Memory Unit : A memory unit is a collection of storage cells together with associated circuits needed to transfer information in and out of storage. The memory stores binary information in groups of bits called words. A word in memory is an entity of bits that move in and out of storage as a unit. A memory word is a group of 1’s and 0’s and may represent a number, an instruction code, one or more alphanumeric characters, or any other binary-coded information. There are two types of memories are used in computer systems: 1) Random-Access-Memory(RAM) 2) Read-Only-Memory
Types of ROM : The required path in a ROM may be programmed in three different ways. The first, mask programming, is done by the semiconductor company during the last fabrication process of the unit. The procedure for fabricating a ROM requires that the customer fill out the truth table that he or she wishes the ROM to satisfy. The truth table may be submitted in a special form provided by the manufacturer or in a specified format on a computer output medium. The manufacturer makes the corresponding mask for the paths to produce the 1’s and 0’s according to the customer’s truth table. This procedure is costly because the vendor charges the customer a special fee for custom masking the particular ROM. For this reasons mask programming is economical only if a large quantity of the same ROM configuration is to be ordered.
For small quantities it is more economical to use a second type of ROM called a programmable –read only memory or PROM. When ordered, PROM units contain all the fuses intact, giving all 1’s in the bits of the stored words. The fuses in the PROM are blown by application of current pulses through the output terminals for each address. A blown fuse defines a binary 0 state, and an intact fuse gives a binary 1 state. This allows user to program PROMs in their own laboratories to achieve the desired relationship between the input addresses and stored words. Special instruments called PROM programmers are available commercially to facilitate this procedure. In any case, all procedures for programming ROMs are hardware procedures even though the word “programming” is used.
The hardware procedure for programming ROMs or PROMs is irreversible, and once programmed, the fixed pattern is permanent and cannot be altered. Once a bit pattern has been established, the unit must be discarded if the bit pattern is to be changed. A third type of ROM available is called erasable PROM or EPROM.
The EPROM can be restructured to the initial value even though its fuses have been blown previously. When the EPROM is placed under a special ultraviolet light for a given period of time, the shortwave radiation discharges the internal gates that serve as fuses. After erasure, the EPROM returns to its initial state and can be reprogrammed to a new set of words. Certain PROMs can be erased with electrical signals instead of ultraviolet light. These PROMs are called electrically erasable PROM or EEPROM. Flash memory is a form of EEPROM in which a block of bytes can be erased in a very short duration. Example applications of EEPROM devices are :
1. Storing current time and date in a machine.
2. Storing port statuses
Examples of flash memory device applications are:
1. storing messages in a mobile phone.
2.storing photographs in a digital camera.
Types of ROM : The required path in a ROM may be programmed in three different ways. The first, mask programming, is done by the semiconductor company during the last fabrication process of the unit. The procedure for fabricating a ROM requires that the customer fill out the truth table that he or she wishes the ROM to satisfy. The truth table may be submitted in a special form provided by the manufacturer or in a specified format on a computer output medium. The manufacturer makes the corresponding mask for the paths to produce the 1’s and 0’s according to the customer’s truth table. This procedure is costly because the vendor charges the customer a special fee for custom masking the particular ROM. For this reasons mask programming is economical only if a large quantity of the same ROM configuration is to be ordered.
For small quantities it is more economical to use a second type of ROM called a programmable –read only memory or PROM. When ordered, PROM units contain all the fuses intact, giving all 1’s in the bits of the stored words. The fuses in the PROM are blown by application of current pulses through the output terminals for each address. A blown fuse defines a binary 0 state, and an intact fuse gives a binary 1 state. This allows user to program PROMs in their own laboratories to achieve the desired relationship between the input addresses and stored words. Special instruments called PROM programmers are available commercially to facilitate this procedure. In any case, all procedures for programming ROMs are hardware procedures even though the word “programming” is used.
The hardware procedure for programming ROMs or PROMs is irreversible, and once programmed, the fixed pattern is permanent and cannot be altered. Once a bit pattern has been established, the unit must be discarded if the bit pattern is to be changed. A third type of ROM available is called erasable PROM or EPROM.
The EPROM can be restructured to the initial value even though its fuses have been blown previously. When the EPROM is placed under a special ultraviolet light for a given period of time, the shortwave radiation discharges the internal gates that serve as fuses. After erasure, the EPROM returns to its initial state and can be reprogrammed to a new set of words. Certain PROMs can be erased with electrical signals instead of ultraviolet light. These PROMs are called electrically erasable PROM or EEPROM. Flash memory is a form of EEPROM in which a block of bytes can be erased in a very short duration. Example applications of EEPROM devices are :
1. Storing current time and date in a machine.
2. Storing port statuses
Examples of flash memory device applications are:
1. storing messages in a mobile phone.
2.storing photographs in a digital camera.