Memory
Memory - Storage Area. Location where data is stored and retrieved.
Memory can be classified into two groups. They are primary memory and secondary memory. Primary memory is a smaller segment of memory for storing program and data. The internal memory of a computer system uses electronic semiconductor devices called ROM and RAM. Secondary memory is used for long term storage of data. Floppy disk and hard disk are examples.
RAM – Read/ Write Memory
The read/write memory of a micro computer system is known as Random Access Memory. It has the facility to read the contents of any memory location as well as to write fresh data in any memory location. RAM is the memory part of a computer that stores information temporarily while the computer is on. It is this region into which programs are loaded before the CPU processes those informations. RAM is volatile in nature, which means that the contents are lost once the power supply is switched off. Two types of RAM are: Static RAM, Dynamic RAM.
Static RAM – In static RAM, binary information is stored in flip flops, one bit for each flip flop. A large number of flip flops are arranged in the form of a matrix. A ‘0’ or ‘1’ can be stored temporarily in a flip flop.
Dynamic RAM – In Dynamic RAM, binary 1s and 0s are stored as an electric charge or no charge on a tiny capacitor. Since these capacitors take up less space on a chip than a flip flop would, a Dynamic RAM chip can store many more bits than the SRAM of same size. But there is a disadvantage in DRAM. The charge stored in each capacitor tends to leak off and hence must be refreshed every 2 milliseconds or so. A device called a dynamic RAM refresh controller can be used to do this job in a microcomputer.
ROM – It is a Read Only Memory. Program or data stored in the ROM can only be read; they cannot be altered. ROM is non-volatile which means that the contents are not changed when the power is switched off.
Types of ROMs: EPROM, EEPROM
EPROM – Erasable Programmable Read Only Memory. The contents can be erased by passing high intensity, short wave length ultra violet light through a quartz window
Provided on top of the IC for about 20 minutes. However we cannot erase the contents of single memory location and reload it with fresh data. The entire contents are erased when exposed to UV light.
EEPROM is known as Electrically Erasable Programmable Read only Memory. Ultra violet light is not used for erasure. Electric signal is used for programming and erasing.
