Simh is an open source, highly portable collection of multi-system emulators.

SIMH implements simulators for:

• Data General Nova, Eclipse
• Digital Equipment Corporation
  • PDP-1, PDP-4, PDP-7, PDP-8, PDP-9, PDP-10, PDP-11, PDP-15
  • VAX
• GRI Corporation GRI-909
• IBM 1401, 1620, 1130, 7090/7094, System 3
• Interdata (Perkin-Elmer) 16b and 32b systems
• Hewlett-Packard 2116, 2100, 21MX
• Honeywell H316/H516
• MITS Altair 8800, with both 8080 and Z80
• Royal-Mcbee LGP-30, LGP-21
• Scientific Data Systems SDS 940

Here is a demo to run the original Unix System V7 on a pdp11 emulator:

 slug:~$ sudo ipkg update
 slug:~$ sudo ipkg install simh
 slug:~$ mkdir -p test; cd test
 slug:~/test$ wget http://simh.trailing-edge.com/kits/uv7swre.zip
 slug:~/test$ unzip uv7swre.zip
 slug:~/test$ pdp11
 PDP-11 simulator V3.6-1
 sim> set cpu u18
 Disabling XQ
 sim> set rl0 RL02
 sim> att rl0 unix_v7_rl.dsk
 sim> boot rl0
 @boot
 New Boot, known devices are hp ht rk rl rp tm vt
 : rl(0,0)rl2unix
 mem = 177856
 # cat /etc/passwd
 root:VwL97VCAx1Qhs:0:1::/:
 daemon:x:1:1::/:
 sys::2:2::/usr/sys:
 bin::3:3::/bin:
 uucp::4:4::/usr/lib/uucp:/usr/lib/uucico
 dmr::7:3::/usr/dmr:
 # mkdir /usr/dmr
 # chown dmr /usr/dmr
 # chgrp 3 /usr/dmr
 # mkdir /tmp
 # chmod 777 /tmp
 # ^D
 Restricted rights: Use, duplication, or disclosure
 is subject to restrictions stated in your contract with
 Western Electric Company, Inc.
 Thu Sep 22 05:51:05 EDT 1988

 login: dmr
 $ ed hello.c
 ?hello.c
 a
 main()
 {
    printf("Hello World!\n");
 }
 .
 w
 40
 q
 $ cc hello.c
 $ ./a.out
 Hello World!
 $ sync
 $ sync
 $ ^E
 Simulation stopped, PC: 016552 (JSR PC,2440)
 sim> quit
 Goodbye
 slug:~/test$

References:

• http://wandel.ca/homepage/unixdemo.html
• http://simh.trailing-edge.com/pdf/simh_swre.pdf