The primary memory in the computer is divided into two part ROM and RAM
ROM :Read only memory
In this type of memory the information is stored permanently, its is not lost after the system shuts down. The advantage of using this memory is that there are some complex functions like translators that are used repeatedly . These functions are permanently stored in the ROM and these instructions are hard wired in ROM so the execution is very fast.
There are different types of ROM . They are described below
- PROM:Programmable random access memory. In this the ROM is programmable but only once. Its just like burning a CD using CD recorder. You have a blank CD you can record what ever you want ,after recording you can read it any number of times.
In a strict ROM the device is constructed with the program already inside. But PROM is produced blank. It is programmed only later. The advantage of using PROM is that a company can change the program it is going to store when ever they want with out loss of hardware.
- Erasable programmable ROM(EPROM): This type of PROM can be erased and reprogrammed a number of times. Here the chip that holds the memory is exposed through a glass widow. A strong UV light of a particular frequency if shined through this glass window will erase the EPROM
- Electrically erasable programmable ROM(EPROM): This PROM can be reprogrammed under software control.
Unlike the ROM this is type of memory is volatile and can erased every now and then. Data can be written to it and may be retrieved from it according to the address given. There are two types of RAM dynamic RAM and Static RAM.
- Static RAM: Retains its content as long as the power supply is there. Once the power supply is lost it looses its contents.
- Dynamic RAM: Cannot retain the data even if constant power supply is there. The life time of data in it is milliseconds.
DRAM controller is used to refresh the data in Dynamic RAM before it starts to fade. So with DRAM controller we can make Dynamic RAM function just like Static RAM.