Several types of memory are used in computers: Such as Read-and-write memory (RAM), Read-only memory (ROM) and Mass storage.
Read-and-write memory (RAM) is used for storing data, instructions, and results during execution of a program. Semiconductor RAM consists of one or more silicon integrated circuits (each of which has many storage cells) and control logic so that information can be transferred into or out of the cell specified by the address.
Usually, the information that is stored in RAM is lost when power is removed. Thus, we say that RAM is volatile. Originally, the acronym RAM meant random-access memory, but the term has changed its meaning over time. As the term is used now, RAM means volatile semiconductor memory. (Actually, RAM is also available with small batteries that maintain information in the absence of other power.)
The time required to access data in RAM is the same for all memory locations. The fastest RAM is capable of access times of a few nanoseconds. No time penalty is incurred by accessing locations in random order.
There are two types of RAM in common use. In static RAM, the storage cells are SR flip-flops that can store data indefinitely, provided that power is applied continuously.
In dynamic RAM, information is stored in each cell as charge (or lack of charge) on a capacitor. Because the charge leaks off the capacitors, it is necessary to refresh the information periodically. This makes the use of dynamic RAM more complex than the use of static RAM. The advantage of dynamic RAM is that the basic storage cell is smaller, so that chips with larger capacities are available. A relatively small amount of RAM is needed in most control applications, and it is simpler to use static RAM.
- RAM
Read-and-write memory (RAM) is used for storing data, instructions, and results during execution of a program. Semiconductor RAM consists of one or more silicon integrated circuits (each of which has many storage cells) and control logic so that information can be transferred into or out of the cell specified by the address.Usually, the information that is stored in RAM is lost when power is removed. Thus, we say that RAM is volatile. Originally, the acronym RAM meant random-access memory, but the term has changed its meaning over time. As the term is used now, RAM means volatile semiconductor memory. (Actually, RAM is also available with small batteries that maintain information in the absence of other power.)
The time required to access data in RAM is the same for all memory locations. The fastest RAM is capable of access times of a few nanoseconds. No time penalty is incurred by accessing locations in random order.
There are two types of RAM in common use. In static RAM, the storage cells are SR flip-flops that can store data indefinitely, provided that power is applied continuously.
In dynamic RAM, information is stored in each cell as charge (or lack of charge) on a capacitor. Because the charge leaks off the capacitors, it is necessary to refresh the information periodically. This makes the use of dynamic RAM more complex than the use of static RAM. The advantage of dynamic RAM is that the basic storage cell is smaller, so that chips with larger capacities are available. A relatively small amount of RAM is needed in most control applications, and it is simpler to use static RAM.
- ROM
In normal operation, read-only memory (ROM) can be read, but not written to. The chief advantages of ROM are that data can be read quickly in random order and that information is not lost when power is turned off. Thus, we say that ROM is nonvolatile (i.e., permanent). ROM is useful for storing programs such as the boot program, which is executed automatically when power is applied to a computer. In simple dedicated applications such as the controller for a clothes washer, all of the programs are stored in ROM. Several types of ROM exist. For example, in mask-programmable ROM, the data are written when the chip is manufactured. A substantial cost is incurred in preparing the mask that is used to write the data while manufacturing this type of ROM. However, mask-programmable ROM is the least expensive form of ROM when the mask cost is spread over a sufficiently large number of units. Mask-programmable ROM is not a good choice if frequent changes in the information stored are necessary, as in initial system development.
In programmable read-only memory (PROM), data can be written by special circuits that blow tiny fuses or leave them un-blown, depending on whether the data bits are zeros or ones. Thus, with PROM, we write data once and can read it as many times as desired. PROM is an economical choice if a small number of units are needed.
Erasable PROM (EPROM) is another type that can be erased by exposure to ultraviolet light (through a window in the chip package) and rewritten by using special circuits. Electrically erasable PROMs (EEPROMs) can be erased by applying proper voltages to the chip. Although we can write data to an EEPROM, the process is much slower than for RAM.
Flash memory is a nonvolatile technology in which data can be erased and rewritten relatively quickly in blocks of locations, ranging in size from 512 bytes up to 512 Kbytes. Flash memory has a limited lifetime, typically on the order of 10 thousand to 100 thousand read/write cycles. Flash is a rapidly advancing technology and may eventually replace hard drives for mass storage in general purpose computers.
Mass-storage units include hard disks and flash memory, both of which are read/write memory. Another type is CD-ROM and DVD-ROM disks, which are used for storing large amounts of data. Mass storage is the least expensive type of memory per unit of capacity. With all forms of mass storage except flash, a relatively long time is required to access a particular location. Initial access times for mass storage range upward from several milliseconds, compared with fractions of a microsecond for RAM or ROM. However, if mass-storage locations are accessed sequentially, the transfer rate is considerably higher (but still lower than for RAM or ROM). Usually, data and instructions need to be accessed quickly in random order during execution of a program. Thus, programs are stored in RAM or ROM during execution.
The main considerations in choosing the type of memory to be used are:
1. The trade-off between speed and cost.
2. Whether the information is to be stored permanently or must be changed frequently.
3. Whether data are to be accessed in random order or in sequence.
In general-purpose computers, programs and data are read into RAM before execution from mass-storage devices such as hard disks. Because many different programs are used, it is not practical to store programs in semiconductor ROM, which would be too expensive for the large memory space required. Furthermore, information stored in ROM is more difficult to modify compared to data stored on a hard disk. We often find a small amount of ROM used for the startup or boot program in general-purpose computers, but most of the memory is RAM and mass storage.
On the other hand, in embedded microcontrollers, programs are usually stored in semiconductor ROM and only a small amount of RAM is needed to store temporary results. For example, in a controller for a television receiver, the programs for operating the TV are stored in ROM, but time and channel information entered by the user is stored in RAM. In this application, power is applied to the RAM even when the TV is “turned off.” However, during a power failure, the data stored in RAM are lost (unless the TV has a battery backup for its RAM). Usually, we do not find mass-storage devices used in embedded computers.
In programmable read-only memory (PROM), data can be written by special circuits that blow tiny fuses or leave them un-blown, depending on whether the data bits are zeros or ones. Thus, with PROM, we write data once and can read it as many times as desired. PROM is an economical choice if a small number of units are needed.
Erasable PROM (EPROM) is another type that can be erased by exposure to ultraviolet light (through a window in the chip package) and rewritten by using special circuits. Electrically erasable PROMs (EEPROMs) can be erased by applying proper voltages to the chip. Although we can write data to an EEPROM, the process is much slower than for RAM.
Flash memory is a nonvolatile technology in which data can be erased and rewritten relatively quickly in blocks of locations, ranging in size from 512 bytes up to 512 Kbytes. Flash memory has a limited lifetime, typically on the order of 10 thousand to 100 thousand read/write cycles. Flash is a rapidly advancing technology and may eventually replace hard drives for mass storage in general purpose computers.
- MASS STORAGE MEMORY
Mass-storage units include hard disks and flash memory, both of which are read/write memory. Another type is CD-ROM and DVD-ROM disks, which are used for storing large amounts of data. Mass storage is the least expensive type of memory per unit of capacity. With all forms of mass storage except flash, a relatively long time is required to access a particular location. Initial access times for mass storage range upward from several milliseconds, compared with fractions of a microsecond for RAM or ROM. However, if mass-storage locations are accessed sequentially, the transfer rate is considerably higher (but still lower than for RAM or ROM). Usually, data and instructions need to be accessed quickly in random order during execution of a program. Thus, programs are stored in RAM or ROM during execution.
- SELECTION OF COMPUTER MEMORY
The main considerations in choosing the type of memory to be used are:1. The trade-off between speed and cost.
2. Whether the information is to be stored permanently or must be changed frequently.
3. Whether data are to be accessed in random order or in sequence.
In general-purpose computers, programs and data are read into RAM before execution from mass-storage devices such as hard disks. Because many different programs are used, it is not practical to store programs in semiconductor ROM, which would be too expensive for the large memory space required. Furthermore, information stored in ROM is more difficult to modify compared to data stored on a hard disk. We often find a small amount of ROM used for the startup or boot program in general-purpose computers, but most of the memory is RAM and mass storage.
On the other hand, in embedded microcontrollers, programs are usually stored in semiconductor ROM and only a small amount of RAM is needed to store temporary results. For example, in a controller for a television receiver, the programs for operating the TV are stored in ROM, but time and channel information entered by the user is stored in RAM. In this application, power is applied to the RAM even when the TV is “turned off.” However, during a power failure, the data stored in RAM are lost (unless the TV has a battery backup for its RAM). Usually, we do not find mass-storage devices used in embedded computers.
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