More About Disk Drives

Floppies – Although floppy drives are being phased out in some new computers, there are still millions of them out there and you should know something about them. The floppy drive has a little slot on the face of the computer cabinet, and into this slot you can slide a floppy diskette like the one shown here. One of the reasons floppy drives are still around is that it is very easy to take a floppy diskette from one system to another.
Inside the floppy diskette is a round flat disk coated with iron oxide on each side so that data can be stored on it magnetically. This disk is called a platter, and it spins underneath an electro-magnet called the write head that puts data onto the platter surface. There is another head called the read head that copies data from the platter.
Once the disk has made one complete revolution, data is written all the way around. That is called a track. The head then moves a bit and writes another circle of data to create a second track. Altogether, there are 80 tracks on each side, for a total of 160. Altogether, the floppy can hold 1.44 MB (megabytes) of data.
If we are looking for just a few bytes out of 1.44 million, it’s not enough to know which track it is in. To help narrow the search, the track is divided into 18 pieces, calledsectors, which look much like a slice of pie. Each sector holds 512 bytes of data, so if we know the track and sector number of the data we want it won’t be hard to find.
Hard Drives – On a hard drive, data is also organized into tracks and sectors. While each sector still holds 512 bytes, there can be many more tracks and sectors on a platter. There are also multiple platters, one on top of the other like a stack of pancakes. Hard drives can hold much more data than floppies, sometimes into the billions of bytes, calledgigabytes(GB).
Multiple platters require multiple read and write heads, all attached to the same arm so they move together. It’s called an actuator arm. When we are reading track number 10 on the top platter, the other heads are also positioned over track 10 of the other platters, and together all of these track 10s make up a cylinder. To specify the location of data on a hard drive it is necessary to say what cylinder, then the track and sector. Moving the heads from one cylinder to another is called a seek, and the amount of time this takes is the average seek time.
Although hard drives can hold much more data than floppies, the platters are sealed into a metal case that is fastened inside the computer cabinet, so it’s not an easy matter to move from one system to another like you can with floppies. A hard drive is sometimes called a fixed diskfor this reason.
Operating systems use a couple of different methods to keep track of what data is stored where on a drive. One common method uses a table called a File Allocation Tableor FAT, which is a section of the disk with pointers to data locations. There are two versions, calledFAT16 and FAT32. Windows NT, XP and 2000 use a similar method called NTFS.
There are two different interfaces commonly by hard drives to talk to the rest of the system. These are called IDE for Integrated Drive Electronics, and SCSI forSmallComputer System Interconnect. The technical differences are not important at this point, but you should know about the two types because they are not interchangeable.
Figuring out where the heads should go next and then moving them there is the job of some electronic circuitry called the disk controller. Every disk drive has its own controller, which may be on the motherboard or inside the drive itself, depending on the type of drive.
There are a few more things you should know about disk drives before we leave the subject. The first sector of Cylinder 0, Track 0 is called the boot sector, and it contains aMaster Boot Record (MBR) that shows whether the disk contains an operating system and the location of the code. If there is more than one operating system, the drive must be divided into multiple partitions. If not, then the whole drive will be a single partition. All of the disk space assigned to a partition is called a volume.
Another term you will encounter is a disk format. There is a high-level format, which creates a new file allocation table and is done with a FORMAT command. There is also alow-level format that creates a new pattern of sectors. A low-level format must be followed by an FDISK command to create a new Master Boot Record and partitions.
Last, we have the word media. This refers to the actual surface holding the data, which is the platter in the case of a disk drive. Because the floppy platter can be taken out of the drive, it is called removable media, while a hard drive is calledfixed media.
Other Drives – Most systems today, especially home systems, have additional storage drives that use CD or DVD discs. The technology for both is similar but DVDs hold much more data. These drives do not store data magnetically but use optical markings that are read with a laser. They are mostly used just to read data and not to write it. The full name for CD in fact is CD-ROM, which stands forCompact Disc - Read Only Memory. However, there are versions that can be used to write also, and these are called CD-RW and DVD-RW. Even so they are mostly used to write just once for permanent storage, and are not practical for constantly changing data.
Like hard drives, CD-ROM drives can use either an IDE or SCSI interface. The version of IDE for CD-ROM drives is called ATAPI, and for SCSI the CD-ROM version is ASPI.
Because the discs can be removed, CD-ROM and DVD are considered removable media. There are other types of removable media also that are not as common, such as tape drives and Zip disks, which are similar to floppies but with a storage capacity of 100 or 250 MB. Zip disks and tape drives also use the ATAPI interface.