MEMORY

The memory of computer is broadly categories into two categories:

• Internal and
• external

Internal memory is used by CPU to perform task and external memory is used to store bulk information, which includes large software and data.Memory is used to store the information in digital form. The memory hierarchy is given by:

• Register
• Cache Memory
• Main Memory
• Magnetic Disk
• Removable media (Magnetic tape)

Register: This is a part of Central Processor Unit, so they reside inside the CPU. The information from main memory is brought to CPU and keep the information in register. Due to space and cost constraints, we have got a limited number of registers in a CPU. These are basically faster devices. Cache Memory: Cache memory is a storage device placed in between CPU and main memory. These are semiconductor memories. These are basically fast memory device, faster than main memory. We can not have a big volume of cache memory due to its higher cost and some constraints of the CPU. Due to higher cost we can not replace the whole main memory by faster memory. Generally, the most recently used information is kept in the cache memory. It is brought from the main memory and placed in the cache memory. Now a days, we get CPU with internal cache. Main Memory: Like cache memory, main memory is also semiconductor memory. But the main memory is relatively slower memory. We have to first bring the information (whether it is data or program), to main memory. CPU can work with the information available in main memory only. Magnetic Disk: This is bulk storage device. We have to deal with huge amount of data in many application. But we don't have so much semiconductor memory to keep these information in our computer. On the other hand, semiconductor memories are volatile in nature. It loses its content once we switch off the computer. For permanent storage, we use magnetic disk. The storage capacity of magnetic disk is very high. Removable media: For different application, we use different data. It may not be possible to keep all the information in magnetic disk. So, which ever data we are not using currently, can be kept in removable media. Magnetic tape is one kind of removable medium. CD is also a removable media, which is an optical device.   Register, cache memory and main memory are internal memory. Magnetic Disk, removable media are external memory. Internal memories are semiconductor memory. Semiconductor memories are categoried as volatile memory and non-volatile memory. RAM: Random Access Memories are volatile in nature. As soon as the computer is switched off, the contents of memory are also lost. ROM: Read only memories are non volatile in nature. The storage is permanent, but it is read only memory. We can not store new information in ROM. Several types of ROM are available: PROM: Programmable Read Only Memory; it can be programmed once as per user requirements.                    EPROM: Erasable Programmable Read Only Memory; the contents of the memory can be erased and store new data into the memory. In this case, we have to erase whole information.                                   EEPROM: Electrically Erasable Programmable Read Only Memory; in this type of memory the contents of a particular location can be changed without effecting the contents of other location. The main memory of a computer is semiconductor memory. The main memory unit of computer is basically consists of two kinds of memory:      RAM : Random access memory; which is volatile in nature.  ROM : Read only memory; which is non-volatile.  The permanent information are kept in ROM and the user space is basically in RAM.The smallest unit of information is known as bit (binary digit), and in one memory cell we can store one bit of information. 8 bit together is termed as a byte. The maximum size of main memory that can be used in any computer is determined by the addressing scheme. A computer that generates 16-bit address is capable of addressing upto 216  which is equal to 64K memory location. Similarly, for 32 bit addresses, the total capacity will be 232 which is equal to 4G memory location.                                                                                   ...........to be continued