Date   : Wed, 05 Apr 1989 03:56:38 GMT
From   : xanth!rlb@g.ms.uky.edu (Robert Lee Bailey)
Subject: adding a hard disk to your floppy-only system

In article <7316@cadnetix.COM> rusty@cadnetix.COM (Rusty) writes:
>
>Well, I just bought myself a nice hard disk controller for my S100 system.
>Having looked over the CPM manual for a bit, it seems to me that I *should*
>be able to accomplish the task of adding the controller and hard disk to
>my system.
>
>
>So, enough rambling, down to the question for the net:
>
>Has anyone else added a hard disk to a CP/M system which was not originally
>configured for a hard disk?  Can I do the trick that Kaypro did and boot
>from either flops or hard?  Any words of wisdom? (Other than "Don't do
it!" :-)
>

I have a Xerox 820-1 single board CP/M computer.  When I got it, it was
not expandable to allow adding a hard disk.  After checking around a bit,
I found an inexpensive (for the time) 5MB hard disk ($99) and a SCSI
HD controller (also $99).  Now the Xerox 820 did not have a SCSI interface,
so, I had to build my own adapter as well as write my own HD driver.
The hardware adapter was not very sophisticated (i.e. not interrupt driven).
I just wire wrapped an adapter that plugged into the Z-80 socket.  This
allowed me to 'tap' the I/O and data lines that I needed for the SCSI
controller.  I added some TTL logic to decode the address that I wanted
to use for the controller and then began to write the software to access
the SCSI port.

I wrote the necessary routines and added them to my BIOS.  I probably
could have made it boot off of the HD, but, that would have required me
to patch the ROM.  Since I didn't have access to an EPROM burner, I
elected to use a hybrid method.  What I did was to modify the boot tracks
of the floppy so that instead of loading CP/M (BDOS & BIOS) from the
floppy, I loaded the HD driver.  After the driver was in memory, I
loaded CP/M from the hard drive (I SYSGENed a modified boot track onto
the HD).  I also faked out CP/M to make my HD (drive E:) my default drive.
So, when I booted the system, it automatically came up logged into
the HD. Using this method, only a few sectors were loaded from floppy,
and then the rest of the booting occured from the HD.

While this was not the optimum way to do it, for me it proved to be
a viable way to get a HD up and running with minimal modifications to
my existing hardware & software.

               Bob Bailey


End of INFO-CPM Digest
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