Large HD, old BIOS

[bscott at ntisys.com]

On Mon, 3 May 2004, at 10:15am, colet at code-energy.com wrote:
> I dunno - what is LBA?  How do I find out if my drive "is LBA" or not?  

  LBA = Logical Block Addressing.

  The original IBM-PC BIOS (and MS-DOS) used C/H/S "physical block
addressing".  Each block (AKA sector) on the disk was addressed by cylinder
(track), head (side), and sector.  This is a really poor way to do things,
for any number of reasons.  The biggest was that each C/H/S field has a
fairly small number of bits allocated to it, and as drives got bigger, these
limits were reached.

  To work around C/H/S limits, the industry came up with "geometry
translation".  The BIOS (or sometimes, a small "drive overlay" program that
loaded before MS-DOS) would change the presentation of disk blocks around to
make use of space in other fields.  For example, it might present the drive
as having 255 heads to keep it under 64K sectors-per-track.

  Of course, geometry translation is ultimately a kludge.  It can be a real
problem if everything isn't on the same page when it comes to the
translation.  The thing you're likely to care about is that the Linux kernel
doesn't use the BIOS, so it has to guess as to what geometry translation
scheme the BIOS might be using.  If it guess wrong, the disk becomes
scrambled between the two.

  The more intelligent way to solve this problem is LBA.  LBA just addresses
the entire disk as a linear series of blocks, numbered from 0 to
total-number-of-sectors.  Of course, not all software supported LBA.  In
particular, LILO lagged way behind Microsoft in implementing LBA, although
that has been fixed for some time now.

> How do I find out if my BIOS can handle it?

  Well, you could check the manual, or contact the manufacturer for support.  
You can also go poking around in the BIOS setup screen looking for an option
for "LBA" or "Large Disk Support".  This is sometimes a column in the basic
hard disk settings (where you type in C/H/S numbers).

-- 
Ben Scott <bscott at ntisys.com>
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