Subject: Re: QUANTUM PD425S support?? (hp300)
To: Reginald J. Reed <>
From: Terry Lambert <>
List: port-hp300
Date: 12/06/1996 11:46:52
> > PS: The hp disktab says that the disk has/uses/supports? ZBR.
> >     Does anyone know this 'feature'?
> I believe ZBR is just a method of encoding information on the disk, like 
> RLL and MFM.

Zone Boundry Recording.

Basically, all it means is that the number of sectors per cylinder
increases as you move out from the spindle... ie: the rotational
density of cylinders remains constant instead of decreasing.

Instead of using C*H*S as an absolute value, there is some average
value such that:

	H* SUM( Sn) n=0 -> n=C    ~= C*H*S'

And the number of sectors per cylinder is reportes as INT(S').

In general, this means that:

1)	The FFS rotational latency, meant to optimize against head
	seek operations for Sn -> S(n+1), is pretty much useless
	(all of the BSD's put a 0 in for seek latency any more anyway)

2)	If you are using an inferior OS which addresses sectors by
	C/H/S (like DOS and INT 13/INT 21 BIOS) instead of by
	absolute sector (like God addresses his disks), then you
	will lose the ability to get at the last C*H*(S' - INT(S'))

The reason it's in the HP disktab is that HP does *not* turn off
the seek latency "optimization".  When using non-SCSI disks (the
default for most HP systems), the optimization still has meaning,
since it can calculate the real cylinder boundries instead of
guessing wrong most of the time.

					Terry Lambert
Any opinions in this posting are my own and not those of my present
or previous employers.