5
WHAT IS YOUR BET? 100
9
SORRY, YOU LOSE
SORRY, FRIEND, BUT YOU BLEW YOUR WAD.
TRY AGAIN (YES OR NO)? NO
O.K., HOPE YOU HAD FUN!
Ready
SYSTAT
RSTS V06C-03 Vixen status at 31-Oct-76, 09:05 AM Up: 2:34
Job Who Where What Size State Run-Time RTS
1 [OPR] Det ERRCPY 5K SR 5.0 BASIC
2 [SELF] KB0 SYSTAT 8K RN Lck 2.1 BASIC
Busy Devices: None
Disk Structure:
Disk Open Free Cluster Errors Name Comments
DK2 3 324 1 0 VIXEN Pub, DLW
Small Large Jobs Hung TTY's Errors
380 1 2/2 0 0
Run-Time Systems:
Name Ext Size Users Comments
BASIC BAC 16(16)K 2 Perm, Addr:26, KBM, CSZ
RSX TSK 2(28)K 0 Non-Res, KBM
RT11 SAV 4(28)K 0 Temp, Addr:66, KBM, CSZ, EMT:255
RMS11 TSK 4(28)K 0 Non-Res
Message Receivers:
Name Job Msgs Max Senders
ERRLOG 1 0 40 Priv
Ready
BYE/Y
Saved all disk files; 388 blocks in use, 112 free
Job 2 User 11,70 logged off KB0 at 31-Oct-76 09:05 AM
System RSTS V06C-03 Vixen
Run time was 2.7 seconds
Elapsed time was 2 minutes
Good morning
Example boot of RSTS 4B-17
Boot> boot tm0
ROLLIN V07
#DK:<MT:VIXEN/REW
END-OF-FILE DURING READ, TYPE
M TO MOUNT ANOTHER REEL, OR K TO KILL REQUEST: k
#/BO:DK:
RSTS V04B-17 VIXEN
OPTION? <LF>
DD-MON-YY? 24-MAR-75
HH:MM? 9:21
VIXEN - SYSTEM PACK MOUNTED
ENABLE CRASH DUMP? N
CHAIN "INIT"
CATASTROPHIC ERROR
PROGRAM LOST-SORRY
I/O CHANNEL NOT OPEN
Ready
SYSTEM INITIALIZATION PROGRAM
END OF FILE ON DEVICE - INIT ASSUMED COMPLETE
Ready
CAT
LOGIN .BAS 7 60 31-Oct-71 31-Oct-71 06:42 AM
LOGIN .BAC 15 60 31-Oct-71 31-Oct-71 06:42 AM
LOGOUT.BAS 7 60 31-Oct-71 31-Oct-71 06:42 AM
LOGOUT.BAC 15 60 31-Oct-71 31-Oct-71 06:42 AM
PIP .BAS 18 60 31-Oct-71 31-Oct-71 06:42 AM
PIP .BAC 23 40 31-Oct-71 31-Oct-71 06:42 AM
SYSTAT.BAS 12 60 31-Oct-71 31-Oct-71 06:42 AM
SYSTAT.BAC 19 168 31-Oct-71 31-Oct-71 06:42 AM
...
Ready
RUN TTYSET
'TTYSET' TERMINAL CHARACTERISTICS PROGRAM
? LC
? ^Z
Ready
run utilty
'UTILTY' SYSTEM UTILITY PROGRAM
? LOGINS
? HELP
COMMANDS ARE:
MOUNT DEV:ID DISMOUNT DEV:
LOCK DEV: UNLOCK DEV:
NO LOGINS LOGINS
CLEAN DEV: KILL JOB#
SEND KBN: STRING FORCE KBN: [^]STRING
CHANGE [ACCT]PASSWORD QUOTA [ACCT] QUOTA
ZERO DEV:[ACCT] HANGUP KBN:
DATE DD-MMM-YY TIME HH:MM
? ^Z
Ready
run pip
PIP - RSTS V04B-17 VIXEN
#acct.sys
11,70,PDP,0,2,PDP 11/70 EMULATOR
#^Z
Ready
run systat
OUTPUT STATUS TO? kb:
RSTS V04B-17 VIXEN STATUS ON 24-Mar-75 AT 09:22 AM UP: 1:07
JOB WHO WHERE WHAT SIZE STATE RUN-TIME
1 1,2 KB0 SYSTAT 6K RN 0.1
BUSY DEVICES: NONE
DISK STRUCTURE:
DISK OPEN FREE CLUSTER ERRORS COMMENTS
DK0 0 3185 1 0 PUBLIC
SMALL LARGE ERRORS HUNG TTY'S
69 0 0 0
Ready
bye
CONFIRM: y
SAVED ALL DISK FILES; 782 BLOCKS IN USE
JOB 1 USER 1,2 LOGGED OFF KB0 AT 24-Mar-75 09:22 AM
SYSTEM RSTS V04B-17 VIXEN
RUN TIME WAS .1 SECONDS
ELAPSED TIME WAS 1 MINUTE, 20 SECONDS
GOOD MORNING
HELLO
RSTS V04B-17 VIXEN JOB 1 KB0 24-Mar-75 09:24 AM
#11,70
PASSWORD: PDP
RSTS V4B-17 IS NOW AVAILABLE...
NEW OR OLD--
^C
Ready
CAT
PRIME .BAS 1 60 31-Oct-71 31-Oct-71 06:43 AM
PI .BAS 1 60 31-Oct-71 31-Oct-71 06:43 AM
Ready
OLD PI
Ready
LIST
PI 09:25 AM 24-Mar-75
10 REM CALCULATE PI
20 P = 0
30 P1 = 1
40 FOR K%=0% TO 10%
50 P = P + 1.0/P1 * (4.0/(8%*K% + 1%) - 2.0/(8%*K% + 4%) - 1.0/(8%*K% +5%) - 1.0/(8%*K% + 6%))
60 P1 = P1 * 16.0
70 NEXT K%
80 PRINT USING "#.##############",P
99 END
Ready
RUN
PI 09:25 AM 24-Mar-75
3.14159265358979
Ready
Software Sources
The software used in this emulator has been sourced from many places on the Internet and unfortunately many of these seem to have disappeared. If you are looking for something I would suggest the following main sites...
