[PLUG] Why create a boot partition?

[Ed Sawicki]

Since you removed all reference to the message you're
responding to, I'll have to guess that you're responding
to one of my messages.


On Mon, 2003-10-06 at 00:35, Michael C. Robinson wrote:
> What is difference to the computer bios of a primary partition verses an
> extended partition verses a logical volume?  

This is a question that's best answered by the BIOS makers
and computer makers. I've often wondered why BIOS are so
restrictive. Most BIOS will only boot an operating system
from a primary partition but there's no difficult challenge
in booting from a logical drive in an extended partition. To
boot from a logical drive, the BIOS just needs to follow the
chain until it discovers where the logical drive starts. This
is no big deal considering that any disk partitioning program
can do it. 


> Does the bios just assume
> to start reading at sector zero or is there more reason than that why
> certain systems have to be installed at the beginning of the drive?

The BIOS assumes that sector zero contains the boot loader and
partition table. This has nothing to do with the 1024-cylinder
limit. The limitation is caused by the partition table's cylinder
value being limited to 10 bits.

> Why was a limit of four primary partitions imposed?

Because:

1. At the time IBM created the BIOS, Winchester hard disk
drives were weighing in at 5, 10, and 12 megabytes of total
capacity. How many pieces would you want to chop a 10 MB
drive in to? IBM decided that four was enough. I probably
would have made the same decision because I never would have
thought that the design would still be in use over 20 years
later.

2. IBM put both the boot loader and the partition table into
the same 512-byte sector. There wasn't enough space for more
primary partitions. If it was me, I would have used one sector
for the boot loader and another for the partition table. That
would have allowed 16 (primary) partitions. There would have
been no need for the insanity of extended partitions.

Remember that hard disk technology in 1981 was very different
than today's technology. Back then, disk space was expensive.
Using 1 sector rather than 2 was not considered an irrational
design consideration.

You're asking questions about why computers manufactured today
have the same limitations as computers designed in 1981. In
addition to partition table limitations we also have a
limit on the number of video cards that can be supported.

It's because today's computers are the same basic design as
the 1981 computers. The PC design should have been retired
years ago. Instead, it's kept alive by, well, inertia I guess.

Imagine if today's space probes used the same basic design as,
say, Voyager. Now imagine the PC.

Ed