0MEMORY
1.Primary/Main memory 2.Secondary memory
1.i RAM 1.ii ROM Compact disk
Hard disk
Magnetic Tape
In computing, memory refers to the physical device used to store programs or data on a temporary or permanent basis for use in a computer or other digital device.
PRIMARY MEMORY: - it is a part of CPU and holds only those data on which computer is working.it has limited capacity and data is lost when power is switched off.it is made of semiconductor devices.it is faster than secondary memory.
SECONDARY MEMORY: - this type of memory is also known as external memory.they are used for storage of data/information permanently. Contents of this memory are first transferred to main memory and then CPU can access it.They are mainly magnetic and optical memories also known as backup memories.computer may run without secondary memory.
Main memory is divided into two parts-Random access memory (RAM) & Read only memory (ROM).
RAM – A RAM chip is an integrated circuit (IC) made of millions of transistors and capacitors.it is also known as read-write or volatile memory.transistor and a capacitor are combined to create a memory cell which represents a single bit of data.
ROM – (Read-only memory) it is also known as non-volatile memory.writing is not possible. Used to store the instructions of routine type.permanent in nature and used to control or supervise the hardware.
SRAM – (Static RAM). It uses a flip-flop to store each bit of memory so does not need to be periodically refreshed.bit is stored as voltage.each memory cell may contain 3-6 transistor, less density i.e, storage capacity is less, fast and consume low power, expensive.
DRAM – (Dynamic RAM).data stored as charge between gate to substrate of a MOS transistor.as a charge leaks of frequently DRAM must be refreshed frequently, more density i.e, storage capacity is more, slow in operation.
MROM- (Masked ROM). Masking and metallization technique is used to design the memory. Once designed by the manufacturer can not be modified.memory arrays contain diode of transistors.presence of a transistor indicates logic 1, absence indicates zero.
PROM-(Programmable ROM). Memory arrays contain transistors or diode along with polysilicon or nichrome fuses which can be programmed.presence of a fuse indicate 1 and absence (indicate fuse blown off) logic 0.once programmed can not be modified.
EPROM-(Erasable PROM) information can be erased and the chip can be reprogrammed. Memory chip contains a crystal window on it when exposed to UV light, data is erased.one of the DISADVANTAGE is this that it takes 20 min to erase the data completely.
EEPROM-(Electrically erasable PROM) voltage is used to erase the data.data can be erased at register label or completely at a time.it takes 10 microseconds to erase the data byte, but for more byte, it will take more time.
FLASH MEMORY – similar to EEPROM except data can be erased at clock or sector level or completely.
CACHE MEMORY – it is used to store frequently used data or instructions.due to this the speed of processor increases it is very costly and made of very small size (usually 512 KB).it is of two type-Primary cache (inside microprocessor) and Secondary cache (in the motherboard or near the microprocessor).
The probability of finding the item at a particular level of the cache memory is known as Hit Ratio ‘h’. and the probability of not finding is known as Miss ratio ‘1-h’.
OPTICAL STORAGE MEDIA – stores a large amount of data in some formats as much as hard disk reads data by focusing a laser generated a beam of light on disk surfaces. Ex, CD, DVD.
MAGNETIC STORAGE MEDIA – data is recorded on the disk in concrete circular bands called track. Ex, Magnetic Tape, floppy Disk, Hard Disk.
