Class 11 | Memory | COA Notes

01

Memory Hierarchy

Memory hierarchy is organization of memory such it can minimize access time.
It is divided into five level based on speed as well as use.

Fig: Memory hierarchy

02

Levels of Memory

1. Register

Register are present inside the CPU.
Least access time.
Most expensive and smallest in size.
Available in KB size.

2. Cache

Store frequently used instruction & data.
Available in MB size.
Made up of SRAM.

3. Primary or Main Memory

Directly communicates with CPU and auxiliary devices.
Available in GB size.
Made up of DRAM.

4. Secondary Memory

Used as backup storage.
Available in few TB.

5. Tertiary Memory

Used to store removable files.
Cheapest and largest in size.
Available in PB.

03

Different Types of Memory – Cache Memory

Cache memory is very high speed memory.
Acts as buffer between RAM and CPU.
It holds frequently requested data and instruction so that they are immediately available to the CPU when needed.

Fig: Cache memory

Processor first checks for the data in the cache.

If the processor finds that data in the cache, it is called cache hit.
If it doesn’t find, it is called cache miss.

04

Primary Memory & Secondary Memory

S.N.	Primary Memory	Secondary Memory
1.	Primary memory is the main memory of computer system.	It refers to auxiliary memory, external memory or secondary memory.
2.	It is a volatile memory.	It is non-volatile memory.
3.	Data is directly accessed by the CPU.	Data is not directly accessed by the CPU.
4.	It is faster but have less storage.	It is slower but have more storage.
5.	Very expensive.	Significantly cheaper.
6.	Eg. RAM / ROM.	Eg. HDD / SSD.

05

Characteristics of Memory System

1. Location

Memory can be either be stored externally or internally (like RAM).

CPU
Internal or main memory
External or secondary memory

2. Capacity

Capacity of any memory device can be expressed in terms of:

Word size
Number of words

3. Unit of Transfer

It is maximum number of bits read or written into the memory at a time.

It is mostly equal to word size in case of main memory.

4. Access Method

Way of searching the storage device is called access method.

It is four types:

Direct Access Method or Direct Access Memory
Sequential Access Method or Sequential Access Memory
Random Access Method or Random Access Memory
Associative Access Method or Associative Access Memory

Commonly used word size are 1 byte, 2 byte and 4 byte.

If a memory device is given as 4K × 16, this means:

word size = 16

number of words = 4K or 4096.

5. Performance

It is determines by using 3 parameter:

Memory cycle time
Access time
Transfer Rate

06

SRAM and DRAM

S.N.	SRAM	DRAM
1.	It stand for static-RAM.	It stand for Dynamic-RAM.
2.	Made up of Transistor.	Made up of capacitor.
3.	Faster but expensive.	Slower and cheap.
4.	Smaller in size. 2–16 MB	Larger in size. 2–16 GB
5.	Less dense.	Higher dense.
6.	Mostly use in this time.	Rarely use in this time.
7.	Memory position is in the processors.	Memory position is in the motherboard.
8.	Modules are simpler.	Modules are complex.

07

Magnetic Disk and How Does It Work

A magnetic disk is a types of secondary memory that consist of a flat disc with a magnetic coating that store data.
IBM introduced the first magnetic hard drive in 1956. It could only store 5MB data.
Data can be easily edited, remove and provides data access at an time.

Structure of Magnetic Disk

Fig: Magnetic Disk

Internal Structure of Platter

Fig: Internal structure of platter

The disk surface is divided into tracks which are concentric circles.
Tracks are further divided into sectors.
Sectors are used to store data in a disc.
Each track may have 8 or more sectors.
Read/write head is connected to a disc arm that moves the head.
Motor moves the disc at a speed of 100 times per second.

08

Memory Management Unit

MMU is a hardware that control the processor’s memory and caching operations.
It is located within computer’s CPU.
All data request are sent to the MMU which is turn determines from where data need to be retrieved.

Three Main Functions

Hardware Memory Management
Operating System’s Memory Management
Application Memory Management

Fig: Memory Management Unit

09

Memory Address

A memory address is a reference to a specific memory location in RAM to find where is data stored.
The memory location are addressed from 0 to 2^k − 1.

Fig: Memory Address

10

Flash Drive

Permanent storage device which can store data for long period of time.
Plug and play device.
Small in size and easy to use.
Easy to carry from one location to another location.
Faster than optical device.
Large data storage capacity is large in size. (256 GB)
Back up data.
Example: pen drive, Memory card etc.
Flash Memory is connected to computer through USB port.

Fig: Flash Drive

11

DVD – Read Write

DVD-RW is stand for Digital versatile Disc – Read write.
Data are easily rewritable many times.
It looks like compact disc.
It is an optical disc which works on laser beam.
Used to backup data.

12

Error Correcting Codes

When bits are transmitted over the computer network, they get corrupted due to interference and network problems.
Error detection techniques are responsible for checking whether an error has occurred or not in the frame.
For error detection, sender need to send additional bits along with data bits.
Receiver performs necessary check based upon the additional redundant bits.
Among many techniques for detecting error in data frames, most popular parity check.

13

Parity Check

Sender’s End: While creating a frame the sender counts the number of 1s in it and adds the parity bit as follows.

1. In Case of Even Parity

If number of 1s is even the parity bit is 0.
If number of 1s is odd the parity bit is 1.

2. In Case of Odd Parity

If number of 1s is odd the parity bit is 0.
If number of 1s is even the parity bit is 1.

Example data frame: 1001101

Frame with even parity: 0 1001101

Frame with odd parity: 1 1001101

14

Hamming Code

Hamming code is a error correction code that can be used to detect and correct the error.
Developed by RW Hamming.
Sender encodes data bits with redundant bits.

At Sender’s End

Calculate number of redundant bits.
Position the redundant bits.
Calculate value of each redundant bit.

Example: Message = 1001

Position	P1	P2	P4
0	0	0	0
1	1	0	1
2	0	1	0
3	1	1	1
4	1	0	0
5	1	0	1
6	1	1	0
7	1	1	1

Source note: The next pages continue the handwritten Hamming-code working, including parity groups, sender frame, receiver checks, an introduced error, redundant-bit result and reconstructed error-free message.

Sender 1 0 0 1 1 0 0

At Receiver (0) P1 = 1 0 1 ✓ (0) P2 = 1 1 1 ✕ (error) (1) P4 = 0 1 1 ✕ (error)

After Some Error 1 1 0 1 1 0 0

Redundant Bit 110 = (6)₁₀

Error Free msg 1001100

15

Memory Packaging and Memory Modules

Memory Packaging

Memory package is a small circuit board that contain memory chips.
SIMM, DIMM and RIMM are some of the memory package.

16

Memory Modules

1. Single In-line Memory Module (SIMM)

A SIMM consists of memory chips soldered onto a printed circuit Board (PCB).
72 pin, SIMM transfers 32 bits of data a time, therefore in modern micro computers with a 64 bit data bus two SIMMs have to be paired up in order to functions.

2. Dual In-line Memory Module (DIMM)

DIMM is 168 pin package.
It is double sided memory modules.
It supports 64-bit, 72 bit or 80 bit.
It consumes 3.3 volt of power.
Modern computer use DIMM.

3. Rombus Inline Memory Module (RIMM)

RIMM is same as DIMM but it only differs in the pin configuration.
The high bandwidth and low latency application use this memory package.
The consume 2.5 volt of power.