Date: Wed, 6 Sep 2006 16:35:33 -0400 From: "Hilt, Ian" <ihilt@mcgregor-surmount.com> To: "Chuck Swiger" <cswiger@mac.com> Cc: freebsd-questions@freebsd.org Subject: RE: Origin of hard drive parameters Message-ID: <890E919AB0857D4A8A52A4AD5E0F0F6221A1C3@msc-server.msc.mcgregor-surmount.com> In-Reply-To: <945DB39F-ECC9-4872-9F5D-EC45BCD439CE@mac.com>
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> -----Original Message----- > From: Chuck Swiger [mailto:cswiger@mac.com] > Sent: Wednesday, September 06, 2006 4:21 PM > To: Hilt, Ian > Cc: freebsd-questions@freebsd.org > Subject: Re: Origin of hard drive parameters > > 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 > > Thanks for the information, Chuck.help
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