CSci 1000 Secondary Storage Chapter - Bateman

Secondary storage is any place other than RAM or ROM where programs or data 
are stored.  These are places where storage can be long term unlike RAM 
which is constantly changing.  Generally, these devices give slower access
than RAM or ROM.  They are larger in capacity and cheaper per byte.
They are measured in units of 
KILOBYTES, MEGABYTES, GIGABYTES, or TERABYTES (trillions of bytes).

Magnetic Disk Secondary Storage
General Magnetic Disk Concepts
These disks store a 0 or 1 by the way a spot is magnetized on the surface;
there are two magnetic polarities possible at each of these tiny spots.
Examples are hard disks and diskettes.

Disk data can be destroyed by a magnet.  Also, small particles of food or even
smoke can be destructive.  

Each disk has to be formatted.  This process subdivides the surfaces of the 
disks into pie-shaped SECTORS and circular TRACKS.  The area contained in one
sector and track is called a BLOCK (called "track-sectors" in your text).
is the smallest possible area that can be read or written on the disk.  Some
machines use a small number of blocks, called a CLUSTER, as the unit for
reading or writing on the disk.  If a disk has several platters, then the
collection of all tracks with the same number is called a cylinder.  For 
example, the collection of all tracks numbered 10 would be called cylinder #10.
A disk can be formatted in various ways.  A given computer may or may not be
able to read a disk that has a certain format.  

A block is located on a disk by its PHYSICAL ADDRESS, which consists of three
numbers: the sector number, the track number, and the surface number. 

Each file is a group of blocks (clusters) strung together; each block, except
the last, contains the address of the next block. The formatting process
creates a special table on the disk that will contain the names of the files,
their sizes, and their first-block (first-cluster) location. 

The disk drive consists of a motor that turns the disk constantly, access
arms, and read/write heads.  In order to read a block, the access arm must
position the read/write head over the appropriate track.  Then when the
block comes around and lies under the track, it can be read or written.
The time it takes to position the head is called SEEK TIME.  The time it
takes for it to rotate is the ROTATIONAL DELAY.  The time it takes to 
actually transfer the block (or cluster) of data is called the DATA TRANSFER
RATE.   ACCESS TIME refers to the combination of these three times:  seek
time, rotational delay, and data transfer rate.

Hard Disks
Hard disks are higher capacity disks made of metal, ceramic, or glass.
They are called "hard" because of the hard material from which they are made.
There may be one or more such hard platters in a hard disk.  Each platter is
disk-shaped.  

In today's PCs (1997) capacities of 1 GB to 3 GB are common for a hard disk.
A larger computer, such as a mainframe, may have numerous disk drives.

Hard disk drives rotate at about 3600 to 7200 RPM (revolutions per minute).
They have access times around 15 milliseconds.

Diskettes
These consist of a single platter with two usable surfaces.  The density
of the bits determines the capacity.
	DOUBLE-density (DD)	- 720KB
	HIGH-density(HD)	- 1.44MB
	EXTENDED-density (ED)	- 2.88MB

Diskettes can be damaged by heat or sunlight and by extreme cold.

Diskettes rotate at about 360 RPM whenever you are reading or writing data.
Access times for diskettes are from 175 ms to 300 ms.

ZIP & JAZ Drives
These are drives that may be attached to the printer port, or may be a
permanent part of the machine.  ZIP drives use 100MB disks.  JAZ drives
use 1GB disks.

Disk Cache
A part of RAM can be used as a disk cache.  The data most often read is kept
in the cache, so that if it needs to be read it can be read quickly from the
cache rather than slowly from the disk.

Data Compression on a Disk
A file on a disk can usually be compressed into fewer bytes by taking advantage
of redundancy and repetition in the original bytes.  In order to use the data
that has been compressed it is necessary to decompress it.

LOSSLESS Compression allows reductions to about half the original size.  The
original bytes can be restored by decompression.  This is especially important
for high quality musical recordings, art, or text (written verbal data).

LOSSY Compression allows reductions sometimes to 1/200th of the original size.
The original bytes can only approximately be restored by decompression.  This
is good enough for some speech or music recording of low quality where a
little distortion, like a little static, does not make the result useless.

CONTIGUOUS FILES & FRAGMENTED FILES
Sometimes all the data of a file resides in a set of blocks that are CONTIGUOUS
(like the 48 contiguous states, they are connected together).  Contiguous files
are more quickly accessed.  If a file is not contiguous, they it consists of
two or more disconnected pieces called fragments and it is said to be
FRAGMENTED.  Special programs can DEFRAGMENT a disk in order to make the
usage of the disk take less time.

Magnetic Tapes
Thin plastic tapes coated with iron oxides are used for secondary storage.
Each 0 or 1 is stored as a positive or negative polarity on a tiny spot on
the tape.  A file is stored on a tape as a series of bytes.  Each byte is
either 8 bytes straight ACROSS the tape or in a diagonal direction across.
CARTRIDGE TAPE is a tape inside a plastic cartridge.  Backup tape drives often
use this kind.

REEL-TO-REEL TAPE requires a second reel (the takeup reel) permanently
mounted on the tape drive.  Data and programs for a mainframe are often
backed up on reels.  In the past, tape reels were used to distribute 
data and programs through the mail.  To some extent this is being replaced
by distribution on compact disks.

Optical Disks
These are high-capacity secondary storage devices.

A strong laser can be used to burn a microscopic hole on the surface of a hard
plastic disk.  The hole (called a "pit") or its absence (called a "land")
is a way to represent a 0 or 1.  A weaker laser can then be used in the reading
of the disk.  This technology is used in LASER DISKS (around 7 GB) and in
CD-ROMs (around 600 MB).  A single speed CD-ROM has a transfer rate of
300 kilobytes per second.  Quad speed, 8x, and 16x speed are now available;
these have transfer rates that are 4, 8, and 16 times the rate of a single
speed.  Most optical disks are read-only.

WORM devices (Write-Once-Read-Many) are blank optical disks that can be recorded
on one time only.  

MAGNETO-OPTICAL drives combine magnetic and laser technologies to provide
high-capacity erasable disks.

Other secondary storage devices
Solid State	- this is RAM used for secondary storage.
Mass Storage	- this is like a jukebox with many disks or tapes.
PC cards	- used in some digital cameras and some portable PCs.
Memory buttons 	- uses a chip in a small cannister.
Optical card	- a credit card like device