Memory is a mechanism that allows data to be written to and retrieved. An example of memory, maybe that of the eye and the retina, collecting information, and the brain storing the information sent by the retina. However, it the information is not refreshed then the information is lost (forgetfulness).
The computer must be able to have the capacity to store information, to take action, immediately. The type of memory that allows this is immediate access memory.
Below is a list with a summary of different types of memory:
Random Access Memory ( RAM )
This type of memory can be read from and written to and from any location and can be accessed independently. This type of memory is a volatile
Dynamic Random Access Memory ( DRAM) and Static Random Access Memory ( SRAM ). DRAM, in this form of memory, the state is stored as a charge or discharge in capacitors. These capacitors are created by using metal oxide transistors (MOS ).The major problem of this system is that data can be lost with time. Therefore the charge on the capacitor is periodically refreshed. There are two disadvantages with this:
• Additional circuitry is required for the refresh process is required.
• There is an increase in access time.
However the structure of these chips is much simpler than the RAM chips, so there is
a higher packing density.
Static Random Access Memory ( SRAM ) With this type of memory the data is maintained as long as the power is maintained, there is no need for a refresh circuit. The other difference is that a the storage mechanism is formed by logic circuits, and not capacitors as with the DRAM.
Read Only Memory ( ROM )
This type of memory is a non-volatile, i.e. it retains its contents when the power is switched off. Once programmed the data is retained. The disadvantage is that, once programmed, making changes once inserted into a system is difficult. ROM, finds use where the operating (i.e. system programs ) can be permanently implanted into a computer system.
Programmable Read - only Memory ( PROM )
A programmable read - only memory PROM, is once programmed, it cannot be erased or re-written.
Erasable ( PROM ) or EPROM
Can be programmed a number of times, erased with Ultra violet light. First introduced in 1971
Electrically erasable ( PROM ) or EEPROM
This type of device can be erased with application of an electrical pulse. This device has a lower density than ROM and EPROM
Flash Memory
Flash memory is a cheap dense form of the EEPROM. Data is erased in blocks and not byte by byte. All storage areas of a specific block are erased in a flash. This form of IC, usually takes the form a card. This is new introduction to the memory market and the advantage that there are no moving parts, ideal in use with mobile computers.
Cache
In general there are certain operations that are performed over and over again. These instructions/operations can be placed in the cache memory. it works at a high speed and works at the speed of the processor, typical storage capacity is 256K.
