Messages in this thread | | | Date | Fri, 18 Apr 2008 15:37:13 +0200 | From | Seewer Philippe <> | Subject | Re: Disk geometry from /sys |
| |
Mark Lord wrote: > That can sound a bit misleading. The complete story, for ATA/SATA drives, > is that the disk has two geometries: an internal physical one, with a > fixed number of heads and cylinders, but variable sectors/track > (which normally varies by cylinder zone). > > Software *never* sees or knows about that geometry, so ignore it. > > The second geometry, is the one that the drive reports to software > as its "native" geometry. This is what you see from "hdparm -I" > and friends, and this geometry is what has to be used by software > when using cylinder/head/sector (CHS) addressing for I/O operations. > The hardware interface has a limit of 4-bits for the head value, > so the maximum number of heads can never be more than 16. > > Nobody uses CHS addressing for I/O operations, at least not on > any hardware newer than at least ten years old, so this geometry > is also unimportant for most uses. > > That's what the drive knows about. > > Software, for compatibility with the MS-DOS partition table scheme, > sometimes uses a "logical" geometry, where we "pretend" that a drive > can have up to 255 heads, which then allows more of the disk to be > described within the limitations of the partition table data layout. > That's where one frequently sees "255 heads", even though the drive > underneath uses 16 at the interface level, and probably as only 2 > or 4 real heads inside the shell.
Aye. Though I prefer the term virtual geometry. But thats cosmetics. Sorry for beeing unclear, and many thanks for untangling my post.
If anyones interested in even more Details about C/H/S adressing and so on, there's a very good document about that to be found here: http://www.mossywell.com/boot-sequence/
| |