From: "Chuck Swiger" <[EMAIL PROTECTED]>
On Sep 6, 2006, at 1:06 PM, Hilt, Ian wrote:
The hard disk has an on-board controller which answers the ATA
"IDENTIFY DEVICE" command with the hard drive parameters used by the
BIOS, assuming that the BIOS is operating in the legacy C/H/S mode
rather than the newer LBA mode which uses absolute block numbers.
Ok. Maybe the better question is: in either case, C/H/S or LBA mode,
where are these parameters stored?
At one time, probably on an EEPROM within the hard drive; nowadays,
probably nowhere-- the drive controller computes some numbers
dynamically depending on whether the C/H/S versus LBA mode jumper is
set, or whether the BIOS makes the extended Int13H call to do LBA
mode (or whatever the exact mechanism there is)....
They flat out are not stored anywhere. There is a standard algorithm
published by the VESA people, I believe, that provides the data for
all SCSI drives and modern IDE/ATA/SATA drives. Inside the drives
only one number is normally of interest to the computer operating
system, the total number of available blocks on the drive per its
current formatting. Spare blocks and cylinders, variable numbers of
blocks per track, and various oddball formattings that at LEAST go
back as far as the old 20 meg Miniscribe SCSI drives make any CHS
that a drive could deliver meaningless. (That old Miniscribe had
spares at the end of a cylinder that were to be applied anywhere
within the cylinder. Thus there was no constant blocks per track
within a cylinder. It had spare tracks scattered around the
drive so that you could recover if a whole track was scratched. And
so forth. I struggled for some (wasted) time trying to find an optimal
CHS geometry I could feed the operating system (Amiga at the time) to
speed up disk accesses. That old thing was impervious to optimization.
Ever since I've strongly advised people to ignore CHS entirely unless
they have a real live ESDI or ST-506 drive in their possession. I
suppose it might matter for IDE drives nearly that old. But anything
likely to be alive today has CHS as a pure fiction that is not all
that particularly useful even at the filesystem optimization levels.)
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