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Q: How do disks greater than 10 Gbyte work ( Answered 5 out of 5 stars,   2 Comments )
Subject: How do disks greater than 10 Gbyte work
Category: Computers > Hardware
Asked by: vaac-ga
List Price: $5.00
Posted: 09 Mar 2005 20:26 PST
Expires: 08 Apr 2005 21:26 PDT
Question ID: 490818
Could anybody explain to me, or refer me to a website, book, or
journal article, either free or to be paid for, which explains, how
the new computers handle hard disks greater than 10 gigaherz. It seems
to me that if a hard disk has 1023 cylinders, 244 sides and 63
sectors, which equals 1023*244*63= about 10000000 sectors this is all
that interrupt 13 can handle.
Subject: Re: How do disks greater than 10 Gbyte work
Answered By: maniac-ga on 14 Mar 2005 18:41 PST
Rated:5 out of 5 stars
Hello Vaac,

I believe you are referring to the concept of
  "cylinder", "head", and "secton" (CHS)
to refer to a high capacity disk drive.

Yes, indeed the CHS method has a limit to roughly 8 Gbytes of data on
a disk. However, several years ago, the disk vendors (and OS and BIOS
vendors) started to use a concept called "Logical Block Address" (LBA)
to access the sectors on a disk. The older 28 bit LBA is sufficient to
address up to roughly 137 Gbytes and the newer 48 bit LBA is
sufficient to address over a hundred terabytes of data on a disk.

For more information, see pages like:
(published in 2000) which has a nice summary of the concepts involved or
a page in the middle of a hard disk explanation explaining the limit
of the "Int 13" and extensions made since that time (go to the next
page...) or more quickly at
which is an index of SEVERAL disk limits over a pretty broad range. Another site
has a similar summary with pretty much the same conclusions.

So in brief, the old Interrupt 13 limit is worked around in a variety
of methods that require some combination of capability in the BIOS
and/or OS. For example, if you use a large disk on an old PC, it may
be necessary to put the boot loader and OS in a partition near the
start of the disk (so the BIOS can load them) and then the OS accesses
the disk directly (bypassing the BIOS) to get the full capacity of the
disk drive. There is no real need to make a large number of partitions
in this case - I've worked with as few as three (boot partition, swap
partition, OS & data partition) under Linux and it works just fine.

Search phrases used
  cylinder head sector disk capacity
  CHS LBA comparison

vaac-ga rated this answer:5 out of 5 stars
very good answer

Subject: Re: How do disks greater than 10 Gbyte work
From: filip04-ga on 09 Mar 2005 21:16 PST
first of all 10,000,000 sectors doesn't mean 10,000,000 bytes,
therefore not 10gigabytes. There is usually 512bytes per sector, so
for your number of 10million sectors, that would be 5,120,000,000
bytes, or 5,000,000kb or about 4880mb or about 4.7GB.

Although harddrive manufactuers don't usually specify the amount of
tracks, heads, and sectors/track, they usually tell you how many
sectors the whole harddrive has. for example: WD CaviarŪ WD3200JD has
625,142,448 sectors (320GB)

Here is a little info on barriers:
Subject: Re: How do disks greater than 10 Gbyte work
From: vaac-ga on 13 Mar 2005 21:15 PST
Thanks,filip04-ga. for your comment and link
It got me to a website explaining that interrupt 13 gets away with the
difficulty by havig "extensions" Would you or anybody know where I can
find out more about these extensions and how to partition a large disk
into as many as possible partition and what the number of sectors per
cluster must be in each of such such partitions?

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