Unit1-chap-3-primary memory

contentsIntroductionMemory hierarchyRandom Access Memory

introductionComputers are used to perform various tasks in

science, engineering , business, education, entertainment and many other fields.

They work at high speed and can handle huge amount of data with great accuracy, can perform operation in sequence without any human interventions.

Computer requires memory to process data, hold intermediate results and to store the output.

Computer memory refers to the electronic holding place for instructions and data where the processor can reach quickly.

Two categories : primary and secondary memory.

Primary memory is used to process data and instructions

Secondary memory is used to store outputThe major limitations with the primary

memory is it is volatile in nature i.e. when power is turned off the data is lost forever.

To store data permanently a computer requires some non-volatile storage medium like a hard disk, this kind of memory is called secondary storage

Memory representationAll quantities are measured in units.Length is measured in meters, time in

seconds, weight in kgs.Digital computers work in two states,

ON(1)OFF(0)

O represents a false value, +5 volts represents a true state i.e. 1.

Each of these values is called binary digit or bit.

Group of eight bits is called a byte.

Bit: smallest unit of data on a machine and a single bit can only hold one of the value 0 or 1.

Byte: A unit of eight bits is known as byteKilobyte: in decimal system, kilo stands for

1000 but in binary system kilo refers to 1024. Kilobytes is 1024 bytes.

Megabytes: 1024 kilobytesGigabytes: 1024 MBTerabytes : 1024GB

Memory hierarchyThe memory in a computer system is of three

fundamental types.Internal processor memory:

Is placed within the CPU or it is attached to a special fast bus.

Includes cache memory and special registersUsed to compensate the speed gap between

the primary memory and the processor.Primary memory

RAM , ROM come under this categoriesAlso known as main memory.BIOS

Secondary memory Also called as auxiliary memory. Provides a backup storage for instructions and data Eg: magnetic disk and tapes Least expensive and have large physical storage. Data stored in these, are permanently stored forever. They can be removed if the user wants to or it is

destroyed. They can be used as overflow memory, when the

primary memory is full The data and instructions have to be shifted from

secondary to main memory, later it is shifted to the processor.

Memory hierarchyWhen data comes from permanent storage ,

they first go into RAM.CPU has to access the hard disk constantly to

retrieve every piece of required data , it would operate slowly.

When the data is kept in primary storage, it can accessed quickly.

Memory hierachy

Random access memoryIt allows computer to store data for

immediate manipulation, and keep track of what is currently being processed.

Its place where the computer keep operating system, application and data in current use, so that the processor can access it quickly.

Data in RAM in it, only till computer is ON.When the computer goes down, the data is

lost which present in RAM.When the computer is started again , the

operating system and files are loaded into RAM.

Interaction of memory and storage with processorWhenever the user enters a command from

keyboard , the CPU interprets the command and instructs the hard disk to “load” the command or program into the main memory

Once the data is loaded into the main memory, the CPU is able to access it faster than secondary storage.

Types of RAMTwo types of RAM

Static RAM (SRAM)Dynamic RAM (DRAM)

SRAMThe word static indicates that memory retains

the content as long as power is suppliedWhen power goes down, the data stored in it is

lost forever.Does not need to be refreshed periodicallyVery fast but much more expensive than DRAMUsed as cache memory due to its high speed.

DRAMNamed so because it is unstable in natureThe data continue to move in and out of the

memory as long as power is availableIt has to be continuously refreshed in order to

maintain the data in it.Done by placing the data in refresh circuit that

rewrites the data several hundred times per second.

Used in most of the system, as its inexpensive and small in size.

Difference between SRAM and DRAMBASED On SRAM DRAMLife of data Data is present as

long as power is there, power is turned off, data is lost

It is continuously refreshed for every 15 microseconds, even when there is continuous power supply

Circuitry Dense Simple, no. of cells present in DRAM is less than SRAM

Profit More beneficial, its fast and has low latency, no need of refreshing

Slower and have longer latency then SRAM

Cost Expensive, requires more power to operate, produces lots of heat

Cheaper, simple, small and space efficient

Access time 25 nanoseconds 60 nanoseconds