Primary memory is classified into two, Random Access Memory (RAM) and Read Only Memory (ROM). The primary memory holds the data which is to be processed by the CPU and the set of instructions to be executed next. The CPU can access the instructions in the primary memory only.
1. Random Access Memory (RAM):
RAM is used to store data and instructions needed by the CPU for processing. RAM can be used for both reading and writing of data so it is called Read and Write memory. It is a volatile memory, that is contents of the RAM will be lost when the power is turned off.
The invention of integrated circuits (chips) increased the memory capacity and reduced the size and cost. Static RAM, Dynamic RAM, Synchronous Dynamic RAM (SDRAM) are the various types of RAM.
2. Read Only Memory (ROM):
We can read this memory but we cannot write into this memory. It is a nonvolatile memory, that is its contents will not lost when the power is turned off. This memory is stored in the ROM chip at the time of manufacture ing itself hence it is called firmware.
ROM contains instructions to check the hardware components connected to the system, perform some basic input/output operations (BIOS), initiates loading of essential software.
The different categories are PROM, EPROM and EE PROM.
(a) PROM (Programmable Read Only Memory):
It is just like WORM. That means the instructions are write once but read many. But we cannot change the instructions.
(b) EPROM (Erasable Programmable Read Only Memory):
It functions just like PROM. But by using ultraviolet light we can erase the old data and can write new data.
(c) EEPROM (Electrically Erasable Programmable Read Only Memory):
Here instead of ultraviolet light electric signals are used to erase old data and write new data under software control. It is highly expensive than regular ROM chips.
