Simh

 

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Simh is not as fast or complete as E11 as far as fidelity to the PDP-11 hardware, but this is still the real winner.

This page was originally written around 2003. Some of the information below is obsoleted by later improvements to Simh. The newer versions of simh solve these problems in a more elegant manner.

Simh comes with source, and you have to compile it yourself. It also includes simulators for a bunch of other antique computers. Here are the VisualC project files to compile the PDP-11 version. simh.dsw simh.dsp.

Simh with 2.9BSD

Using simh, I have been able to boot and configure 2.9BSD, and I am now using it to do development work, just like a real PDP-11. Here is the ini file for 2.9BSD: bsd.ini. The disks I have configured are:

bullet/dev/rl0 (simh rl0): an RL02 containing the disk image rl02_2.9BSDroot.dsk, which you can obtain from the PUPS archive. Simh will boot from this image. You need to type: rl(0,0)rlunix. to get it to boot.
bullet/dev/rl1 (simh rl1): an RL02 mounted as /users. This is where I put user account files. To create this disk, I made a copy of the root image, then used mkfs to clear it.
bullet/dev/rl2 (simh rl2): another RL02 image, used as the swap space. Initialized like rl1.
bullet/dev/xp0h (simh rp0): this is an rp06 mounted as /usr. This is the destination of the usr.tar file which comes on the UNIX distribution "tape".
bullet/dev/xp1h (simh rp1). I could not get any virtual tape to work with 2.9BSD, so I mount things like usr.tar on simh's rp6 in the bsd.ini file. Once in UNIX, I can say "tar xvf /dev/xp1h" to fetch data off the tar file.

I had to create /dev/rl2, /dev/xp1h, etc. The "h" in /dev/xp1h means that the disk is viewed as a single, unpartioned sequential file.

Simh with 2.11BSD

I have 2.11BSD running under simh, but the current setup is extremely clumsy. I hear the latest version of simh has support for MSCP drives. After I convert to the new version, I'll document how to setup for 2.11BSD.

Because 2.11BSD expects disks to be partitioned using disklabel, you cannot use a disk as a virtual tape. The virtual tape device does work with 2.11BSD. The virtual tape format has markers in it to simulate tape records, so you have to use a translation utility on the DOS side to convert from files to tape format and back.

I wrote quick-and-dirty utilities to do this: tapify and dtapify. They are short on error checking, so don't make any mistakes.

Telnet interface for simh

One of the brilliant new concepts in Windows Millennium is that they have eliminated all the support problems that arose from the use of DOSish features such as TSRs and ansi.sys by deleting these features. One result is that Millennium users no longer have a way to run programs that assume that the terminal is a VT100 (such as some compiles of MicroEMACS).

This also affects simh. Therefore, I have added a section to scp-tty.c that implements the console terminal as a TELNET server. After running simh, you can telnet to "localhost", and you are connected to the console of the PDP-11. Since most telnet clients emulate VT-100, the Millennium problem should be solved. This may not be entirely debugged yet, but it seems to work.

I added an output buffering mechanism that uses the time since the last  sim_putchar to flush the output buffer. It's a bit clumsy, and could probably be done better, but it seems to work. It results in a slight delay to echoed characters that is not too annoying.

I thought that by buffering the output I could speed up the console output from 2.9BSD under simh. I was assuming that the slowness of console output (compared to E11) was due to doing I/O a single character at a time. This assumption was evidently incorrect -- the console output rate is about the same. (Extremely slow output from 2.9BSD can be fixed by a stty setting).

Here is the modified scp_tty.c, and the executable. In addition you need to #define USE_SOCKETS and add wsock32.lib to the linker command. This is for simh 2.6.

I was hoping that vi would now come up in VT100 full screen mode, but it doesn't (yet). I "set term=vt100" and "tset" according to one VI tutorial, but that didn't help. Maybe this will work better with a up-to-date version of simh.

simh.exe

simh.zip

 

Last modified: December 31, 2009