chap3 primary memory
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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
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