CLASSIFICATIONS OF INTERNAL STORAGE
Up to this point, you have learned some of the general functions of the cpu, the physical characteristics of memory, and how data is stored in the internal storage section. Now, we will explain yet another way to classify internal (primary or main) storage. This is by the different kinds of memories used within the cpu: read-only memory, random-access memory, programmable read-only memory, and erasable programmable read-only memory.
READ-ONLY MEMORY (ROM)
In most computers, it is useful to have often used instructions, such as those used to bootstrap (initial system load) the computer or other specialized programs, permanently stored inside the computer. Memory that enables us to do this without the programs and data being lost (even when the computer is powered down) is called read-only memory. Only the computer manufacturer can provide these programs in ROM and once done, they cannot be changed. Consequently, you cannot put any of your own data or programs in ROM. Many complex functions such as routines to extract square roots, translators for programming languages, and operating systems can be placed in ROM memory. Since these instructions are hard wired (permanent), they can be performed quickly and accurately. Another advantage of ROM is that your computer facility can order programs tailored for its needs and have them permanently installed in ROM by the manufacturer. Such programs are called microprograms or firmware.
RANDOM-ACCESS MEMORY (RAM)
Another kind of memory used inside computers is called random-access memory (RAM) or read/write memory. RAM memory is rather like a blackboard on which you can scribble down notes, read them, and rub them out when you are finished with them. In the computer, RAM is the working memory. Data can be read (retrieved) from or written (stored) into RAM just by giving the computer the address of the location where the data is stored or is to be stored. When the data is no longer needed, you can simply write over it. This allows you to use the storage again for something else. Core, semiconductor, and bubble storage all have random access capabilities.
PROGRAMMABLE READ-ONLY MEMORY (PROM)
An alternative to ROM is programmable read only memory (PROM) that can be purchased already programmed by the manufacturer or in a blank state. By using a blank PROM, you can enter any program into the memory. However, once the PROM has been written into, it can never be altered or changed. Thus you have the advantage of ROM with the additional flexibility to program the memory to meet a unique need. The main disadvantage of PROM is that if a mistake is made and entered into PROM, it cannot be corrected or erased. Also, a special device is needed to "burn" the program into PROM.
ERASABLE PROGRAMMABLE READ-ONLY MEMORY (EPROM)
The erasable programmable read-only memory (EPROM) was developed to overcome the drawback of PROM. EPROMs can also be purchased blank from the manufacturer and programmed locally at your command/activity. Again, this requires special equipment. The big difference with EPROM is that it can be erased if and when the need arises. Data and programs can be retrieved over and over again without destroying the contents of the EPROM. They will stay there quite safely until you want to reprogram it by first erasing the EPROM with a burst of ultra-violet light. This is to your advantage, because if a mistake is made while programming the EPROM, it is not considered fatal. The EPROM can be erased and corrected. Also, it allows you the flexibility to change programs to include improvements or modifications in the future.
Q.17 In what type of memory are often used instructions and programs permanently stored
inside the computer?
The last kind of memory we will briefly introduce here is called secondary storage or auxiliary storage. This is memory outside the main body of the computer (cpu) where we store programs and data for future use. When the computer is ready to use these programs and data, they are read into internal storage. Secondary (auxiliary) storage media extends the storage capabilities of the computer system. We need it for two reasons. First, because the computer's internal storage is limited in size, it cannot always hold all the data we need. Second, in secondary storage, data and programs do not disappear when power is turned off. Secondary storage is nonvolatile. This means information is lost only if you, the user, intentionally erase it. The three types of secondary storage we most commonly use are magnetic disk, tape, and drum.
The popularity of disk storage devices is largely because of their direct-access capabilities. Most every system (micro, mini, and mainframe) will have disk capability. Magnetic disks resemble phonograph records (round platters), coated with a magnetizable recording material (iron oxide), but their similarities end there. Magnetic disks come in many different sizes and storage capacities. They range from 3 inches to 4 feet in diameter and can store from 2.5 million to 600 million characters (bytes) of data.
They can be portable in that they are removable, or they can be permanently mounted in the storage devices called disk drive units or disk drives. They can be made of rigid metal (hard disks) or flexible plastic (floppy disks or diskettes) as shown in figure 2-6.
Figure 2-6. - Various types and sizes of magnetic disk storage.
Music is stored on a phonograph record in a continuous groove that spirals into the center of the record. But there are no grooves on a magnetic disk. Instead, data is stored on all disks in a number of invisible concentric circles called tracks. Each track has a designated number beginning with track 000 at the outer edge of the disk. The numbering continues sequentially toward the center to track 199, 800, or whatever the highest track number is. No track ever touches another (fig. 2-7). The number of tracks can vary from 35 to 77 on a floppy disk surface and from 200 to over 800 on hard disk surfaces.
Figure 2-7. - Location of tracks on the disk's recording surface.
Data is written as tiny magnetic bits (or spots) on the disk surface. Eight-bit codes are generally used to represent data. Each code represents a different number, letter, or special character. In chapter 4, you'll learn how the codes are formed. When data is read from the disk, the data on the disk remains unchanged. When data is written on the disk, it replaces any data previously stored on the same area of the disk.
Characters are stored on a single track as strings of magnetized bits (0's and 1's) as shown in figure 2-8. The 1 bits indicate magnetized spots or ON bits. The 0 bits represent unmagnetized portions of the track or OFF bits. Although the tracks get smaller as they get closer to the center of the disk platter, each track can hold the same amount of data because the data density is greater on tracks near the center.
Figure 2-8. - A string of bits written to disk on a single track.
A track can hold one or more records. A record is a set of related data treated as a unit. The records on a track are separated by gaps in which no data is recorded, and each of the records is preceded by a disk address. This address indicates the unique position of the record on the track and is used to directly access the record. Figure 2-9 shows a track on which five records have been recorded. Because of the gaps and addresses, the amount of data we can store on a track is reduced as the number of records per track is increased. Records on disk can be blocked (grouped together). Only one disk address is needed per block, and as a result, fewer gaps occur. We can use the blocking technique to increase the amount of data we can store on one track.
Figure 2-9. - Data records as they are written to disk on a single track.