Re: C/H/S from user space

[linux-os (Dick Johnson)]

On Fri, 17 Feb 2006, Jeff V. Merkey wrote:

>
> Dick,
>
> The model Netware uses for large drives for calculating C/H/S addressed
> this problem years ago and
> provides more useful output, since C/H/S is converted into a hieristic
> which acts almost like an MD5 hash
> and for a given dimension, does provide unique output for even very
> large drives.
>
> Code Attached.
>
> Jeff
>
>
>
> ULONG SetPartitionTableGeometry(BYTE *hd, BYTE *sec, BYTE *cyl,
>                ULONG LBA, ULONG TracksPerCylinder,
>                ULONG SectorsPerTrack)
> {
>    ULONG offset, cylinders, head, sector;
>
>    if (!cyl || !hd || !sec)
>       return -1;
>
>    cylinders = (LBA / (TracksPerCylinder * SectorsPerTrack));
>    offset = LBA % (TracksPerCylinder * SectorsPerTrack);
>    head = (WORD)(offset / SectorsPerTrack);
>    sector = (WORD)(offset % SectorsPerTrack) + 1;
>
>    if (cylinders < 1023)
>    {
>       *sec = (BYTE)sector;
>       *hd = (BYTE)head;
>       *cyl = (BYTE)(cylinders & 0xff);
>       *sec |= (BYTE)((cylinders >> 2) & 0xC0);
>    }
>    else
>    {
>    *sec = (BYTE)(SectorsPerTrack | 0xC0);
>    *hd = (BYTE)(TracksPerCylinder - 1);
>    *cyl = (BYTE)0xFE;
>    }
>
>    return 0;
> }
>
> ULONG SetPartitionTableValues(struct PartitionTableEntry *Part,
>                  ULONG Type,
>                  ULONG StartingLBA,
>                  ULONG EndingLBA,
>                  ULONG Flag,
>                  ULONG TracksPerCylinder,
>                  ULONG SectorsPerTrack)
> {
>
>    Part->SysFlag = (BYTE) Type;
>    Part->fBootable = (BYTE) Flag;
>    Part->StartLBA = StartingLBA;
>    Part->nSectorsTotal = (EndingLBA - StartingLBA) + 1;
>
>    SetPartitionTableGeometry(&Part->HeadStart, &Part->SecStart,
> &Part->CylStart,
>                  StartingLBA, TracksPerCylinder, SectorsPerTrack);
>
>    SetPartitionTableGeometry(&Part->HeadEnd, &Part->SecEnd, &Part->CylEnd,
>                  EndingLBA, TracksPerCylinder, SectorsPerTrack);
>
>    return 0;
>
> }
>
>
>
[SNIPPED...]

Yes, it's a very good model, in fact what I've been talking about.
However, several people who refused to read or understand, insisted
upon obtaining the exact same C/H/S that the machine claimed to
use when it was booted.

So, since Linux doesn't destroy that information remaining in
the BIOS tables, I show how to make it available to a 'root' user.
Observation over several machines will show that the BIOS always
uses the same stuff for large media and, in fact, it has no choice.
Basically, this means that the first part of the boot-code, the
stuff that needs to be translated to fit into the int 0x13 registers,
needs to be below 1024 cylinders, 63 sectors-track, and 256 heads.
Trivial... even LILO was able to do that! Once the machine boots
past the requirement to use the BIOS services, it's a CHS=NOP.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.15.4 on an i686 machine (5589.53 BogoMips).
Warning : 98.36% of all statistics are fiction.
_


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