Writing single density disks on a PC?

From: Fred Cisin <cisin_at_xenosoft.com>
Date: Mon May 19 21:51:00 2003

On Mon, 19 May 2003, Dwight K. Elvey wrote:
> Hi Fred
> I tried 22DISK for my M20 but it didn't work. The tables
> were wrong, as well as that my computer didn't deal
> with single density. Since it had no way of ignoring
> track 0, easily, I gave up on it.
> I found that the single density,
> on track zero, was solved by using the M20 to pre-format
> the disk and then used my routines to write the double
> density stuff to the remaining tracks. If Geoff has the
> same issue, he might be able to deal with the single
> density the same way. I found that the first track
> being single density was just a boot compatability issue.
> The first track would just have enough code to switch
> to double density and then the main loader was on
> track 1. This meant that the code was basically the
> same on all of the track zero's.
> I'd forgotten that the BIOS wouldn't deal with the
> single density. I was thinking more in terms of number
> and size of sectors.
> Dwight

Some chips such as the 37C65 handled single density internally, and if you
spoke directly to the FDC, you could create appropriate commands. But the
BIOS (13, 1E) were locked in to MFM. There were a few disk controller
boards for the PC and XT that could do single density, but often they used
mutually incompatible additional ports and commands to switch, naking it
unfeasable to handle them in a commercial product. ("your product
supports Maynard, how come it won't work with my Vista controller, which
is 'just like it'") For a NON-commercial product, it could be done
without too much hassle.

However, since the goal in THIS case was TRS-80 Model 4, it might be
feasable to use Trakcess on the TRS-80 to do the job.
Received on Mon May 19 2003 - 21:51:00 BST