MAGNETIC DATA STORAGE AND
A. POOJA SHUKLA
M.Tech (NANO) Ist Yr
A data or information found in a particular address
A data can be randomly selected or easy to find in
the given array with the help of the given address
Types of data addressing,
• Basic Input Output System.
• Is a set of initial code to run your PC.
• Intel have designed their CPUs always to begin
execution of code at address FFFF0.
• The first instruction is a jump instruction to the main
part of BIOS code which could be "anywhere" else
• As well as testing and initializing the hardware in the
Power On Self Test (POST)
For the purpose of booting up the computer, the BIOS is
used for three main functions:
– Providing a set of assembler subroutines that can be
called by the operating system and whose function is
to access the hardware components of the computer
such as hard disk.
– Initiates Power On Self Test (POST).
– Allowing changes to the low level setup options.
– Initiating the boot sequence after a hardware Reset.
A set of standards called the ATA (AT-Attachment) came
into being in 1989 that effectively put a boundary on
what was possible in terms of cylinder, head and sector
• ATA-5 says (amongst other things) that:
– The cylinder number must be represented by 16
– The head number must be represented by 4 binary
– The sector number must be represented by 8 binary
• A total of 28 bits.
• CHS, Method for giving addresses to each physical
block of data on a hard disk drive
• It can be low level formatted to different capacities
• Identifying individual sectors on a disk by their
position in a track
• The track is determined by the head and cylinder
• For addressing the sector in smallest unit
Hard Disk Geometry
Heads on a Platter
A stack of Rings or "Tracks"
Where the data is physically on the disk?
• “disk controller” allows us to specify only the
Cylinder, Head and Sector that we want to access
• The disk controller calculates where the data is
physically on the disk and hands the data back to
• More accurately CHS is a pointer in memory to
How the physical disk is accessed
If we wanted to access one particular sector, we could
reference it by specifying
which head it was on,
which cylinder it was on and finally
which sector it was on.
That would then uniquely identify the sector that we
wanted to access.
• 16-bit Cylinder value (C)
• 4-bit Head Value (H)
• 8-bit Sector Value (S)
• Old BIOS:
– 10-bit C
– 8-bit H
– 6-bit S
– Limited to 528MB disk
Used for specifying the location of blocks of data stored
Generally in secondary storage system eg, hard disks
It is an simple linear addressing scheme
Blocks are arranged by an integer index
example : LBA 0, LBA 1
The five most important things to note about LBA are:
• LBA is a translation method based on the CHS
• LBA was not (as many web sites claim) invented to
overcome any disk size limits
• It is simply a different way of accessing the hard disk
• LBA does not require the BIOS to know anything about
the disk geometry to access the disk
• LBA is used to address data
• It is also called as large mode CHS
• It uses BIOS translation method
• It is inherited by CHS mode to be in given size barrier
• The BIOS coders agreed on a way round the 504 MB
limitation. They came up with a system called “Enhanced
CHS”, or ”ECHS”.
• ECHS is a simple translation method written on the end of
the interrupt INT13h.
In summary, this is what happens:
O/S --> INT13h --> ECHS Translation --> CHS call --> Disk
• Consequently, the requirements for this type of disk
access are as follows:
An operating system that can make INT13h calls.
A BIOS that understands INT13h calls.
A BIOS that can perform the ECHS translations.
A BIOS that can make CHS calls.
A disk controller that understands the (translated)
CHS values referenced.