Table of Contents
COMPUTER MEMORY ORGANIZATION
The memory unit of a computer is used to store data, instructions for processing data, intermediate results of processing and the final processed information. The memory unit of a computer are classified as primary and secondary memory.
PRIMARY MEMORY
The primary memory is available in the computer as built-in unitof the computer. The primary memory is represented as a set of location occupying 8 bits. Each bit in the memory is identified by a unique address. The data is stored in the machine understandable binary form in these memory locations.
THE COMMONLY USED PRIMARY MEMORY ARE :
Random Access Memory
This is the primary memory from where data & instructions can be received in a random manner (RAM). It is volatile memory in which the contents are lost once the power is turned off. The kind of memory is used to store the data temporarily during the computer operations.
- Static Random Access Memory (SRAM): It is a volatile memory based on traditional transistors using flip-flop gates to hold data as long as the power is on. The contents are lost once the power is turned off. It is very fast and that’s why used in cache memory. The SRAM takes more space and is expensive too, but it is easy to It does not need to be refreshed periodically and synchronizes itself with the timing of CPU.
- Dynamic Random Access Memory (DRAM): It is a volatile memory based on the capacitors that hold data as long as the power is on. Due to discharging capacitors, the DRAM is refreshed periodically. This refreshing is done automatically, and due to the time consumed in refreshing. The DRAM is slow. It is inexpensive and takes less space; therefore, DRAM is used as the main memory.
Read Only Memory
ROM is a special type of memory which can only be read and contents of which are not lost even when the computer is switched off. ROM chips are used not only in computers, but in most other electronic items such as washing machines, microwave ovens, calculators, laser printers, media players etc. It is not limited to electronic chips, CDROM and DVD ROM. It is categorized as follows:
- Programmable Read Only Memory (PROM): This is a kind of ROM has not been pre-recorded by the manufacture, but is supplied empty. The user of this ROM can store programs on it using a special tool. Once the empty ROM is programmed, it behaves like any other ROM, that is, it cannot be rewritten.
- Erasable Programmable Read Only Memory (EPROM): This is a ROM which has not been pre-recorded by the manufacture, but it is supplied empty. The user of this ROM can store programs on it using a special Once the empty ROM is programmed, it can be rewritten repeatedly after erasing the previously written entire contents using ultraviolet light of specific frequency. EPROM is more expensive than PROM.
- Electrically erasable programmable read only memory (EEPROM): This is a ROM which has not been pre-recorded by the manufacturer, but it supplied empty. The user of this ROM can store programs on it using a special tool. Once the empty ROM is programmed, it can be rewritten repeatedly after erasing the previously written contents using electric charge. This kind or ROM requires erasing the entire previously written content, but it allows erasing one byte at a time before writing the new content onto it. The EEPROM is more expensive than PROM, but it need not be removed from computer for rewriting. This is the most flexible type of ROM, which is now commonly used for storing BIOS
SECONDARY MEMORY
The secondary memory are the storage devices in which the data can be stored for longer duration, and it is not lost even when the power is turned off. The hard disks, flash drive, floppy disks, CD-ROMs and DVDs, etc., are examples of secondary memory. This memory has greater storage capacity than the primary memory . Also, it is inexpensive, but slow.
The secondary storage device can be classified as:
Magnetic Storage device
The magnetic storage devices store information that can be read, erased and rewritten a number of times. These include floppy disk, hard disk and magnetic tapes.
Optical Storage Device
The optical storage devices are secondary storage devices that use laser beams to read the stored data. These include CD-ROM, rewritable compact disk (CD-RW). Digital video disks with read-only memory, etc.
Magneto-Optical Storage Device
The megneto-optaical devices are generally used to store information, such as large programs, files and back up data. The end user can modify the information stored in megneto-optaical devices multiple times. These devices provide higher storages capacity as they use lease beams and magnets for reading and writing data to the device.
Cache Memory
The cache memory is faster than the CPU registers and slower than the main or primary memory. It is a SRAM placed between the CPU and the main memory, and when CPU needs any data or instruction is found in the cache memory, it is retrieved by the CPU for processing. Otherwise, the main memory is searched for the same information. The most frequently used instructions and data are placed in the cache memory; therefore, the overall speed of the computer is increased.
MEMORY HIERARCHY
The computer requires different kinds of memory for its proper functioning. Since the fast memories are very expensive, therefore they cannot be used in excess. There is a hierarchy of memories considering their speed and cost. The registers are the fastest storage devices; even faster than the cache memory (SRAM), which is faster than the main memory.
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