Date: Wed, 6 Sep 2006 13:20:39 -0700 From: Chuck Swiger <cswiger@mac.com> To: "Hilt, Ian" <ihilt@mcgregor-surmount.com> Cc: freebsd-questions@freebsd.org Subject: Re: Origin of hard drive parameters Message-ID: <945DB39F-ECC9-4872-9F5D-EC45BCD439CE@mac.com> In-Reply-To: <890E919AB0857D4A8A52A4AD5E0F0F6221A1C2@msc-server.msc.mcgregor-surmount.com> References: <890E919AB0857D4A8A52A4AD5E0F0F6221A1C2@msc-server.msc.mcgregor-surmount.com>
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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).... >> Note that the answer the drive controller gives will normally be a >> fabricated geometry which does not have anything to do with the >> actual geometry of the physical device, in part because drives >> nowadays keep a variable number of sectors per track rather than >> using a CAV layout. > > If CAV == Constant Angular Velocity, I thought this layout stored a > variable number of sectors per track, as opposed to CLV which stores > data at a constant density over the platters. CAV == Constant Angular Velocity. It's the format used by data CD's which gives less storage space but better random access-- tracks near the center have the same # of sectors as tracks on the outside, which means the outer tracks are spread out more; versus CLV, which stores more data on the outer tracks by slowing down the rotational speed to keep a constant density under the heads. -- -Chuck
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