Basic - short description ..
When created the disk surface is blank
A Low level format process writes 'sector' defining entries onto the blank
media
Each head position on the drive is known as a Cylinder
( on the assumption that there are many platters ( physical discs ) spinning
A 'Cylinder' is composed of the 'Track' around each of the platters at a
single head position
On the track the data can be written as 1 continuous block of data
Or it can be written as a series of blocks with 'gaps' between them
Generally, a low level format will write blocks with gaps, making the blocks
big enough to hold 512 bytes of data
plus a header identifying the block ( number in the format Cylinder (or head
position) , track ( or head number) , sector on the track
and a 'CRC' hash-check and parity type recovery value for that block
(That's how MS get/got more than 1.44Mb on a 3½" floppy disk
They made each data block use the entire track, so didn't have to have
multiple id's, CRC's or gaps on a track)
Now - splitting up the block id into 'fields' limits the number of
cylinders, heads/tracks and sectors
If you just treat the field as a single number then you don't hit those
limitations -
That mode of identifying the blocks is known as LBA
Right - so each block has an id, and the physical drive access facility
knows the size of the drive
in real Cylinders, Heads and bytes that can be written to a track
Your program asks for block 4 of a file,
The OS knows the LBA of the file portions (It gets that from the Partition
ID, the Directory, and the Space Allocation table for that partition
The read me LBA____ request is passed to the drive management electronics
and that divides the LBA by the appropriate values to get the block to read,
Reads that block,
checks the CRC and other markers - one of which is the 'This is a bad
sector/track' marker
If the marker is set it takes the data it managed to read, and interprets
that as a new (substitute) LBA to go to for the data
---------
Now hidden Cylinders -
Either the tracks can be marked as bad - with NO alternate/substitute LBA
so the drive electronics will not normally use that part of the drive
( that can be over-ridden by commands similar to those used in the low-level
format)
Or - the entries that make up the drive's partition can be changed so that
they do not include parts of the drive
e.g. indicate the drive is only 120Gb when it is actually 140Gb
or - remembering that each partition has at it's start the part of the table
that describes it, and the location of the next partition/table
A partitions entries can simply be changed to point to the partition after
the next one, rather than the next one
( A whole partition hidden)
The tables can also be altered to point to an area within the partition
where an image of a partition has been created
-
Yes the system can be 'adjusted' to hide data, simulate extra partitions -
CD drives etc,
and if you adjust the entries you can stop the whole 'tweaked' setup from
being acceptable to the OS and/or it's initiator
JimB
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