How the memory is organised in computer systems(Memory Organization)

 By:Sakib Shabir


Memory:   A memory is the collection of storage units or devices together.The memory unit stores the binary information together in the form of bits.
Generally, memory storage is classified into two categories
1.Volatile memory:This loses its data ,when power is switched off.
2.Non-Volatile:This is a permanent storage and does not loses any data when power is switched off. 


Memory hierarchy:
The total capacity of a computer can be visualised by hierarchy of components.The memory hierarchy system consists of all storage devices contained in a computer system from the slow Auxiliary memory to fast main memory and to the smaller cache memory.As we go from top to bottom speed and cost decreases but size increases.Memory hierarchy is nothing but various labels of memory
Auxiliary memory access time is generally 1000 times that of  the main memory ,hence it is at the bottom of the hierarchy
The main memory occupies the central position because it is equipped to communicate directly with the CPU and with auxiliary memory devices through I/P processor
when the program not residing in main memory is needed by the CPU,they are brought in from auxiliary memory.Programs not currently needed in main memory are transferred into auxiliary memory to provide space in main memory for other programs that are currently in use .                          
Cache Memory:
The cache memory is used to store program data which is currently being executed in the CPU .Approximate access ratio between cache memory is about 1 to 7~10.The data or contents of the main memory that are used again and again by CPU,are stored in the cache memory so that we can easily access that data in shorter time.
Whenever the CPU needs to access memory,it first checks the cache memory,if the data is not found in cache memory then the CPU moves onto the main memory.It also transfers block of recent data into the cache and keeps on deleting the old data in cache to accommodate the new one.

Main Memory:
Main memory is made up of RAM and ROM,with RAM integrated circuit chips holding the major share.The memory unit that communicates directly within the CPU, Auxiliary memory and Cache memory, is called main memory

RAM:(Random Access Memory)
It is the best known form of computer memory.RAM is considered “Random Access” because you can access any memory cell directly if you know the row and column that intersect at that cell.
RAM has also its types:

  • DRAM:Dynamic Ram .it is made up 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.(ex.flash memory)

ROM:(Read only Memory)
It is non-volatile and is more like a permanent storage for information .It is also stores the bootstrap loader program,to load and start the operating system when computer is turned on.
PROM and EPROM are some commonly used Rom’s

Secondary Memory:
Secondary memory is not accessed directly by the CPU.Instead,data accessed from a secondary memory is first load into RAM and is then sent to the processing unit
Secondary memory is slower in data accessing.Typically primary memory memory is six times faster than the secondary memory.Secondary memory provides a feature of being non-volatile which means it can hold on its data with or without electric power supply

Types of Secondary Memory:
Fixed storage:
A fixed storage is an internal media device that is used by a computer system to store data,and usually these are referred to as the fixed disks drivers or the hard drives
Fixed storage devices are literally not fixed,obviously these can be removed from the system for repairing work,maintenance purpose,and also for upgrade etc.But in general,this can’t be done without a proper toolkit to open up the computer system to provide physical access ,and that needs to be done by an engineer .
Technically,almost all of the data that is being processed on a computer system is stored on some type of a built-in fixed storage device.
Types of fixed storage device:
    • Internal flash memory
    • SSD (solid state disks) units
    • Hard disk drives(HDD)

Removable Storage:
A removable storage is an external media device that is used by a computer system to store data,and usually these are referred to as the removable disk drives or the external drives
Removable storage is any type of storage device that can be removed/ejected from a computer system is running
Examples are CD’s,DVD’s and Blu-Ray disk drives as well as USB drives.
Removable storage makes it easier for a user to transfer data from one computer system to other.In a storage factors,the main benefit of removable disks is that they can provide the fast data transfer rates associated with storage area networks
    • Optical disks(CD’s,DVD’s)
    • Memory disks
    • Floppy disks
    • Disk packs
    • Magnetic tapes