Path: utzoo!utgpu!jarvis.csri.toronto.edu!torsqnt!tmsoft!list-injector
From: brian@ncrcan.Toronto.NCR.COM
Newsgroups: can.usrgroup
Subject: Re: Is it the interleave?
Message-ID: <891013224519.14689@tmsoft.uucp>
Date: 13 Oct 89 02:00:52 GMT
Reply-To: brian@ncrcan.Toronto.NCR.COM
Distribution: ont
Lines: 88
In-Reply-To: <1989Oct10.221219.3571@eci386.uucp>; from "Chris Lewis" at Oct 10, 89 10:12 pm
X-Mailer: ELM [version 2.2 PL9]

> |>>This is done on PC's with Perstor Controllers (31 Sectors/track) and using
> |>>the 1140 disks as if they were 2190's (ie, 1224 cylinders instead of the 
> |>>918 as published by Maxtor).  I have yet to come across a Maxtor 1140 on
> |>>which this couldn't be done.
> |
> |Note that Brian's also suggesting that there are more tracks really on
> |the disk.
> 
> I don't think so.  This is why:

Yes there is more tracks on the drive.  You can also use the 1140 
as a 1224 cyl, 17 sect/track unit, if you wish. 

> 
> 	- ST506 normally allows 1024 cylinders.  If the XT1140 was >988,
> 	  Maxtor certainly would have at least said 1024.

I didn't think there was a limit imposed on the cylinder count by the ST506
interface.  Heads, yes, since there is only 4 head select lines, limiting you
to 16 heads.  (Note that most fast (ie voice-coil actuated head assembly) ST506
drives allow 15 heads, as the 16th head is used internally by the drive logic
to read the servo tracks on the reserved servo platter.  Stepper motor drives
can have a full complement of 16 heads).

The actual cylinder positioning is done with the STEP and DIRECTION 
signals, using the TRACK00 signal for synchronization.  Therefore, you can
STEP the heads in as far as the drive will allow it.  In the case of the
Maxtor 1140, which is voice coil actuated, there is enough servo tracks
on the servo platter to allow 1224 cylinders. There may be more, I've 
never tried and the Perstor doesn't support any more.  And why push
my luck any further anyways :-)

> 	- A controller is *not* able to change head step distances (as they
> 	  were, say, with Apple II floppies), because to the controller,
> 	  an ST506 disk has a discrete number of head positions that
> 	  you issue "step head out" and "step head in" and "recalibrate head" 
> 	  commands to.  Eg: you can't 1/2 step the head.  (Apple II's used
> 	  this for software copy protect...  but some floppy drives couldn't
> 	  do it at all....  grumble)

Agreed.  

> 
> What I think that the Perstor is really doing is getting the number of
> sectors per track > 31, running into a DOS limitation, and then spoofing the
> geometry so that the number sectors per track remains legal, but the
> number of cylinders is upped instead.  This is analogous to the way
> the WD1007 handles ESDI disks with 34 sectors/track...

Most modern AT BIOSES are able to handle large number of sectors/track.
The DOS limitation lies in the fact that it uses a 10 bit number to represent
the track number internally in it's logical block calculations, thus 
limitting DOS's usable number of cylinders to 1024.  When using the 
Perstor and Maxtor at 1024 cyls with 31 spt, there is no problem at all, and
no drive mapping going on.  In fact, I don't even think that the Perstor
has the BIOS software or hardware required to map the drive to 17 spt,
which is the way the WD and DTC controllers do it.

To gain access to the additional 200 cylinders, a software driver is loaded
at boot time.  I suspect that this driver is doing some sector mapping,
but again note that the drive runs fine at 1024 cylinders, 31 spt, no
additional drivers required.  Norton, PCTOOLS, PCKWIK disk cache, all 
report 1024 cylinders, 31 spt! 

> 
> You're right though, *part* of the way this is done could be that the
> Perstor formats each track as one *long* physical block, thus you
> have relatively little header/trailer lossage.

Actually, the Perstor uses a technique developed by them called ARRL, 
Advanced RLL, which is similar to 2,7 RLL developed by IBM.  It uses a
different grouping scheme when writing the flux transitions onto the surface
of the disk.  The grouping chosen is very specific, in that it will yield
a data rate of 9 megabits/sec (31 spt) or 10 megabits/sec (34 spt), yet
the actual code rate to the drive will be only be 5 megabits/sec, which 
gives a FTPI (flux transitions per inch) rate the same as that seen
with standard MFM at 5 megabits/sec.  In this way, it is working within
the limits of the ST506 interface and the drives built to that standard. 
In other words, it does not place any additional demands on the drive
than normal ST506 MFM would.  This is unlike standard RLL, which
operates at a code rate of 7.5 Mb/s, or 7.5 million FTPI.  (which is
why RLL requires plated media drives.  ARLL does not!)

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