COMPUTER MEMORY

Computer Memory: 

A memory is just like a human brain. It is used to store data and instructions. Computer memory is the storage space in the computer, where data is to be processed and instructions required for processing are stored. The memory is divided into large number of small parts called cells

A memory is a device which store information that is currently being utilized/used by the operating system, Application software , Hardware devices etc.

Types of Computer Memory

There are two main types of computer memory

  1. Primary Memory/Main Memory
  2.  Secondary Memory
Primary Memory/Main Memory
The Primary storage unit (main memory) holds data and instructions between processing steps and supplies them to the CU and ALU during processing. All data and programs must be placed into main memory before they can be processed. The primary storage unit consists of microelectronic semiconductor memory chips. 
This includes 
  • RAM (random access memory) for loading programs and data before execution, and 
  • ROM (read only memory) for permanent  programs that are supplied with the computer hardware.

Main memory is where programs and data are kept when the processor is actively using them. It hold data temporary. Main memory is closely connected to the processor, so moving instructions and data into and out of the processor is very fast. Workable example of main memory is RAM(Random Access 
Memory)
RAM is considered as temporary or volatile
memory; this means that the contents of RAM are lost when the computer power is turned off.


Random Access Memory (RAM)
RAM: Is the main memory space of your 
computer which is considered as temporary or volatile memory, this means that the contents of RAM are lost when the Computer power is turned off.


The RAM is the workspace of your computer. If your computer has more RAM, it can open more and larger programs and documents simultaneously. The documents you are currently editing, typing, formatting, and the programs your computer is using are stored in the RAM. Note: RAM capacity is expressed in ―Megabytes‖ (MB)
Characteristics of RAM
  • It is located near the CPU and hence store data immediately after CPU process them.
  • RAM is a Primary storage.
  • RAM is Volatile storage.
  • RAM has little storage capacity.
  • RAM is a temporarily storage.
  • RAM has high speed compared to ROM
  • It is electrically dependent.
Types of RAM
  1. DRAM
  2. SRAM
DRAM: 
DRAM stands for Dynamic RAM, and it is the most common type of RAM used in computers. 
  • The oldest type is known as single data rate (SDR) DRAM, 
  • Newer computers use faster dual data rate (DDR) DRAM. 
DDR comes in several versions including DDR2 , DDR3, and DDR4, which offer better performance and are more energy efficient than DDR. However different versions are incompatible, so it is not possible to mix DDR2 with DDR3 DRAM in a computer system. DRAM consists of a transistor and a capacitor in each cell.

SRAM: 
SRAM stands for Static RAM, and it is a particular type of RAM which is faster than DRAM, but more expensive and bulker, having six transistors in each cell. For those reasons SRAM is generally only used as a data cache within a CPU itself or as RAM in very high-end server systems. A small SRAM cache of the most imminently-needed data can result in significant speed improvements in a system
The key differences between DRAM and SRAM 
  • SRAM is faster than DRAM – perhaps two to three times faster – but more expensive and bulkier. 
  • SRAM is usually available in megabytes, while DRAM is purchased in gigabytes.
  • DRAM uses more energy than SRAM because it constantly needs to be refreshed to maintain data integrity, while SRAM  – though volatile – does not need constant refreshing when it is powered up.
Read Only Memory (ROM)
ROM: Is non- volatile memory; this means that the contents of ROM are not lost when the computer power is turned off. So ROM stores data permanently. Data stored in ROM cannot be altered easily; since it was created by the manufacture of the computer.ROM has low speed compared to RAM.
Characteristics of ROM
  • ROM stores data permanently
  • ROM is non-volatile storage.
  • ROM has low speed compared to RAM
  • Data stored in ROM cannot be altered easily

Types of ROM

ROM is available in several different types, including 
  1. PROM, 
  2. EPROM, and 
  3. EEPROM.

PROM 
PROM stands for Programmable Read-Only Memory, and it is different from true ROM in that while a ROM is programmed (i.e. has data written to it) during the manufacturing process, a PROM is manufactured in an empty state and then programmed later using a PROM programmer or burner.

EPROM 
EPROM stands for Erasable Programmable Read-Only Memory, and as the name suggests, data stored in an EPROM can be erased and the EPROM reprogrammed. Erasing an EPROM involves removing it from the computer and exposing it to ultraviolet light before re-burning it.

EEPROM 
EEPROM stands for Electrically Erasable Programmable Read-Only Memory, and the distinction between EPROM and EEPROM is that the latter can be erased and written to by the computer system it is installed in. In that sense EEPROM is not strictly read-only. However in many cases the write process is slow, so it is normally only done to update program code such as firmware or BIOS code on an occasional basis
Confusingly, NAND flash memory (such as that found in USB memory sticks and solid state disk drives) is a type of EEPROM, but NAND flash is considered to be secondary memory.

Differences between RAM and ROM

ROM:
  • Non-volatile
  • Fast to read
  • Usually used in small quantities
  • Cannot be written to quickly
  • Used to store boot instructions or firmware
  • Relatively expensive per megabyte stored compared to RAM
RAM:
  • Volatile
  • Fast to read and write
  • Used as system memory to store data (including program code)  that the CPU needs to process imminently
  • Relatively cheap per megabyte stored compared to ROM, but relatively expensive compared to secondary memory
Secondary Memory
This type of memory is also known as external memory or non-volatile. It is slower than the main memory. These are used for storing data/information permanently. CPU directly does not access these memories, instead they are accessed via input-output routines. 
The contents of secondary memories are first transferred to the main memory, and then the CPU can access it. For example, disk, CD-ROM, DVD, etc.
Characteristics of Secondary Memory
  • These are magnetic and optical memories.
  • It is known as the backup memory.
  • It is a non-volatile memory.
  • Data is permanently stored even if power is switched off.
  • It is used for storage of data in a computer.
  • Computer may run without the secondary memory.
  • Slower than primary memories.
Cache Memory
Cache memory is a very high speed semiconductor memory which can speed up the CPU. It acts as a buffer between the CPU and the main memory. It is used to hold those parts of data and program which are most frequently used by the CPU. 


The parts of data and programs are transferred from the disk to cache memory by the operating system, from where the CPU can access them.


Advantages of cache memory
The advantages of cache memory are as follows −
  • Cache memory is faster than main memory.
  • It consumes less access time as compared to main memory.
  • It stores the program that can be executed within a short period of time.
  • It stores data for temporary use.
Disadvantages
The disadvantages of cache memory are as follows −
  • Cache memory has limited capacity.
  • It is very expensive.

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