The storage system inside a PC, be it a diskette, a tape storage or a hard disk, consists of three logical parts. First part is the medium itself, that is the diskette, the cartridge tape or the hard disk. The second part is the mechanism that operates this medium—spins the disk or spools the tape. This is called the drive. The various kinds of drive are the floppy disk drive, the tape drive and the hard disk drive. The third part is the electronics, that controls the drive through commands given by the computer system. This is called the controller. In this section, you will learn about the floppy disk and hard disk systems.
Floppy Disk System
Floppy disks come in two sizes: 5¼ inch and 3½ inch. The 5¼ inch diskette is circular in shape with a hole in the center, called the clamping hole. It is covered with a square protective jacket. The jacket has an oval slot, called the head slot, which exposes a portion of the diskette surface. There is also a small hole near the center, called the index hole, which is the starting point of the diskette.
5¼ inch Floppy Disk
The diskette is inserted into the floppy disk drive, which is a sealed enclosure. It is clamped to the drive through the hub. The drive contains a spindle mechanism for rotating the diskette. It also contains another delicate piece of hardware called the read-write head that accesses the disk surface through the head slot and retrieves or stores data. The drive uses the index hole to sense the beginning of a track. The drive controller card supplies the drive with the necessary signals for its operations. For example, the controller tells the drive when to move the read-write head, when to start rotating the disk, and so on.
A 3½ inch diskette, also called as the microdisk, is also circular in shape and is covered with a square protective jacket. The media cover is pushed to expose the diskette surface. The operations are similar to those of 5¼ inch diskettes. A 3½ inch diskette is shown in Figure.
A 3½ Inch Diskette
A diskette is divided into a series of concentric circles called tracks. The tracks are identified by numbers, with the outermost track as track zero. The tracks do not spread across the whole width of the recording surface, but take up a small area. The track is divided into sectors. Each sector is identified by a number that is assigned to it, starting with zero. Each type of floppy has a sector of a fixed size. A disk has two sides. Data can be recorded on either one or both sides of the disk.
The storage capacity of a disk is determined by factors like the number of tracks, sectors and the number of sides on which data is recorded. Depending upon the capacity of storage, the 5¼ inch diskettes are divided into double-density (360 KB) and high-density (1.2 MB).
The microdisk may be double-density (720 KB) or high-density (1.44 MB). Accordingly, disk drives can be double-density or high-density drives. A high-density drive can read and write on to a double-density diskette but the reverse is not possible.
Hard Disk System
The hard disk is made up of a collection of disks known as platters. It is sealed in a vacuum container, and is located inside the system unit. When the tracks having the same track number on the different platters of a hard disk are joined together, they form a cylinder.
The read-write mechanism in a hard disk is slightly different from that of a floppy disk. Here, the read-write arm (containing twice as many read-write heads as there are platters, since each platter has two recording surfaces) is positioned over the recording surface and the disk is rotated. The read-write head then picks up the necessary data, which is transported to the RAM through the data bus. The hard disk controller performs functions similar to that of a floppy disk controller.
Display Adaptors
Various kinds of monitors are available—black and white or color—some displaying sharper images than others. Images are formed on the monitor's screen by a card called the display adapter card.
The key elements of a display adaptor are the video controller and the memory. As you know, the results of your interaction with the PC are displayed on the screen of the monitor. The information to be displayed on the monitor need to be stored somewhere so that it can be displayed. The memory on the adaptor card holds this data. The video controller is responsible for forming the image on the screen using this data. This memory is also accessible by the microprocessor so that changes can be made to the data when required. The time taken for reading the display memory and displaying a new picture on the screen is roughly one-fiftieth of a second. If a program wants new information to be displayed, it places the data in the display memory and it is reflected almost instantaneously on the screen.
Several kinds display adaptors exist in the market, each with its own special features. Some of the parameters that are used to distinguish between the various display adaptors are:
- Display in text mode
- Resolution in graphics mode
The PC can represent data on the screen in two modes. One is the text mode in which only text or individual characters can be displayed. The other is the graphics mode, where graphics (pictures) can be shown by constructing an image on the screen using dots.
In the text mode, the VDU screen is divided into specific character positions—usually 80 columns—of characters across the width of the screen and 25 lines of characters from top to bottom. Most, but not all, display adaptors support display of text on the screen.
In the graphics mode, the screen is treated as an array of tiny dots called pixels (short for picture elements), and anything that appears on the screen is shown using these dots. Each of these dots can be set on or off on the screen.
In the graphics mode, a parameter that can further differentiate between various display adaptors is resolution. Resolution of the screen indicates the number of dots or pixels on the screen. A typical high resolution display has 640 columns of dots across and 480 rows of dots down the screen. Besides, the display adaptors, that support graphics, have different resolutions. The higher the resolution (i.e. more the number of pixels), the better the quality of pictures displayed on the screen.
Yet another parameter used to differentiate between display adaptors is whether they support color or not. There are two types of monitors—monochrome (i.e. black and white) and color.
The monitor you have needs to be compatible with the adaptor card. It would be no use having a color monitor with a display adaptor that does not support color; or having an adaptor that supports high resolution with a low resolution monitor.
Since the resolution and the number of colors increase with increasing sophistication of display adaptors, the pixels require increasing amount of memory. Thus, the amount of memory on the display adaptor has to be increased.
Having considered the main parameters used to evaluate the display adaptors, you will now study some of the popular display adaptors supported by the PC.
=> Enhanced Graphics Adaptor adaptors (EGA)
Enhanced Graphics Adaptor (EGA)
EGA or Enhanced Graphics Adaptor is an effort to unify features of a variety of display adaptors. The EGA combines all the features of the above adaptors and provides higher resolutions. It supports upto 16 colors at a time. The EGA could have been a replacement for the other adaptors except for the fact that it is expensive and is not supported by specific software provided by some vendors. An EGA usually has a resolution of either 640 x 200 pixels or 640 x 350 pixels. In order to make the best use of an EGA, you need to use a high quality color monitor.
Enhanced Graphics Adaptor (EGA) Card
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