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From - Fri Aug  4 05:59:43 2000
Newsgroups: comp.unix.sco.misc
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From: stephen@bokonon.ussinc.com (Stephen M. Dunn)
Subject: Re: 1024 Cyls or Mb's?
Organization: Followers of Bokonon
Date: Fri, 4 Aug 2000 02:24:05 GMT
Message-ID: <FyqxC5.47s@bokonon.ussinc.com> 
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In article <8mch8l$h1s$1@neptunium.btinternet.com> "Steve Morris" <Stephen.Morris@btinternet.com> writes:
$Thanks for the info, but how exactly do I determine where on my disk the
$1024th cyc lies? (i.e what Mb on the disk).

   First, some background so that you understand the answer I'm
giving - it's usually easier to remember stuff if you can understand
it.

   In the Good Old Days, when the numbers of heads, cylinders, and
sectors per track printed on the hard drive and/or its manual were
what the software used, this was straightforward.

   With SCSI, though, the geometry the OS sees is entirely unrelated
to the actual geometry of the hard drive.  From a SCSI perspective,
the hard drive is a one-dimensional stream of blocks numbered from
0 through whatever, rather than a three-dimensional array of
blocks numbered from (0,0,0) through whatever.  This actually makes
more sense; it permits zone-bit recording (where outer tracks have
more sectors to make better use of the physical size of the tracks)
and automatic substitution of bad tracks (no more bad track lists
to handle).  It could also enable me to devise a storage medium
that acts like a hard drive but is fundamentally different;
as long as I can represent it as a collection of blocks, any
SCSI host adapter can use it as if it were a hard drive.

   The cylinders, heads, and sectors the OS sees are whatever the
host adapter presents.  In the old days, most host adapters
mapped things so that each cylinder was a megabyte, so the
first 1024 cylinders were the first gigabyte.

   As hard drives got larger, and it became more common for
people to use hard drives that were over 1024 logical cylinders
with this mapping, host adapter manufacturers started playing
around with larger logical cylinders.  If your host adapter's
BIOS or setup utility gives you the option of DOS vs. extended
mappings, that's why - the DOS mapping gives you the traditional
1 MB cylinders (so that DOS is happy) and the extended mapping
gives you larger cylinders (to make more advanced OSes happy).  The
manual probably won't tell you exactly what the mapping is.

   ATA drives in LBA mode work much like SCSI drives, but at
least you usually get to set and/or examine their mappings
in your machine's BIOS.

   If you've already got Unix installed, or if you can wait
until you start the installation, Unix will tell you what the
logical layout of your hard drive is.  The following lines are
from ATA drives but you'll see something similar for SCSI:

%disk     0x01F0-0x01F7  14   -  type=W0 unit=0 cyls=1025 hds=255 secs=63
%disk     0x0170-0x0177  15   -  type=W1 unit=1 cyls=944 hds=14 secs=40

   The first drive is an 8 GB (approx.) ATA drive in LBA mode.
The PC reports it as 1025 cylinders x 255 heads x 63 sectors.
So on this drive, I'm free to put the boot filesystem anywhere
except the very end.

   The second drive is a 270 MB (approx.) ATA drive in traditional
CHS mode.  The PC reports it as 944 cylinders x 14 heads x 40
sectors.  Back when this was my boot drive, I was free to put the
boot filesystem anywhere I wanted on it.

   If you were to see a line like this:

%disk     0x01F0-0x01F7  14   -  type=W0 unit=0 cyls=2200 hds=255 secs=63

that would be an 18 GB (approx.) drive, and you could put the boot
filesystem anywhere in the first 8 GB of it.
-- 
Stephen M. Dunn                       <stephen@bokonon.ussinc.com>
>>>------------> http://staff.ussinc.com/~steved/ <------------<<<
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