[BBC-Micro] Hard or Soft Sectored Drives

Pete Turnbull pete at dunnington.plus.com
Fri Dec 22 10:20:45 GMT 2006


On 22/12/2006 09:21, David Harper wrote:

> Hard sectored disks had a physical mark at each sector. This meant a whole 
> ring of index holes, one for each sector, rather than just a single hole to 
> mark the start of the track.
> 
> I have not seen one of these disks for a very long time. IIRC the technique 
> was used for the old 8-inch floppy disks. (We are talking early 1970's 
> technology here, before 5.25-inch disks were thought of.)

Some 5.25" disks use the same technique, and were available in either 
10-sector or 16-sector types.  8" disks typically used either 26 
sectors, or occasionally 32 sectors, though the earliest used only 8. 
Even with 8" disks, though, soft-sectoring was more common.  BTW, the 
holes were referred to as sector holes, not index holes (though they 
look the same).  For floppies, the same drives are used for both hard- 
and soft-sectored disks, only the media and controllers differ.

One advantage of hard sectoring is that you don't need such big gaps 
between sector header and data; since there is a permanent indication of 
where the sector should start, you can write a new sector identifier at 
the start of the data every time you write a sector, and omit the gap 
and clock sync bits that usually separate them.  With soft-sectoring, 
each sector begins with a series of clock pulses (usually 128 '0' bits) 
used to synchronise the clock circuit in the controller during reading, 
then some special bit pattern to mark the start of the header, the 
header itself (which contains the track number, head number, sector 
number and a CRC check), then a gap (often a sequence of '1' bits) so 
that there is some latitude allowed in the exact start position of the 
data (to allow for minor drive speed variations), then more sync bits, 
the data, a CRC byte, and another gap.  When you write new data, you 
only write the data block and CRC (and the preceeding sync bits), so the 
gaps are needed in case the new data starts a little early or continues 
a little late.  With hard-sectoring, you always have a signal from a 
physical marker to determine where the sector starts, so you can 
eliminate one gap and the following sync and always write both header 
and data, in a smaller space.

It's a bit different on hard drives, as they don't have index holes.  In 
fact, on some hard drives, like the 12" Fujitsu winchesters on one of my 
PDP-11s, they can be programmed for any of several numbers of sectors, 
and different sector sizes.  On some multiple-platter disks, this is 
done by having one surface dedicated to holding sector identifiers.

-- 

Pete						Peter Turnbull
						Network Manager
						University of York



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