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Understanding Disk Drive Terminology, Part Two

Copyright©2001,2004 by Daniel B. Sedory
NOT to be reproduced in any form without Permission of the Author!

Disk Terminology, Page One.


Disk Parameters
for drives over 8.4 GB and having a
BIOS with Extended INT 13 Functions

Disk Drives over 8.4 GB should reply to most direct access inquiries (using ATA commands and coded I/O instructions) as having a drive geometry of:  16,383 Cylinders, 16 Heads and 63 SPT. (NOTE: This is not the same as the "geometry" used by computer BIOS, most utility programs or the Partition Table itself to refer to drives over 8.4GB; see below!)

This should be true for all disk sizes over 8.4 Gigabytes!  To find their true capacities, you can only use the Total number of Sectors (or LBA number). As a matter of fact, the 20 GB drive we'll be examining lists exactly these three values (16,383, 16 and 63  as its own Cylinders, Heads and SPT). I have no idea why Maxtor® did this, since this tells you almost nothing about the actual capacity of the drive!

Western Digital®, however, stopped using the terms Cylinder, Head and Sector on their drive cases a long time ago, switching to just a single LBA ( Logical Block Addressing ) number; for example, one of their 40 GB drives is labeled: “LBA 78165360” (which means there's a total of 78,165,360 512-byte sectors available on this drive — for a capacity of: 40,020,664,320 bytes).  That's very useful information compared to the Maxtor® drive!

In order to confirm our test drive's size, we had to search the Maxtor® web site for the exact model number and found that the drive's Number of Cylinders was: 39,704. But, that's based upon only 16 Heads and 63 SPT, so, after multiplying them together, that finally gave us an LBA of: 40,021,632 sectors. Hopefully, Maxtor (and any drive manufacturers like them) will begin using this single LBA number in the future!

The Useless "Geometry" Values: Programs which use only the CHS method for calculating partition sizes (including the CHS entries in the Partition Table of any MBR or EBRs (Extended Boot Recrods) should now list all Starting or Ending Locations beyond about 8.4 GB as: 1024 Cylinders, 255 Heads and 63 Sectors (this means the values shown in the Partition Table itself will be: 1023, 254 and 63, since the Cylinder and Head values begin counting from zero). The point here is that you can no longer compute the capacity of a drive's partitions from its CHS values if any portion of the partition extends beyond 8.4 GB; you can only use the Total Number of Sectors to obtain its size in such cases. [See the example Partition Table below.]

NOTE: Various utility programs (such as PowerQuest's PartInNT for Windows) will display values beyond the standard 1024 cylinders!
( You need to note that this is not what's really inside of your Partition Table, since it's physically impossible for a 16-byte Partition Table entry to hold a number larger than 1023 for its cylinder value! There are only 10 bits available for the Cylinder value, and 11 1111 1111 in binary = 3FF in hex = 1023. )  If a utility program explains that it's only displaying an interpretation of the data found in the Partition Table, then I'd have no complaints about it. Unfortunately, many utility programs, including all of PowerQuest's utilities and TestDisk, assume you know the difference; so be aware of this.

However, if you ever see such values being displayed in a Disk Editor (as I recall seeing at least once), that could totally confuse you!   It's my opinion that a Disk editor should always show exactly what's inside of a sector (even when it's interpreting the CHS values for you), so you know for sure what data is on that disk!

PQ Utility Users, Note: Various versions of PowerQuest's Partition Magic always complain about incorrect CHS values for partitions that end or begin after 1024 cylinders! This is very confusing to users of their software! Though we're not exactly sure why PQ decided to do this (since their program can easily convert the data in the Partition Table into a usable cylinder value), it may be that they expected BIOS manufacturers to already be making chips that would do this; and Partition Magic and the BIOS chip should agree on the same cylinder value before proceeding! (If you find an answer from PQ, let me know.)

After obtaining the data from many different computers with various BIOS chips in them, sometimes we've seen a cylinder count of only 1023 instead of 1024 for entries ending or beginning beyond 8.4GB; I'm not quite sure why that should occur, but thought you should be aware that some variations still exist in the real world! As a matter of fact, it has recently been found that some new Linux distribution installation programs under certain settings will alter the Head count in a Partition Table to only 16; completely messing up the "geometry" of a drive for programs that depend upon those values!

Now we'll attempt to examine and account for every sector of a more recent hard drive that has a capacity of about 20 GB and was partitioned into a number of file systems for three different OSs!

(The yellow highlighting and Linux labels are not in the original view.)

Partition Table Data for the Whole Disk Drive


Sorry, but this page is still under construction!

Last Update: May 4, 2005. (04.05.2005)

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