HP 9816 CP/M-68K

Glen Slick glen.slick at gmail.com
Mon Feb 12 14:52:16 CST 2018

On Mon, Feb 12, 2018 at 12:34 PM, Eric Smith via cctalk
<cctalk at classiccmp.org> wrote:
> Format a track with the sector size that occurs later on the track, with
> dummy sectors ahead of them and gap sizes selected to position them
> properly.
> Start formatting with the sector size for the earlier sectors. Abort the
> format at the time when the desired number of sectors have been written.
> I'm not sure whether there's any way to abort a track format on a PC. I did
> it on a machine that had control over the μPD765 reset pin.

Bit 2 of the Digital-Output Register at 0x3F2 resets the FDC when the
bit is clear.

I used that on my 5150 back in the day when I bought it new to
reproduce a copy protected disk that used sectors with bad CRCs as one
of its protection mechanisms. Feed the sector data a byte at a time to
the FDC in PIO mode so that the timing is controlled, then toggle the
FDC reset bit in the DOR just at the time when the CRC is being

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