[Simh] ImageDisk support for SIMH

[Howard M. Harte]

I added support for Dave Dunfield's "ImageDisk" disk image file format for
soft-sectored floppies to the SIMH/AltairZ80 simulator.  

 

Dave Dunfield has many ImageDisk (IMD) disk image files on his website at:
http://www.classiccmp.org/dunfield/img/index.htm

 

ImageDisk is an excellent format for storing disk images, because the disk's
metadata is also retained by the format, allowing the image to be written
back to a floppy.  I have used SIMH/AltairZ80 to create a disk image for an
S-100 system that I had, that I had no boot floppy for.  I added support for
the disk controller to SIMH, then debugged the disk I/O routines, tested it
on the simulator, and wrote the image to a real 8" floppy and it booted on
the actual hardware on the first try. 

 

The ImageDisk support is implemented as a generic module that has a simple
sector read/write/format API.  It supports all features of the IMD file
format, including non-standard sector numbering, and skewing.  The support
is contained in two files, sim_imd.c, and sim_imd.h.  In addition to the
ImageDisk library, I also wrote two disk controller cores, an Intel 8272
(NEC765) and a Western Digital 179x.  This has been tested under Windows,
Mac OS/X, and Linux, as well as big-endian Mac.

 

Recently, I realized that DEC RX-01 floppies can be imaged using ImageDisk
since they are in the regular IBM single-density format.  I added support to
the PDP-11 and PDP-8 RX02 disk simulation and was able to boot RT-11 with
it.  I would like to contribute these enhancements to the main SIMH
distribution, and would welcome any feedback on the implementation.  The
PDP-11 RX02 support has not been well tested.  While RX-02 floppies cannot
be imaged by ImageDisk (due to the non-standard formatting) being able to
read and write RX-01 floppies is very useful for transferring data to
PDP-11's and for possibly creating bootable media using the simulator, which
can then be used to revive actual hardware.

 

The sim_imd support can be obtained by downloading the source archive at:
http://www.schorn.ch/cpm/intro.php

If anyone wants the PDP-8 and PDP-11 RX files, please let me know and I will
make them available.

 

Using ImageDisk, I have recovered many 8" floppy disk images, including
86-DOS, MP/M-86, Concurrent CP/M-86, OASIS, OASIS-86.  While I have a lot of
these running in AltairZ80, I have imaged many more disks (about 200) that I
will get to doing something with as time permits.  The important thing is
that the information can be preserved in a format that will not degrade with
time like the original floppy media.  I have even found that a very new
Intel Core2 Duo Motherboard can still write single-density 8" floppies, much
to my surprise.  Many PC motherboards cannot do this, but given that a
readily available $99 motherboard can do it, I think there is an opportunity
to save a lot more disks via imaging before the original media decays
further.

 

-Howard

 

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