Primary Memory (Main Memory)
Primary memory holds only those data and instructions on which the computer is currently working. It has a limited capacity and data is lost when power is switched off. It is generally made up of semiconductor device. These memories are not as fast as registers. The data and instruction required to be processed resides in the main memory. It is divided into two subcategories RAM and ROM.
- RAM: Random Access Memory
- DRAM: Dynamic RAM, is made of capacitors and transistors, and must be refreshed every 10~100 ms. It is slower and cheaper than SRAM.
- SRAM: Static RAM, has a six transistor circuit in each cell and retains data, until powered off.
- NVRAM: Non-Volatile RAM, retains its data, even when turned off. Example: Flash memory.
- ROM: Read Only Memory, is non-volatile and is more like a permanent storage for information. It also stores the bootstrap loader program, to load and start the operating system when computer is turned on. PROM(Programmable ROM), EPROM(Erasable PROM) and EEPROM(Electrically Erasable PROM) are some commonly used ROMs.
Characteristics of Main Memory
- These are semiconductor memories.
- It is known as the main memory.
- Usually volatile memory.
- Data is lost in case power is switched off.
- It is the working memory of the computer.
- Faster than secondary memories.
- A computer cannot run without the primary memory.