UNIX V4 workshop at Low Resource Computing

UNIX V4 workshop at Low Resource Computing

At the Low Resource Computing 2026 workshop at Dartmouth, I hosted an interactive session on UNIX V4. This version of UNIX was recently recovered from a 1974 magnetic tape found at the University of Utah and is the earliest complete machine-readable snapshot of UNIX. I taught how to do software development using the tools of the day, particularly the ed text editor with a teletype, and participants solved a coding challenge, all signed into the same machine.

This repository reproduces the disk image and terminal logs from the workshop, as well as my setup.

Terminals

Before the workshop, I modified the kernel to support up to 32 simultaneous terminal connections, raised from the previous 20. It uses 16 KL terminals and 16 DC terminals. I would have also configured 16 DH terminals, bringing the total up to 48 terminals, the theoretical limit for this version of UNIX, but the PiDP-11 fork of the SIMH emulator does not support this multiplexer.

I had two physical terminals connected (a Silent 700 teleprinter and a Heathkit CRT terminal), 29 telnet connections from participants' laptops, as well as the main console on the PiDP-11.

Find the terminals you used in this list:

I describe my configuration for reproducibility, though the notes are mostly intended for myself. I will rework it into a tutorial for configuring V4–V7 later.

Getting started

I handed out printed manuals to teach the system in the manner of the day. The ed manuals were particularly helpful. Alex even came back the next day using ed commands I didn't teach, from having read it all. I had also hoped to setup the man command, but didn't get it working in time.

• 15x ed(I) manual (V4)
• 10x "A Tutorial Introduction to the UNIX Text Editor" (V7)
• 10x "UNIX For Beginners" (V7)
• 4x as(I) manual (V4)
• 4x "Assembler Reference Manual" (V7)
• 4x "Setting Up UNIX" (V4)

First, we created user accounts for everyone. Since this was accomplished by editing /etc/passwd, I handled this for most people, to avoid people overwriting others' changes. They were creative with their user IDs.

% cat /etc/passwd
root::0:1::/:
bin::3:1::/bin:
thalia::4:4::/usr/thalia:
ben::5:5::/usr/ben:
alex::9:9::/usr/alex:
newt::64:64::/usr/newt:
auberon::18:18::/usr/auberon:
ncb::12:12::/usr/ncb:
joe::88:88::/usr/joe:
cody::25:25::/usr/cody:
hash::31337:31337::/usr/hash:
dnm::55555:55555::/usr/dnm:
ty::1990:1990::/usr/ty:
justin::70:70::/usr/justin:
yang::16::16::/usr/yang:
doug::8:::/usr/doug:
amitb::6:6::/usr/amitb:
voytilla::7:7::/usr/voytilla:
kevin::10:10::/usr/kevin:
michael::30:30::/usr/michael:
bx::42:42::/usr/bx:
hacker::256:256::/usr/hacker:
jordan::69:69::/usr/jordan:
dominic::111:111::/usr/dominic:

Quirks

Perhaps the largest adjustment was that everyone instinctively reached for backspace to fix typos, but it would instead send a literal ASCII backspace character. This happened hundreds of times. Early UNIX was designed for printing terminals and, of course, text cannot be cleared once it's printed on paper. Instead, it used # to erase the last character and @ to erase the current line, borrowed from Multics. One participant marveled at how instinctive this was for me. amitb found that the erase character could be configured to backspace with stty erase '^h', but alas that only worked in later versions of UNIX.

Another pervasive surface difference was that cd was named chdir until V7. Participants ran into cd: not found about 100 times.

But by the end, this was no longer a limitation and someone hacked the shell to laugh at you if you typed cd:

% cd dir
haha
% ed /usr/source/s2/sh.c
10267
/cd/;/}/p
                if(equal(cp1, "cd")) {
                        prs("haha\n");
                        return;
                }

A confusing quirk was that login would sometimes use an all-caps mode for compatibility with the Teletype Model 33. It cycles between terminal settings until one works, so if you can't sign in, it attempts under all-caps mode. This was especially prone to happen for the KL terminal sessions. That driver assumes that the terminal is always connected and doesn't wait for it to be ready, so the initial login: prompt would always be lost. After pressing return, it would then cycle to the next setting and present an all-caps prompt, making your whole session uppercase. This happened about 14 times.

LOGIN INCORRECT.
NAME: ROOT
# CD /USR
CD: NOT FOUND
# CHDIR /USR

Paper

I brought my TI Silent 700 Model 707/1200 teleprinter to demonstrate computing on paper. Before the world transitioned to CRT terminals, teletypes printed your session onto paper. The paper was your monitor and cut/paste was done with scissors and tape.

Alex was particularly fond of the teletype and was usually found sitting behind it. Auberon found the long roll of paper so amusing that she will show this photo of me holding it to her students to show how computing has progressed.

We used up the last bit of paper by printing a meter-long Saturn V rocket ASCII art, preceded by a countdown of ten beeps. Streaks of red in the paper warned us of the impending end, and the rocket looked bloodied.

A photo of the Heathkit H19 displaying LRC 2026 text art with the o8 FIGlet font became the unofficial icon of the workshop, and it was printed onto stickers.

Challenge

I challenged participants to backport the winning program of the inaugural International Obfuscated C Code Contest, mullender.c, from UNIX V7 in 1984 to V4 in 1974. It exploits quirks of the early cc and backporting it touches surprisingly deep into the system for something so short. This was one of the first things I did with UNIX V5, before we recovered V4, so I figured it could be doable for new users.

This program prints a :-) smiley face scrolling across the screen, but is written in PDP-11 and VAX machine code. It is as follows:

/* Portable between VAX11 && PDP11 */
short main[] = {
        277, 04735, -4129, 25, 0, 477, 1019, 0xbef, 0, 12800,
        -113, 21119, 0x52d7, -1006, -7151, 0, 0x4bc, 020004,
        14880, 10541, 2056, 04010, 4548, 3044, -6716, 0x9,
        4407, 6, 5568, 1, -30460, 0, 0x9, 5570, 512, -30419,
        0x7e82, 0760, 6, 0, 4, 02400, 15, 0, 4, 1280, 4, 0,
        4, 0, 0, 0, 0x8, 0, 4, 0, ',', 0, 12, 0, 4, 0, '#',
        0, 020, 0, 4, 0, 30, 0, 026, 0, 0x6176, 120, 25712,
        'p', 072163, 'r', 29303, 29801, 'e' };

I placed the program at /usr/thalia/mullender.c and hints at /usr/thalia/hints/, then set them loose.

Three users solved the challenge: alex, kevin, and jordan.

Alex and Travis collaborated and used Travis' HP calculator to convert between bases. Jordan used dc (desk calculator) to perform arithmetic.

% dc
_30419
15
-
p
    -30434
q

The original uses a non-existent system call to add a delay between prints, but this doesn't work in V4, as non-existent syscalls trap with Bad system call -- Core dumped instead. The solution involves replacing it with some other syscall that does not produce an effect. Alex and Kevin matched what I did, switching it to getuid, but Jordan instead chose smdate. This syscall originally would set the modified time of a file, but it was removed, as it caused issues for tools that observed file times. Its nullsys handler has no effect, but it still consumes an argument, so it skips the sob branch and prints extra fast.

% ed /usr/sys/ken/sysent.c
1957
/45/
        0, &nosys,                      /* 45 = tiu */
/getuid/
        0, &getuid,                     /* 24 = getuid */
/smdate/
        1, &nullsys,                    /* 30 = smdate */

Alex scripted the hexadecimal syntax fixes by using the old behavior of the Thompson shell: Commands that consume stdin would read from the script file until they exited, so commands and input were interleaved with no quoting like the later heredocs.

% cat /usr/alex/hint3b
ed mullender.c
/0xbef/s//05757/
/0x52d7/s//051327/
/0x4bc/s//02274/
g/0x9/s//011/
/0x7e82/s//077202/
/0x8/s//010/
/0x6176/s//060566/
w
q
% sh hint3b
469
469
% 

auberon, bx, and amitb successfully backported the syntax changes and cody almost finished that.

bx learned the db debugger and traced program execution.

amitb formatted it artistically:

/* Portable between VAX11 && PDP11 */
int main[] {
        277, 04735, -4129, 25, 0,
    477, 1019, 3055, 0, 12800, -113,
  21119, 21207, -1006, -7151, 0, 1212,
 020004, 14880,               10541,
 2056, 04010,                 4548,
 3044, -6716,                 9, 4407,
 6, 5568,                     1,
 -30460, 0,                   9, 5570,
 512, -30419,                 32386,
 0760, 6, 0, 4, 02400, 15, 0, 4, 1280,
  4, 0, 4, 0, 0, 0, 8, 0, 4, 0, 44,
   0, 12, 0, 4, 0, 35, 0, 020, 0, 4,
    0, 30, 0, 026, 0, 24950, 120,
      25712, 112, 072163, 114,
          29303, 29801, 101
};

After this, they had become quite proficient in ed.

Playing

Besides programming, people also found the games in /usr/games. Hunt the Wumpus was rather popular and someone discovered a difference in bj from the traditional Blackjack rules.

% /usr/games/wump
Instructions? (y-n) y

Welcome to 'Hunt the Wumpus.'

The Wumpus lives in a cave of 20 rooms.
Each room has 3 tunnels leading to other rooms.

A legend

Doug McIlroy visited on the third day of the workshop. He was the first-ever UNIX user, and responsible for it growing beyond the initial research group. He gave a talk on bare m4, a single-operation version of the m4 macro processor with only define, which is nonetheless Turing-complete.

Afterwards, he setup an account, doug, for himself on UNIX V4. He was accustomed to later versions of ed that allow omitting the closing slashes and the later passwd format, but he still had the muscle memory.

We chatted about the early years of UNIX: They used a Teletype Model 33 ASR as the console teletype until the end, even though they didn't use it for programming, as a hardcopy terminal was useful. Ken and Dennis worked at night, out of his sight, and although they had terminals setup at home, they worked better together and dutifully came into the Labs. Early UNIX distributions were produced with the GE 635, since it had a magtape drive, but the PDP-11 didn't. Bob Morris and Ken wrote a program that printed a million digits of e and used a roll of paper. Doug's speech synthesizer wasn't widely distributed, but I pointed out that V4 had a Screw Works driver. Doug wrote the roff for Multics.

It was a privilege sharing a UNIX system with Doug. Since research progressed so quickly, this would have been the first time in 52 years that he had used this version of UNIX.

Hacking

Ben was particularly curious about the limits of the user and group IDs and changed his several times.

ben::5:5::/usr/ben:
ben::65535:65535::/usr/ben:
ben::-1-1::/usr/ben:
hacker::256:256::/usr/hacker:

He first tried unsigned -1, then signed -1. He hypothesized that it simply used scanf, so a sign would be accepted, but the parsing was simpler than that and still produced a usable uid. However, his entry was missing a colon, so he couldn't sign in anyways.

% ed /usr/source/s1/login.c
2884
/uid =/;.+2p
        uid = 0;
        while (*np != ':')
                uid = uid*10 + *np++ - '0';

After reading the shell source, he realized that any uid equal to 0 mod 256 was a super user, so he made an account with a uid of 256 and was treated as root.

% ed /usr/source/s2/sh.c
10267
/acname =/;/}/p
        acname = "/usr/adm/sh_acct";
        promp = "% ";
        if(((uid = getuid())&0377) == 0) {
                promp = "# ";
                acname = "/usr/adm/su_acct";
        }
q
% ^D
login: hacker
# 

Messaging

We had at least 11 users active simultaneously, though there were probably more at other times:

# who
root    tty0 Aug 18 10:36
ty      tty1 Aug 18 10:52
hash    tty2 Aug 18 10:50
root    tty8 Aug 18 10:53
ty      ttyc Aug 18 10:54
newt    ttyk Aug 18 10:43
cody    ttyl Aug 18 10:48
alex    ttym Aug 18 10:48
newt    ttyn Aug 18 10:39
dnm     ttyq Aug 18 10:49
ncb     ttyt Aug 18 10:43

Some messaged themselves:

# write root

␇␇␇Message from root...
hiiiiiii
hiiiiiii

and to others:

# 
␇␇␇Message from alex...
hey! what ar you doing???
EOT

# 
# write alex
{not much!!! hacking the planet)
# 

Mail was sent:

% mail kevin
is this thEOT
ing on

% 

and I wrote directly to others' terminal devices:

% who
% cat > /dev/tty1
hello
how
are
you
% 

Finishing

By the end, we had become acquainted with the system, learned ed, programmed a bit in C, played some games, mailed each other, and used up a whole roll of paper. Great success!

login: thalia
% cat thanks
thank_you
from_darpa_dan