What is Memory ?
Memory unit is that Component of a computer system, which is used to store the data, instruction and information and information before, during and after the processing done by ALU. It is actually a work area within the computer, where the CPU stores the data and instructions. Sometimes unallocated space of the hard disk is used as virtual memory also known as page file. This type of memory is a temporary memory.
Types of Main Memory ?
There are two types of main memory :-
- RAM (Random Access Memory)
- ROM (Read Only Memory)
RAM (Random Access Memory) :- It is Popularly Known as RAM, is the Key Working Area of the memory that is Used for our Problems and Data. This Area is Often Called the User Memory.
The Main Feature of RAM is that it can be read or written onto any Location and can be Accessed Randomly. The Contents of RAM are Available as long as the Computer is not Switched off i.e., it loses its Contents the moment, the Power is Turned off. Hence, it is Called Volatile Memory.
- Random Access Memory :- Holds its Data as Long as the Computer is Switched on.
- All data in RAM is Lost when the Computer is Switched off.
- Described as being Volatile.
- It is Direct Access as it can be both written to or read from in any order.
Types of RAM :-
- Dynamic Random Access Memory (DRAM)
- Contents are constantly refreshed 1000 times per second.
- Access time 60- 70 nanoseconds.
Note : A Nanosecond is one billionth of a Speed.
- Static Random Access Memory (SRAM)
- Doesn’t need refreshing.
- Retains contents as long as power is Applied to the chip.
- Access time is around ten nanoseconds.
- Used for Cache Memory.
- Also for Data and Time Settings as Powered by small battery.
ROM (Read Only Memory):- ROM as the name itself, implies, holds permanent data or instructions that can only be read from, but not written onto, i.e., information is permanently recorded, so that it cannot be changed by programmers. It is non- Volatile in nature meaning that the Contents of ROM are not lost, when the computer is switched off, it remains there, it is merely deactivated.
ROM contains instructions to get the computer started when switched on. Other uses of ROM includes holding instruction and data that control various peripheral units of the computer, such as graphic displays and disk drive etc.
- ROM holds program and data permanently even when computer is switched off.
- Data can be read by the CPU in any order so ROM is also direct access.
- The contents of ROM are fixed at the time of manufacture.
- Store a Program called the bootstrap loader that helps start up the computer.
- Access time is between 10 – 50 nanoseconds.
Types of ROM :-
- Programmable Read Only Memory (PROM)
- Empty of data when manufactured.
- May be Permanently programmed by the user.
- Erasable Programmable Read Only Memory (EPROM)
- Can be programmed, erased and reprogrammed.
- The EPROM chip has a small window on top allowing it to be erased by shining ultra-violet light.
- After reprogramming, the window is covered to prevent new contents from being erased.
- Access time is around 45- 90 nanoseconds.
- Electrically Erasable Programmable Read Only Memory (EEPROM).
- Reprogrammed Electrically without using ultraviolet light.
- Must be removed from the computer and Placed in a Special Machine to do this.
- Access time between 45 and 200 nanoseconds.
- Flash ROM
- Similar to EEPROM
- However, can be reprogrammed while still in the computer.
- Easier to upgrade programs stored in flash ROM.
- Used to Store Programs in devices for example, modems.
- Access time is around 45 – 90 nanoseconds.