technical info on hard drives-1

[abeetha.m]

Some
Technical Info on Hard Drives

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Filename:
HDTECH.TXT


WWW URL:
http://www.firmware.com/support/bios/hdtech.htm


FTP URL: ftp://ftp.firmware.com/text/hdtech.txt


Author: Terry Slade, Micro Firmware Technical
Support


Revision: 07/27/98


Summary: Discusses some technical aspects of hard
drives such as CHS


addressing scheme, boot sectors,
partition tables, FAT, etc.


This file is not really in a finished state, but hopefully
is still of some use. TLS

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Hard drives are the primary storage devices used in most
computers. Hard drives are also called hard disks, hard disk drives, fixed
disks, fixed disk drives, and (very rarely) Winchester drives. Programs and data are stored on the drive in the
form of binary 1's and 0's, represented by magnetic flux patterns. This is
accomplished by read/write heads using electromagnetism, the same basic type of
technology used in audio tape and video tape (like in cassette tape decks and
VCRs). The media that data is stored on consists of one or more rigid platters
coated with magnetic material. The platters and the head assembly are in a
sealed compartment called the HDA (Head Disk Assembly). The platters spin at a
rate of 3600RPM on older IDE drives up to 5400RPM and faster on newer drives.
Modern IDE drives also include the controlling electronics.


Note that everything in this file applies to hard drives as
used in PCs. Things may be completely different in the worlds of Macintoshes,
or RISC systems, or mainframes or other types of computers. One thing that
defines a PC is the standards that the ROM BIOS is written to. Another factor
defining a PC is the CPU. PCs use x86 CPUs - this is the family of CPUs
including Intel (and compatible) 8086, 286, 386, 486, and Pentium CPUs.

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Cylinders, Heads, and Sectors


Data is written to and read from the drive in units called
sectors. A sector is 512 bytes. Sectors are organized on a drive in a matrix of
cylinders, heads, and sectors. The cylinder/head/sector (CHS) values used by a
drive are also referred to as the drive's geometry. At one time this addressing
system corresponded directly to the actual physical makeup of the drive. This
is no longer true, but the same system is still used, so it is useful to
understand the theory behind it.


A hard drive consists of several round platters stacked on
top of each other with tracks defined as concentric circles on both sides of
each platter. The read/write heads are part of an assembly that moves all of
the heads together, with one head for each side of each platter. In the cylinder/head/sector
system, cylinder refers to the set of tracks (concentric circles) that line up
on top of each other, head refers to a side of a platter, and a sector is the
portion of a track that holds 512 bytes of data.


To illustrate this more clearly we can draw a picture of a
hypothetical drive with CHS values of 3 x 2 x 8. Draw a circle and draw two
smaller concentric circles inside it. Then draw four lines to divide the
picture into eight sections just like slicing a pie. Then draw another picture just
like it to represent the other side of the platter. The three circles are
tracks. Each track is divided into eight sectors. The first picture represents
one head (or platter), the other picture is the other head. The outer track on
each picture is one cylinder, the middle tracks are another cylinder, and the
inner tracks are another cylinder. Cylinders and heads are numbered beginning
with 0, sectors are numbered beginning with 1. So on this imaginary tiny drive
we have 3 cylinders (numbered 0, 1, and 2), 2 heads (numbered 0 and 1), and 8
sectors (numbered 1 - 8).


We can apply this scheme to an older MFM drive with CHS
values of 306 x 4 x 17. If we multiply the CHS values together, we get the
number of sectors. If we multiply this number by 512 (the number of bytes in a
sector), we see that this is a 10MB drive. This drive would have 2 platters (4
heads) with 306 tracks on each one and each track would be divided into 17
sectors.


Although newer drives still have to use the CHS system,
since BIOSes and operating systems require it, these values no longer
correspond to the physical makeup of the drive. Modern drives may have only one
or two platters but will usually report that they have 15 or 16 heads. Also,
modern drives will have different numbers of sectors on each track, since the
outer tracks can hold more. The drive will internally and automatically handle
the translation from the CHS values to the actual layout of sectors on its
platters.


When sectors are numbered sequentially, they are ordered in
a particular way. First count all sectors on the first cylinder and head, then
count the sectors on the next head until all heads in the cylinder are counted,
then move to the next cylinder and start over at the first head and sector.
This would normally be the order in which an operating system would write files
to a disk, since there is less head movement required if all sectors in a
cylinder are used before moving to the next cylinder. Also, the first cylinder
is normally at the outer edge of the platters and the first head is normally
the one on top.

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