This document discusses computer memory and the memory hierarchy. It begins by introducing primary and secondary memory. Primary memory, like RAM, is used to process data and instructions directly, while secondary memory, like hard disks, is used for long-term storage. It then describes the memory hierarchy, with internal processor memory and cache at the top for fastest access, primary/RAM memory next, and secondary disk storage at the bottom. The document focuses on RAM, describing its purpose to temporarily store active data and programs, and how it must be refreshed to maintain data in dynamic RAM unlike static RAM.

2. contents
 Introduction
 Memory hierarchy
 Random Access Memory

3. introduction
 Computers 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.

4.  Primary memory is used to process data and
instructions
 Secondary memory is used to store output
 The 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

5. Memory representation
 All 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.

6.  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 byte
 Kilobyte: in decimal system, kilo stands for 1000 but in
binary system kilo refers to 1024. Kilobytes is 1024
bytes.
 Megabytes: 1024 kilobytes
 Gigabytes: 1024 MB
 Terabytes : 1024GB

7. Memory hierarchy
 The 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 registers
 Used to compensate the speed gap between the primary
memory and the processor.
 Primary memory
 RAM , ROM come under this categories
 Also known as main memory.
 BIOS

8.  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.

9. Memory hierarchy
 When 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.

10. Memory hierachy

11. Random access memory
 It 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.

12. Interaction of memory and storage
with processor
 Whenever 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.

14. Types of RAM
 Two types of RAM
 Static RAM (SRAM)
 Dynamic RAM (DRAM)
 SRAM
 The word static indicates that memory retains the
content as long as power is supplied
 When power goes down, the data stored in it is lost
forever.
 Does not need to be refreshed periodically
 Very fast but much more expensive than DRAM
 Used as cache memory due to its high speed.

15.  DRAM
 Named so because it is unstable in nature
 The data continue to move in and out of the memory as
long as power is available
 It 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.

16. Difference between SRAM and
DRAM
BASED On SRAM DRAM
Life 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