How Memory Works

• Physical Example

0

Water Tank

1

Water Tank

Empty Full

How Memory Works

• Physical Example

0

ElectronicCircuit

1

ElectronicCircuit

Discharged Charged

Practical Dynamic Memory

• Use capacitors to store the charge to represent “0” and “1”

+++ +++

_ _ _ _ _ _

Practical Static Memory

• Transistors play the main roles

•

Base

Collector

Emitter

Memory

• Memory is the second important component in modern computers.

1001111000011001

0000000000000000000000000000000100000000000000020000000000000003

0001101000011011

Content

Address

1111111111111111

Memory

• Memory is the second important component in modern computers.

10011110000110010001101000011011

Content

Address

0000h0001h0002h0003h

FFFFh

Memory Related Terms

• Bit------------------0 or 1

• Byte----------------8 bits

• Word---------------16 bits-----2 bytes

• Doubleword------32 bits-----4 bytes

• Quadword---------64 bits-----8 bytes

Range of Unsigned Integers

• Unsigned byte------------0 to 255

• Unsigned word-----------0 to 65535--64KB

• Unsigned doubleword--0 to 4,294,967,295

• 0 to -----4.3GB

• Unsigned quadword----

• 0 to 18,446,744,073,709,551,615

• 0 to 18,446,744,073GB

Memory Related Terms

• RAM -----Random Access Memory

• ROM -----Read Only Memory

• PROM--Programmable Read Only

• Memory

• EPROM--Erasable PROM

• EEPROM--Electrical Erasable PROM

• Dynamic RAM---Need to be refreshed

• very often (every few milliseconds)

• Static RAM---Expensive cache memories

Memory Related Terms

Memory Module

• .

DATA Bus

.

.

.

Address Bus

Control Bus

Memory Related Terms

MemoryModule

• .

DATA Bus

Address Bus

Control Bus

Memory Related Terms

• By Packaging Styles

• SIMM---Single Inline Memory Module

• DIMM---Dual Inline Memory Module

• SO-DIMM--Small Outline DIMM

Memory Organization

• For the same amount of capacity, there are many different layout patterns.

• Such as, for 1Kbits memory, we can have:• a. 1,024 cells and 1,024 addresses, each • stores a “0” or “1”• b. 128 cells and 128 addresses, each cell • has 8bits or I byte• c. 1 cell and 1 address, the cell has 1024bits

Number of bits per cell forSome Commercial Computers

• Burroughs B1700 1

• IBM PC 8

• DEC PDP-8 12

• IBM 1130 16

• DEC PDP-15 18

• CDC 3600 48

• CDC Cyber 60

Cache Memory

• Why do we need cache memory?• Main memory is always slower than CPU• Main memory is far away from CPU• Faster memory can be made but neither

economic nor practical• Small amount of expensive faster memory

made close to the CPU will solve most of the problems

Cache Memory

• *** The most often used memory words are kept in the cache.***

CPU Main Memory

Cache

Bus

h-Hit Ratio of Cache Memory

• h = (k-1)/k where;

• k --- a word is written or read k times in a short interval and only need to reference the main memory 1 time.

• 1 - h is called miss ratio

• mean access time = c + (1 - h)m, where m is the time to reference the main memory

Byte Ordering

• The big endian and the little endian

0

4

8

12

0

4

8

12

0 1 2 3 0123

45674 5 6 7

8 9 10 11

12 13 14 15

891011

12131415

Address Address

The big endian stores higher digits in the lower bytes& The little endian stores higher digits in higher bytes

Byte Ordering

• The big endian and the little endian

0

4

8

12

0

4

8

12

J I M

T

JIM

SMITS M I

H H

Address Address

The big endian stores higher digits in the lower bytes& The little endian stores higher digits in higher bytes

Big Endian & Little Endian

• Address store number 1234567h

• 100 101 102 103 104 105

• 01 23 45 67 xx xx Big

• 67 45 23 01 xx xx Little

What wrong with the two systems?

• There is nothing wrong when each system works alone (they are both internally consistent).

• However, when transfer information over the network, we have problems.

• We will also have problems when using a software from one machine to another.

• There is no easy way to make both compatible without a time consuming conversion.