Unix Fourth Edition, released in November 1973 by Bell Labs, marked a fundamental shift in operating system design. Ken Thompson and Dennis Ritchie rewrote the kernel in C, proving that a high-level language could be used for systems programming. This decision would influence virtually every operating system that followed.

Prior to V4, operating systems were written in assembly language, tightly coupled to their hardware. The C rewrite made Unix portable—a revolutionary concept that enabled its spread across different computer architectures.

The tape was discovered in July 2025 by Aleks Maricq in a storage closet at the University of Utah's Flux Research Group. It had belonged to the late Professor Jay Lepreau and was originally sent to Martin Newell—the computer scientist famous for creating the Utah Teapot, the iconic 3D test model still used today.

On December 19, 2025, Al Kossow at the Computer History Museum successfully read the decades-old nine-track tape using modified hardware and Len Shustek's readtape tool. The recovery made headlines across the technology world.

$ uname -a
FreeBSD nexus 14.3-RELEASE FreeBSD 14.3-RELEASE releng/14.3-n271432-8c9ce319fef7 GENERIC amd64

FreeBSD provides two SimH packages. I chose open-simh for its active development:

$ pkg install open-simh

This installs version 4.1.0.20240303, which includes emulators for the entire PDP family (PDP-1 through PDP-15), VAX systems, and many other historic computers.

The pdp11 binary is installed at /usr/local/bin/pdp11.

The essential files are hosted at squoze.net:

wget http://squoze.net/UNIX/v4/unix_v4.tap   # Original tape image
wget http://squoze.net/UNIX/v4/disk.rk       # Pre-made RK05 disk image
wget http://squoze.net/UNIX/v4/boot.ini      # SimH boot configuration
wget http://squoze.net/UNIX/v4/install.ini   # SimH install configuration

The tape image is also available from the Internet Archive at https://archive.org/details/utah_unix_v4_raw (file: analog.tap, identical content with MD5 2e9dc71cbd24001484dc38904e609cba).

My first approach was to automate the boot process using Expect scripts. This led to hours of frustration as inputs seemed to be swallowed or misinterpreted by the emulator.

The boot process involves a chain of programs:

1. SimH loads and executes the tape boot loader (mboot)
2. mboot presents a menu of available programs
3. uboot loads the Unix kernel from disk
4. The kernel initializes and presents a login prompt

Every time SimH attached the tape, it displayed concerning warnings:

%SIM-INFO: TM0: Tape Image 'unix_v4.tap' scanned as SIMH format
%SIM-INFO: After processing 2191360 bytes of tape data (4280 records, 0 tapemarks)
%SIM-INFO: Read Tape Record Returned Unexpected Status: error in record
%SIM-INFO: 346332 bytes of unexamined data remain in the tape image file
%SIM-INFO: A potentially unreasonable number of record sizes(1) vs tape marks (0)
%SIM-INFO: The tape format (SIMH) might not be correct for the 'unix_v4.tap' tape image

I initially suspected the tape image was corrupted. Analysis with xxd confirmed the file began with =0002 0000=—a valid SimH tape format header indicating 512-byte records. The warnings, it turned out, are informational rather than fatal. The tape functions correctly despite them.

The most frustrating issue involved mcopy, the utility that copies the Unix filesystem from tape to disk. The squoze.net documentation shows:

=mcopy
'p' for rp; 'k' for rk
k
disk offset
0
tape offset
75
count
4000

This appears straightforward: select k for RK05 disk, then provide three numeric values. My Expect scripts faithfully sent these inputs, yet the copy always failed with incorrect parameters.

After switching from Expect to tmux for better visibility, I captured the actual terminal output:

'p' for rp; 'k' for rk
k
disk offset

tape offset

The revelation: disk offset appears WITHOUT a trailing newline. The cursor sits at the end of that line, not on a new line. When I pressed Enter after typing k, I was unknowingly submitting an empty response to disk offset.

This caused a cascade failure:

• disk offset received empty string (defaulted to 0, but incorrectly)
• tape offset received my intended disk offset value (0)
• count received my intended tape offset value (75)
• My count value (4000) went to the next prompt

The result was copying only 75 blocks from the wrong tape position—a useless disk image.

The correct input sequence requires NOT pressing Enter after the disk type:

=mcopy
'p' for rp; 'k' for rk
k0        ← type 'k' then '0' then Enter
75        ← Enter
4000      ← Enter

In tmux terms:

tmux send-keys -t unixv4 'mcopy' Enter
sleep 1
tmux send-keys -t unixv4 'k'           # NO Enter here!
tmux send-keys -t unixv4 '0' Enter     # disk offset
tmux send-keys -t unixv4 '75' Enter    # tape offset
tmux send-keys -t unixv4 '4000' Enter  # count

A similar issue affects uboot. After loading, it expects:

1. A single character for disk type (k for RK05)
2. The kernel filename (unix)

Again, no Enter between the disk type and filename:

tmux send-keys -t unixv4 'uboot' Enter
sleep 1
tmux send-keys -t unixv4 'k'           # NO Enter!
tmux send-keys -t unixv4 'unix' Enter  # Kernel name

# date
Wed Jun 12 19:51:34 EDT 1974
# who
root    tty8 Jun 12 19:51
# ls /
bin
dev
etc
lib
mnt
tmp
unix
usr

The date shows June 12, 1974—preserved from when the tape was created.

# cat /etc/passwd
root::0:1::/:
bin::3:1::/bin:

Only two users defined. Note the empty password fields—security was simpler then.

# ls /bin
ar    cmp   dump  if    ls    od    sh    su    uniq
as    cp    echo  kill  mail  pr    size  sum   who
bas   date  ed    ld    mkdir ps    sort  sync  write
cat   db    exit  ln    mv    restor strip stty
cc    dc    fc    login nm    rm    su    time
...

# ls /usr/games
bj      chess   cubic   moo     ttt     ttt.k   wump

The system includes cc (C compiler), as (assembler), ed (the original line editor), and games including chess and wump (Hunt the Wumpus).

The historic kernel source lives in /usr/sys:

/usr/sys/ken/    - Ken Thompson's kernel code
  main.c         - Kernel initialization
  slp.c          - Sleep/wakeup mechanisms
  trap.c         - Trap handling
  sys1.c-sys4.c  - System calls
  ...

/usr/sys/dmr/    - Dennis Ritchie's device drivers
  bio.c          - Block I/O
  tty.c          - Terminal handling
  rk.c           - RK05 disk driver
  ...

The separation into ken and dmr directories reflects the division of labor between the two Unix creators.

The SimH command d sr 1 sets the switch register to 1. This tells the boot loader to enter interactive mode rather than attempting an automatic boot.

Before exiting, always sync the filesystem:

# sync
# sync
# sync

The triple sync is a Unix tradition—ensuring all buffered writes complete. Use Ctrl-E to return to the SimH prompt, then exit.

The RK05 was a 2.5MB removable disk pack. The disk image is 2,494,464 bytes (4,872 512-byte blocks). The mcopy count of 4000 blocks copies the filesystem portion; the remaining space is unused.