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Computer Memory and its Classification
Whenever a program executes inside a computer then at first the program has to be loaded on to the computer’s primary memory. And also when a does not execute then also the program must remain loaded in computers secondary memory. This memory unit works as an extra block. In a program in memory, the entire set of performed operations is written using the keyboard as a sequence of instructions. This collection of instruction is called a program. Also, the data on which the program is to operate is entered with the help of the keyboard and can be stored in memory. Memory can also store intermediate and final results. This design of a stored program computer where program, data, and result reside in-memory is because of Von Neumann.
In a broad sense, memory can be classified as –
Main Memory– Main memory has the disadvantage of high-cost and low-capacity storage. But its advantage or facility is the high speed of data transfer. The control unit can be able to directly communicate with the main memory. Main memory can be generally classified into random-access memory (RAM) and read-only memory (ROM). It is a volatile memory. Due to the absence of power, the content of this memory will be lost.
Secondary Memory– Secondary memory is also frequently known as auxiliary memory. The control unit can’t directly communicate with the secondary memory. It is a non-volatile memory.
Random Access Memory (RAM) – RAMcontains of a various number of memory locations wherein each location typically 8-bits are stored. It can be possible to read from a RAM location, as well as write to a RAM location. The drawback ofRAM is that it is volatile. That means, when the power supply to the RAM is switched off, the information in the RAM will be lost.
From the memory, data can be accessed in two different ways – SequentialAccess and Random Access.
Sequential Access − Sequential access and random access are two types of accessing information. In sequential access, it is mandatory to access information strictly in order. If there are 4000 memory locations, it has to be accessed in the order of 1, 2, 3,…,4000. Thus, it takes minimum time to access information from location 0 and at most time to access information from location 4000. Magnetic Tape is an example that employs sequential access.
Random Access − In a random access technique, it can be possible to access a memory location in any order. For example, one can read from the 4000locations in the order of 1500, 1210, 3060, 1640, 1352, and so on. Second, it takes a similar time to read from a memory location irrespective of its position. In a RAM, the access method is random, and its name, in fact, is derived based on the method of access.
Read Only Memory (ROM) – ROM consists of a number of memory locations wherein each location typically 8-bits are stored like a RAM. A ROM also uses random access method just like a RAM. The advantage of ROM is that it is non-volatile in nature. That means, though the power supply to theROM is switched off, the information in the ROM will not be lost. However, only reading operation is possible from a ROM location. Thus, in a computer, ROM is used for storing information which is not lost when power is switched off.
Let us discuss different versions of ROM which are provided by semiconductor manufacturers for storing information –
Mask-Programmed ROM – It derives this name because the information is written to this type of ROM at the time of manufacture by applying a suitable mask. Once the manufacturer writes this type of ROM, it cannot be possible to change this information even by the manufacturer. So that the information entered is permanent. It is cheap with compared to the other types of ROMs when the cost per unit quantity is considered. But the user has to place an order for a huge number at a time. Thus, ROMs are used in equipment that is produced in huge quantities. To give an analogy, mask-programmed ROMs are compared to books that are printed in huge volumes.
Programmable Read Only Memory(PROM) – The user writes information to this type of ROM with the help of PROM programmer equipment. Once the user writes this type of ROM, it cannot be possible to change this information anymore. Like Mask-Programmed ROM, the information entered is permanent. Though itis cheaper than the other types of ROMs but it is costlier than a mask ROM. The user can buy even a single piece of PROM from a local shop. Thus, PROMs are implemented in equipment those are produced in small quantities.
Erasable Programmable Read Only Memory (EPROM) – Information is written to this type of ROM by the user with the help of EPROM programmer equipment. As its content is erasable and rewritable, so the user can change this information in a number of times. Thus, although the information entered is not lost when power is switched off, it can be possible to erase it and then write new information. With the exposure of strong ultraviolet (UV) light from a very close distance on the quartz window of the EPROM chip, we can erase the content of the EPROM. Then the contents of the entire EPROM are entirely lost. The user can purchase even a single piece of EPROM from the market read and write its content for several numbers of times as well.
Electrically erasable (or Alterable) Programmable Read Only Memory (EEPROM or EEPROM) − Information is has written to this type of ROM by the user with the help of EPROM programmer equipment. After the user writes this type of ROM, it can be possible to change this information a number of times. Thus although the information entered is not lost at the time of power is switched off, it can be possible to erase it and then write new information. Unlike EPROM where erasing of data is to be done using ultra Violet rays, but in the case of EEPROM, erasing of its content is done with the help of electrical signals. Also, the contents of the EEPROM can be erased selectively. It is costlier than the other types of ROMs and is gaining in popularity. The user can purchase even a single piece of EEPROM or EAPROM from the market and read and write its content for several numbers of times as well.
| Types of ROM | Info.entered by | Possible to change info? | Cost per chip | Min.order quantity | Method of erasing info. | Application area |
|---|---|---|---|---|---|---|
| Mask-programmed ROM | Manufacturer | No | Cheapest | Large | Not possible | Large-scale production |
| PROM | User | No | Costlier | One | Not possible | Medium-scale production |
| EPROM | User | Yes | Even more costly | One | Using strong UV source | R& D |
| EEPROM | User | Yes | Costliest | One | Using electrical signals | R& D |
Secondary memory − Whenever there is a need to have the huge amount of information that is possible to store in the main memory. In this case, it is stored in the secondary memory. This type of memory is characterized with the help of virtues of large capacity for storage and low cost per bit of storage. But its drawback is its very low speed for access. The control unit is not capable to directly access secondary storage. Hard disk, floppy disk, and magnetic tape are good examples of secondary memory.
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