Floppy Drives, Optical Storage
November 2 (Day); November 1 (Night)
§ Introduction:
o The floppy drive has been a part of the personal computer since the start. It has moved from being 5 ¼ inch wide to its current width of 3 ½ inches (p. 179).
o Early PCs did not even have hard drives. All programs and data were read from and stored on floppy disks (p. 177).
o Most of the early PCs actually had two floppy drives. One was used to store the program. The other was used to store the data you were working on.
Old Computer with two Floppy Drives
§ Floppy Drive Connections:
o A floppy drive controller is a small hardware device that is used to control the flow of data to and from RAM. Remember that all data on your computer must first go to RAM before it can be processed.
o In olden times, the floppy drive controller was an expansion card that actually plugged into an expansion slot. Today, a generic floppy drive controller is installed on all motherboards.
o Data Cables:
§ The floppy drive connects to its controller (whether it be on the motherboard or controller card) by a 34-pin cable (p. 179).
· Most floppy drive cables have one connector. Some have two (p. 179).
· If you have two connectors, BIOS will recognize the floppy drive on the end of the cable as A: and the floppy drive installed on the second computer as B: (p. 179).
The Floppy Drive You Intend to be Drive A: Needs to Be on End
§ Floppy drives receive power from the power supply through a Mini connector (p. 160).
§ How Data is Physically Stored on a Floppy Drive:
o All floppy disks are coated with a metallic film capable of holding a magnetic charge. The magnetic coating on a newly made floppy disk has no charge until it is formatted, which happens before it leaves the factory (p. 177).
o Formatting is a process that places organizational information on the floppy disk so that it can store data in a useful way by placing magnetic charges on the disk (p. 177).
o More simply, formatting can be stated as the process of mapping tracks and sectors on the disk.
o Tracks are a series of circles. There are eighty tracks on the top and eighty tracks on the bottom of the disk.
o Each track is broken down into nine sectors. Each sector can be broken down into 512 bytes (p. 177).
Tracks and Sectors on a Floppy Disk
o Data is written to the disk as bits. Each bit is actually a small magnetized area on the disk with unmagnetized spots in between. These gaps prevent the magnetic field of one bit from corrupting the magnetism of a nearby bit.
§ Disk Density:
o The smaller the unmagnetized areas, the more closely bits may be placed to one another and the more bits can be fit on a disk. This increases the disk density.
o Double-density means that the tracks are packed together twice as closely as tracks on a single-density disk (p. 179).
o Disk density is increased by using higher quality disk surfaces. The higher quality disk surfaces are less sensitive to magnetic fields. This means that bits may be placed closer together without being corrupted by the magnetic field of a nearby bit.
§ Physical Connection of the Drive:
o Each floppy disk has two read/write heads: one for the top and one for the bottom of the disk.
§ The read/write heads move together. Therefore, the top read/write head and the bottom read/write head will always be positioned in the same place.
§ The read/write heads lightly touch the surface of the disk.
o Each floppy drive has a spring mechanism that pulls back the metal tab on the floppy disk, thus exposing the disk surface so that the read/write heads can make contact with the surface (p. 178).
o Each floppy drive has a motor-driven spindle that makes the disk spin (p. 178).
§ CD-ROM drives and DVD drives are known as optical media.
§ CD-ROM Drives:
o Here are some ways that the CD-ROM drive differs from the floppy drive:
§ Data can be written to a CD-ROM disc only once, because the surface of the disc is actually embedded with data.
§ Instead of storing data in a series of tracks, the surface of the CD-ROM disc is laid out in one continuous spiral patterns.
§ The surface of a CD-ROM stores data as pits and lands (p. 241):
· Pits are recessed areas, or holes, on the disc surface and represent a binary 0.
· Lands are raised areas on the disc surface and represent a binary 1.
Pits and Lands
§ Instead of using read/write heads, CD-ROM drives use a laser beam. The laser beam distinguishes between a pit and a land by the way that light is deflected when the beam hits the surface of the disc.
Link to Demonstration of How a CD Reads Data
at http://static.howstuffworks.com/flash/cd-read.swf
o The first CD-ROM drive transferred data at a rate of 150 KB/sec. This transfer rate was fine for audio CDs, but not good for data CDs. This speed of this first CD-ROM drive is known as 1X (p. 243).
§ The speeds listed for CD-ROMs today are simply multipliers of this first CD-ROM drive speed. A 20X CD-ROM, for example, is twenty times faster than the first CD-ROM drive.
§ CD-R and CD-RW Drives:
o CD-Recordable (CD-R) drives allow data to be written to a disc and be read by all regular CD-ROM drives (p. 244).
§ CD-R drives apply heat to special chemicals on the disc that cause areas on the disc that to reflect light differently than other areas. This process is known as burning (p. 244).
Back of CD-Recordable Disc; It’s Not Silver
o CD-Rewritable (CD-RW) drives allow data to be added to a disc and then changed later (p. 246).
§ CD-RW uses a special chemical that crystallizes when the laser beam strikes it and decrystallizes when higher heat is applied (p. 246).
§ DVD:
o A DVD is capable of storing massive amounts of data. It takes 7 CDs to store a full-length movie, but only 1 DVD.
o CD and DVD share many similarities. Both use tiny pits and lands on their surface to represent bits, which are then readable by a laser beam. They are both 5 in. wide and 1.2 mm thick.
o Here’s how CD and DVD drives differ:
§ The DVD drive uses a shorter wavelength laser, so it can read smaller, more densely packed bits, which increases the disc’s capacity.
§ DVD discs can have two layers of bits, instead of one.
§ A DVD disc can use both the top and bottom of the disc.