Re: RFC: disk geometry via sysfs

[linux-os (Dick Johnson)]

On Wed, 15 Feb 2006, Phillip Susi wrote:

> linux-os (Dick Johnson) wrote:
>> Heads start at 0. Sectors start at 1. Cylinders start at 0.
>> A "lower head" than allowed would be 0xff so the BIOS wouldn't
>> know it was "lower". The BIOS doesn't look at the MBR for
>> normal read/write access! Only while booting does it
>> read the first sector of the master boot record (MBR) into
>> the appropriate physical place (0x7c00). Then it checks to see
>> if there is an 0xaa55 as the last word in the sector. If so,
>> it executes code starting at offset zero. Modern BIOS don't
>> even check the "boot flag" because it may be wrong, preventing
>> a boot.
>>
>
> I'm talking about the geometry of the disk.  If the disk has 16 sectors
> and 8 heads, then the maximum value allowed for any valid address is 16
> in the sector field and 7 in the heads field.  This influences the
> translation to/from LBA.  A sector with LBA of 1234 would have a CHS
> address using this geometry of 9/5/3.  If the disk reports a geometry of
> x/8/16 but the bios is using a geometry of x/255/63, then when you pass
> 9/5/3 to int 13 it will fetch LBA 144902 which is clearly not going to
> give you what you wanted.
>

Wrong! The disk gets an OFFSET!  It doesn't care how that OFFSET
is obtained. That OFFSET is the sum of some variables. Some start
at 0 and some start at 1. The BIOS takes these PHONY things, without
checking to see if they "fit" in some pre-conceived notion of
"geometery" and sums them all up to make an OFFSET. The C/H/S
stuff started and ENDED with the ST-506 interface.  PERIOD.

[Snipped rest]


Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.66 BogoMips).
Warning : 98.36% of all statistics are fiction.
_


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