Date   : Tue, 03 May 1988 17:18:12 GMT
From   : rochester!ur-tut!cwwj@bbn.com (Clarence Wilkerson)
Subject: Stack in the BIOS, and the SB180 format

I will send you some turbo pascal fragments documenting
the h89 formats. If you have the actual DD h89 disk, all this
information is written out on the first sector of the disk
in a manner similar to the BPB of an MSDOS disk. You will
probably not be able to read a disk you wrote elsewhere on
an H89 unless this block is present. The default is SD SS.
The other gotcha is that the 177x controller used on a H89
is not completely compatible with the 765 clone on the SB180.
The incompatiblity is said to arise from the index hole and how
soon data can appear after the hole. According to this theory,
the 177X controllers can write sooner after the index, and the 765
attempting to read this disk cannot recover from the registering the
passage of the index hole to get the first sector.
  Depending on your bios, the index hole may not need to be observed,
so one solution that works sometimes on a PC is to cover the index
hole of the disk. I changed the format program on my H89 to write
more filler bytes before the first sector of data..
 In the other direction, I have observed no problems except that 
pc disks are formatted with no physical skew factor, so the h89
reads them more slowly than its own which have a physical slew
which is invisible after the format.
Clarence Wilkerson