Today's Byte Size is all about exploring heads, tracks, cylinders and sectors.
[beeping and music]
[Typing SFX]
If I wanted to explain directions to my house, I might say something like travel to the furthest eastern regions of England
continue north on winding and desolate country roads and take a right by the lady with a shopping trolley full of old plastic bags
in order to follow these directions you'd need to understand what East, North, Right and so forth meant
although used to a much lesser extent nowadays the words cylinder, head. sector, and track, are much like directions concerning disk operations
data is stored on disks in bytes and traditionally those bytes are organized into 512 byte groups known as sectors
although modern hard drives use 4,096 byte sectors
but given since I like concentrating on nostalgia we will be concentrating on earlier forms of IBM PC-compatible hard disk
commonly used in the 1980's
which exposed the cylinder, head, sector method of addressing
to the operating system
allowing the OS to mount, format, and organise each disk accordingly
a sector is the minimum use of data you can read or write to a disk
not to be confused with blocks
a sector refers to the physical area on the disk
whereas a block denotes a small chunk of data
you may also hear the term cluster
this is a term found in DOS and simply refers to a sequential number of sectors
these sectors are arranged into tracks which themselves
can be arranged into groups known as cylinders
but before going into further detail
its easier to explain hard drive structure
with a simple example
in this case 5 1/4th inch 360k single density floppy disk
a floppy has two sides
which two heads or surfaces
to read them
each side contains data and each side contains forty
concentric tracks
which in the case of a floppy also equates to 40 cylinders
each of these tracks is seperated
like a pie into nine sectors called wedges
and each sector stores 512 bytes of data
so without sounding like we are delving into a "Three Little Bears" rhyme
lets move back to the harddrive
unlike a floppy is made up of rigid metal discs
and each disc is known as a platter
the original XT 10 megabyte drive
had two platters
but newer drives could hold many more
like a floppy each platter has an electromagnetic read-write head
for each side
and each platter is made up of tracks
although we are talking about several thousands instead of a 40 on a floppy disk
because hard drive heads move in unison
it makes sense to store related data across the same track on each platter
and this is where we get the term cylinder
it essentially refers to a stack
of tracks
and therefore if a platter had 300 tracks
the harddrive by definition will have 300 cylinders
so a harddrive with 10 platters may have the same number of cylinders
with five equivalent platters
but still twice the storage space
a floppy is divided into 40 tracks of 9 sectors each
equating to 700 total sectors per disk
and 360 Kilobytes of total storage
whereas our 10 Megabyte XT harddrive
has 305 cylinders
meaning 305 tracks on each of its surfaces
divided into 17 sectors
equaling 20,740 total sectors
and equating to 10,370 Kilobytes
of storage
and for the operating system then these divisions simply act as road signs
the OS refers to a table of contents stored at the start of the disc
when it requires a particular file
and this table directs the drive heads to the place where that file can be found
so much like a house address is broken down into more and more descending parts in specific order
16, the Drive, Birmingham, England
data on a drive
is organised in increasingly specific locations of cylinder, surface, and sector
of course having fixed areas on the disk mean wastage
outer tracks contain more surface area than inner tracks
and so zone bit recording was introduced which introduced different number of tracks per sector
an example of this is the CD-rom which rather than concentric tracks
data is stored on one continuous spiral
like the grooves of a record
in the case of a CD drive the problem solving this varying number of sectors
is solved by simply varying the speed of the drive motor
depending on which part of the disc is being read
logical block addressing allowed this method to be used in harddrives
first introduced with the SCSI standard
LBA exposed a linear addressing scheme to the operating system
which the drive firmware itself converted to cylinders
heads, and sectors
and thus allowed a spoke number of sectors
to be stored per track eliminating waste
so, although heads, tracks, cylinders, and sectors are a thing of the past
they are also still with us just tucked away
they are like an iPhone but a hard disk, yeah
you get the idea
[Harddrive ticking and music]