Transferring RX01 discs to SIMH

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Dave Dunfield’s ImageDisk software includes two utilities that are very useful for moving a real RX01 disk to a virtual disk file that can be used with the SIMH PDP-11 simulator.

To transfer a real RX01 disc to SIMH, use this process:

  1. Put the RX01 disc in an 8-inch floppy drive attached to an MSDOS machine that has the ImageDisk programs installed
  2. Use the main ImageDisk program (IMD.COM) to “read” the disc and store it as an IMD disk image. Let’s call it “ORIGINAL.IMD”
  3. On the MSDOS machine, use IMDU.COM to convert the IMD disk image to a BINARY disk image, as follows: IMDU ORIGINAL.IMD ORIGINAL.DSK /B
  4. Now transfer the .DSK file to the machine that you run SIMH on. Hint: Setting up the mTCP package on your MSDOS machine and vsftp on a Linux machine will provide a super-fast and reliable way to transfer disk images between your machines

Let’s assume your RX01 disc was a bootable RT-11 disc. A suitable SIMH.INI file to boot your RX01 disc in SIMH is as follows:

set cpu 64k
set cpu 11/23
set throttle 10%
at rx0 original.dsk
set tti 8b
set tto 8b
set console telnet=2222
boot rx0

You should now be able to fire up SIMH, and connect a console to SIMH using a Telnet terminal emulator (Hint: Use PuTTY and connect via the Telnet protocol to IP address port 2222).

If your RX01 disc was bootable, you should now see RT-11’s boot messages and the familiar RT-11 dot-prompt in the terminal emulator.

Once you’ve finished making changes to your RX01 disc in SIMH, press CTRL-E in the SIMH window to return to the SIMH prompt. Type exit and press return to exit SIMH. Now rename your DSK file as “changed.dsk”.

To transfer the DSK file back to a real RX01 disk, use this process:

  1. Transfer the DSK file back to your MSDOS machine that has ImageDisk on it (Hint: consider using the FTP technique described above to do this)
  2. On the MSDOS machine, use BIN2IMD.COM to convert the BINARY disk image to an IMD disk image as follows: BIN2IMD CHANGED.DSK CHANGED.IMD N=77 DM=0 SS=128 SM=1-26 /1 /C
  3. Use the main ImageDisk program (IMD.COM) to “write” the IMD disk image to a real 8-inch floppy disc connected to the MSDOS machine
  4. Move the floppy disc to the RX01 drive on your PDP-11, and boot the PDP-11 with it

A few things to note about this process:

  • It works well with RX01 discs. But it can’t be used with RX02 discs. That’s because RX02 uses a non-standard mixture of single-density and double-density information on every track. ImageDisk is unable to read or write these discs
  • An RX01 disc cannot be booted in an RX02 drive. That’s because the boot tracks of the disc will have a DX.SYS driver in them (to suit the RX01 controller). This won’t successfully boot an RX02 controller (which requires DY.SYS in the boot tracks)
  • You can use double-sided 8-inch discs with the single-sided RX01 and RX02 drives. But there is one catch. The index hole in the jacket of double-sided discs does not line up with the opto-couplers in the RX01/RX02 drives. This will prevent the disc from booting from the KDF11B boot menu (the boot ROM will search for an index pulse, and when none is detected, it will assume that no disc is inserted in the drive). Once you boot the PDP-11 from another drive, you can boot your double-side media from the RT-11 prompt (eg BOOT DX0:). RT-11 doesn’t seem to care whether it sees index pulses or not.


Dave Dunfield’s ImageDisk software

Revision History

1 August 2016: Posted initial version