[PLUG] Why create a boot partition?

Ian Burrell ian at znark.com
Sun Oct 5 14:14:01 UTC 2003


Michael C. Robinson wrote:
> Isn't a drive something that can be looked at as a series of writable
> locations from 0 to some high number where some of these locations 
> need to have partition start and partition end written to them?  Why 
> won't Microsoft and everyone else agree that partitioning should be
> defined outside any operating system where a recognized hardware API
> could then be made to work for everyone wanting to access that drive?  
> I would think this would solve a lot of problems with the OS not 
> looking at the drive geometry correctly. 
>  

There is a standard on PCs for partitioning drives.  It has some 
restrictions, like 4 primary partitions, and cylinder alignment, but is 
used by most PC operating systems.  Other platforms and some OSes like 
the BSDs have other schemes.

There are also two BIOS hardware APIs to access the hard drive.  One is 
the old CHS method that uses the drive geometry.  This is the one with 
the 1024 cylinder limit and 8 GB drive size.  DOS and Windows 95/98 are 
limited by this because they are old.  The other is the LBA method that 
Linux and Windows 2000 uses.  The BIOS does not care one bit about 
partitioning.

> What does partition alignment really mean?  Is it like memory 
> alignment where you use packing of nibbles to create bytes 
> because the architecture has a certain word size or something?  
> If these counters were part of the drive controller would we 
> ever have had the 10 bit counter limit or 1024 cylinders?
> I guess this as a red herring though like the making
> everyone implement PTR records in their dns.
> 

Partition alignment means that the partitions are aligned to cylinder 
boundaries.  The old disk access mode used the disk geometry of 
cylinder, head, and sector (CHS) to address sectors.  For the most part, 
the partitions needed to start on cylinder boundaries.  The CHS access 
mode went all the way to the drive controller.

All modern hard drives and BIOS support the linear block access (LBA) 
mode which treats the drive as a linear sequence of sectors.  The drive 
hides its real geometry.  The partition table also stores the begin, 
end, and size in sectors.  It is usually a good idea to keep the 
partitions aligned with cylinders so older programs (like Windows) don't 
get confused.

  - Ian

-- 
ian at znark.com
http://www.znark.com/





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