Memory Management – Computer Awareness Notes

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Memory Management – Computer Awareness Notes
Memory: Computer memory is the storage space in 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. Each location or cell has a unique address which varies from zero to memory size minus one.

Memory is primarily of two types:
  • Primary Memory/Main Memory: Primary memory holds only those data and instructions on which computer is currently working. It has limited capacity and data is lost when power is switched off.
  • Secondary Memory: This type of memory is also known as external memory or non-volatile. It is slower than main memory. These are used for storing data/Information permanently.
Primary memory consists of mainly two types of memories:
Random Access Memory (RAM): RAM is the internal memory of the CPU for storing data, program and program result. It is read/write memory which stores data until the machine is working. As soon as the machine is switched off, data is erased.  
RAM is volatile, i.e. data stored in it is lost when we switch off the computer or if there is a power failure. Hence a backup uninterruptible power system (UPS) is often used with computers. RAM is small, both in terms of its physical size and in the amount of data it can hold.
There are mainly three types of RAM available:
  • Dynamic Random Access Memory (DRAM): A type of physical memory used in most personal computers. The term dynamic indicates that the memory must be constantly refreshed2 (reenergized) or it loses its contents. This type of memory is more economical.
  • Static Random Access Memory (SRAM): A type of memory that is faster and less volatile than DRAM, but requires more power and is more expensive. The term static is derived from the fact that it does not need to be refreshed like DRAM.
  • Synchronous Dynamic Random Access Memory (SDRAM): A type of DRAM that can run at much higher clock speeds.
Read Only Memory (ROM): The memory from which we can only read but can not write on it . This type of memory is non-volatile. The information is stored permanently in such memories during manufacture. A ROM, stores such instructions that are required to start a computer. This operation is referred to as bootstrap.
There are mainly four types of ROM available:
  • MROM (Masked ROM): The very first ROMs were hard-wired devices that contained a pre-programmed set of data or instructions. These kinds of ROMs are known as masked ROMs which are inexpensive.
  • PROM (Programmable Read only Memory): PROM is read-only memory that can be modified only once by a user. The user buys a blank PROM and enters the desired contents using a PROM program
  • EPROM (Erasable and Programmable Read Only Memory): The EPROM can be erased by exposing it to ultra-violet light for a duration of up to 40 minutes. Usually, an EPROM eraser achieves this function.
  • EEPROM (Electrically Erasable and Programmable Read Only Memory): The EEPROM is programmed and erased electrically. It can be erased and reprogrammed about ten thousand times. Both erasing and programming take about 4 to 10 ms (milli second). 

Cache Memory: It is a very high speed semiconductor memory which can speed up CPU. It acts as a buffer between the CPU and main memory.
Secondary Memory: This type of memory is also known as external memory.It is slower than main memory and non-volatile. These are used for storing data/Information permanently.Contents of secondary memories are first transferred to main memory, and then CPU can access it.  For example: disk, CD-ROM, DVD etc.
Memory can also be categorized on the basis of their material:
Semiconductor memory:-such as RAM, ROM, EPROM, and flash memory.
Magnetic memory:-such as hard disk, floppy disk and magnetic tapes.
Optical memory:-such as computer disk, DVD and blue-ray disk.

A bus, in computing, is a set of physical connections (cables, printed circuits, etc.) which can be shared by multiple hardware components in order to communicate with one another.
The address bus (sometimes called the memory bus) transports memory addresses which the processor wants to access in order to read or write data. It is a unidirectional bus.
The data bus transfers instructions coming from or going to the processor. It is a bidirectional bus.
The control bus (or command bus) transports orders and synchonisation signals coming from the control unit and travelling to all other hardware components. It is a bidirectional bus, as it also transmits response signals from the hardware.