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1 Content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Questions for Primary Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

4 Memory In a Computer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

5 Secondary Memory, In Brief . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

6 Secondary Memory types, In Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

7 Primary Memory, In Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

8 Primary Memory, Characteristics and Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

9 Primary Memory Types, In Comprehensive Detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

10 Primary Memory, Why do computers need it? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

11 Reference and Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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Questions for Primary Memory

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Introduction

A computer, as shown in Figure (1), is the most sophisticated electronic

device that takes input data from a user such as keyboard and mouse as data

entries or microphone and voice recognition software as voice entries, store

the data until it is ready to work on permanent and slowly memory devices

such as hard-drive and flash memory or temporarily and fast memory devices

such as RAM memory and catch memory, then processes and executes the

data using processor such as CPU, and finally produces output through LCD

screen, stereo loudspeakers, inkjet or LaserJet printers. All the components of

the computer, such as the input devices, the processor CPU, the hard-drive,

the output devices, and the operating system, works together as one team,

and the memory is an essential element of the computer in the team.

Figure (1): Components of the Computer

Memory in a computer

Memory is a form of electronic storage that plays an important role in

storing and retrieving data (Agrawal 2012). It is used most often to identify

fast, temporary forms of storage. There are many types of memory that

become an integer part of daily vocabulary such as RAM, ROM, Flash

memory, and Video memory, and many electronic devices that use memory

such as Mobile phones, Microsoft Xbox, Sony PlayStation, iPods, and

computer tablets. In this paper, the focus will be on computer's memory and

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why is it important for a computer to have a primary memory? And without a

primary memory, can the computer perform its functions or might not work at

all?

The memory as it is used in the computers can be further classified as

primary or main memory and secondary or auxiliary memory. The

secondary memory is an external and permanent storage used to hold and

store data until it is erased or overwritten. It is used for two reasons: to

overcome the size and the capacity limitation of the primary memory and the

data is not lost or disappear when the computer power is turned off (Mounika

n.d.).

Figure (2): Primary and Secondary memory

Secondary Memory, In Brief

The secondary memory devices will provide a number of benefits and

features, some of which are (How Computers work: the CPU and Memory):

Portability: It is connected to computers through cables, and the data

can be saved and transported to another location or computer

Security: sensitive data can be copied (backups) and kept in safe

places

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High capacity and large size: It is capable to store huge amount of

information such as operating systems, other applications software,

videos, audios, and pictures. The size ranges between 80 GB

(Gigabyte) up to 4 TB (Terabyte) Hard Disc Drives

Non-volatile: the data is not lost or disappear when the computer

power is turned off

CPU processor: It is very slow in interacting with the CPU when

compared with the primary memory

Cost: It is very cheap and affordable comparing to the primary

storages

Secondary Memory Types, In Summary

There are different types of secondary storage devices, each of them

suitable for a different purpose. Currently the most common forms of

secondary storage device are (Introduction to Information Technology):

Hard disks: Hard disks are designed to store very high volume of

data. Currently hard disks can store gigabytes of data up to 4

terabytes and they are an integral part of the computer. Most operating

systems are stored in hard disks and all materials (application

software, documents, images, music files etc) that are saved on

computers are stored in the hard disk. Hard disks are now can be

connected externally to computers via USB ports called external hard

disks. They are mostly used for back up purposes or transferring of

large amount of data.

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Figure (3): Hard-drive

Floppy Disks: Floppy disks mostly referred to as diskettes and widely

used from the 1970s to the 1990s. Storage capabilities ranged from

1.5 Mb to 200 MB on some versions.

Figure (4): Floppy Disk

Compact Disks (CD): A Compact disk is part of secondary storage

devices called optical disks invented by Sony and Philips. It is also

known as a WORM (write once read many) medium. Usually has a

storage capacity of 700 MB. Currently there exist three basic types of

CDs that you will surely make much use of: CD-ROM, CD-R, and CD-

RW. Basically all of them have the same storage capacity except that

they differ from their read-write capability.

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Figure (5): CD-R

Digital Versatile Disk (DVD): Digital Versatile Disks (DVDs) are now

commonly used for the distribution of movies since they offer high

capacity storage medium with good quality. They have a storage

capacity of usually 4.1 GB. There is also an 8.54-GB dual-layer

version. Currently there exist three basic types of CDs that you will

surely make much use of: DVD ROMs, DVD R, and DVD+RW.

Figure (6): DVD-R

Magnetic Tape: Magnetic tape has been in use for more than 50

years and provides a very effective means for back up purposes of

large amount of data. It is in recent years packaged in

cartridges/cassettes. The average amount of storage is 5 MB to 140

MB for every standard-length reel (2,400 feet).

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Figure (7): Magnetic tapes

USB Drives: A USB flash drive is a type of flash memory storage

device integrated with a universal serial bus interface. Usually portable

and rewritable, some can hold up to 300 GB.

Figure (8): USB Flash memory

Primary Memory, In Detail

The Central Processing Unit (CPU) is a fast and powerful device and it

can access and process data at the speed of about 1 gigahertz in which is

equivalent to billions of bytes per second. The secondary memory is

considered to be the slowest form of memories and for the CPU to read from

the hard-drive, it must go through multiple steps to send a request to the

hard-drive to retrieve the data it wants and this process it might take a good

bit of time, approaching a second. This slowness of the hard-drive might

force the CPU to stop its work and wait for a long time. The CPU needs a fast

memory that can cope with its speed and here comes the need for the

primary memory.

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As seen in Figure (9), the CPU accesses memory according to a distinct

hierarchy, whether the data comes from the hard-drive or the keyboard, the

data goes to the primary memory first. When a user turns on the computer

and until it turns off, the CPU is constantly using memory as follows (Tyson

n.d.) :

Figure (9): Memory hierarchy

When the computer's power is turned on, the CPU runs a set of

instructions kept in the ROM and performs Power on Self Test (POST) to

check the CPU, memory, BIOS, hard-drives, keyboard and mouse to

ensure that they are functioning properly.

The computer then loads the Basic Input/output System (BIOS) from

Read-Only Memory (ROM) to find the Operating System (OS). The OS is

loaded from the (C:) drive or the hard-drive. The BIOS is able to load the

OS from a CD, USB memory or ZIP drive.

The computer loads the OS into Random Access Memory (RAM). When

any application is opened such as interactive games, music, youtube.com

website, or word processing document, it will be loaded into RAM. The

CPU requests the data it needs from RAM, processes it and writes new

data back to RAM in a continuous cycle. In most computers, this shuffling

of data between the CPU and RAM happens millions of times every

second. When an application is closed, it and any accompanying files are

usually deleted from RAM to make room for new data.

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The primary memory is an internal and temporary storage device resides

inside the computer's motherboard and the CPU processor accesses first,

directly and constantly. The primary memory is where the computer stores all

of its instructions, data, and application programs are currently working and

running. This includes several types of memory, such as the CPU processor

registers, the cache memory, the ROM memory, and the RAM memory.

Primary memory of modern computer systems is made of the semiconductor

devices.

Primary Memory, Characteristics and Features

The primary memory devices will provide a number of benefits and

features, some of which are (How Computers work: the CPU and Memory):

Portability: it is connected to the computer via slots and memory bus.

Low capacity and small size: Capacity is total amount of information

that a medium can hold and is usually expressed in bytes; and primary

memory capacity can range from 512 MB (Megabyte) up to 8 GB

(Gigabyte). Size is the estimate or the measure of something's

dimensions; and the primary memory size is very small comparing to

the size of secondary memory.

Non-volatile and volatile: A volatile memory means that the computer

storage does not lose its data or content when the power is turned off.

In contrary with the volatile memory, the non-volatile memory only

maintains its data or content while the computer's power is on; when

the computer is turned off, the memory content or data will be erased

or lost. The primary memory can be either non-volatile such as RAM or

volatile such as ROM.

CPU processor: the primary memory provides very high performance

because it provides a very fast memory that can cope with the speed of

the CPU processor. Data on primary memory is accessed about a

million times faster than data on secondary memory. The access time

to read or write any particular byte can range from a few nanoseconds

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(a thousand millionth of a second) up to 100 nanoseconds (a thousand

millionth of a second).

Cost: the primary memory costs more money than the secondary

memory. As shown in Figure (10), the more the memory becomes

faster, the more it becomes expensive; the price could reach $150 per

MB (per mega byte).

Figure (10): Primary Memory Cost, Speed and Size VS. Cost

Primary Memory Types, In Comprehensive Detail

The following terms comes under the primary memory of a computer are

discussed below:

CPU Registers

CPU registers are the top of the memory hierarchy, as shown in Figure (9),

and the fastest and smallest of all primary memory that are built into the CPU

(Central Processing Unit). They contain specific data needed by the CPU,

particularly the arithmetic and logic unit (ALU). All data must be represented in

the register before it can be processed. For example, if two numbers are to be

multiplied, both numbers must be in registers, and the result is also placed in

a register (CPU Registers).

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Figure (11): CPU Register

Registers are normally measured by the number of bits they can hold, the

number of registers that a CPU has and the size of each (number of bits) help

determine the power and speed of a CPU. The register size can be 16, 32, or

64 bits wide, which allow instructions up to the register size. If a program is

designed to process 64 bit instructions, a CPU processor with a 32 bit register

would not be able to run the program (Computer Registers).

The CPU processor registers with their names, size and functions are

listed below (Computer Registers):

Register Symbol

Register Name # of Bits

Description

AC Accumulator 16 Processor Register

DR Data Register 16 Holds memory data

TR Temporary Register 16 Holds temporary data

IR Instruction Register 16 Holds instruction code

AR Address Register 12 Holds memory address

PC Program Counter 12 Holds address of next instruction

INPR Input Register 8 Holds input data

OUTR Output Register 8 Holds output data

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Cache Memory

Cache memory was invented to solve a significant problem that is the

growing gap between the CPU processor clock speed and performance with

the main memory (RAM memory) speed and performance. As this gap grew,

it became increasingly clear that a new type of fast memory was needed to

bridge the gap (Hruska 2014).

Cache memory acts as a buffer between the CPU processor and the main

memory (RAM memory), and holds a copy of only the most frequently used

information or program codes stored in the main memory (RAM memory)

(Computer Fundamentals Computer - Memory).

Cache memory is extremely fast memory that is built into a computers

central processing unit (CPU) and referred to it as Level 1 (L1) cache, or

located next to it on a separate chip and referred to it as Level 2 (L2). Some

CPU processors have both L1 and L2 cache built-in and designate the

separate cache chip as Level 3 (L3) cache. Cache memory that is built into

the CPU processor is faster than separate cache, running at the speed of the

CPU processor itself. However, the separate cache memory is still roughly

twice as fast as Random Access Memory (RAM). Cache memory is more

expensive than the RAM memory, but it is well worth getting a CPU processor

and motherboard with built-in cache memory in order to maximize system

performance (Davesh n.d.).

Figure (12): L1 and L2 Cache Memory

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In order to understand the need for cache memory, Level-1 (L1) cache,

Level-2 (L2) cache, and Level-3 (L3) cache, someone has to have a full

understanding of how CPU processor works or how it processes. As shown in

Figure (13), the process flows (Davesh n.d.):

Figure (13): Process Flow from CPU to RAM memory

When the CPU processor starts to perform an operation or execute a

command, the first step is to check the registers. If the required data is not

found there, then it looks in (L1) cache memory, and if also not found

there, then it goes further to (L2) cache memory and further to (L3) cache

memory. Whenever the data needed by CPU processor is not found in the

cache memory it is known as CACHE MISS and it leads to delay in the

execution thus making the system slow. If the data is found in cache

memory it is known as CACHE HIT.

If the data needed is not found in any of the cache memory, the CPU

processor checks in RAM memory. And if this also fails then it goes to look

onto the slower storage device such as hard-drive or CD-drive.

So the above process can be graphically summarized as:

The goal of the cache memory is to ensure that the CPU processor

has the next bit of data it will need already loaded into cache

memory by the time it goes looking for it.

(L3) cache memory is faster than RAM but slower than (L2) cache

memory and (L1) cache memory is the fastest of all the cache

memory.

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To conclude, cache memory is used to make the system fast by

making the processing/execution fast.

ROM Memory

Read-only memory (ROM) is a nonvolatile memory, which means that the

data is maintained and lost when the computer's source power is turned off.

The Data stored in the ROM memory is either unchangeable or requires a

special operation to change it. It is located on a Basic Input / Output System

(BIOS) chip, which is plugged into the motherboard (Zandbergen n.d.).

The ROM plays a critical part in starting up the computer and performs

diagnostics, which also referred to as booting up the computer. When

someone turns on the computer, the BIOS wakes up and performs what is

called the power-on self-test (POST), which checks the various components

of the computer to make sure they are all present and functioning properly.

Once this test is completed, the CPU processor takes over and starts

launching the operating system (Zandbergen n.d.).

Figure (14): ROM Chip

The ROM memory is not only used in the computer but also in other

electronic items such as washing machine and microwave oven. ROM

memory is also often used in optical storage media such as various types of

compact discs, including read only memory (CD-ROM), compact disc

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recordable (CD-R) and compact disc rewritable (CD-RW) (Computer

Fundamentals Computer - Memory).

There are different types of ROM and each type has unique

characteristics, but all the types have two things in common: Data is

nonvolatile and unchangeable or requires a special operation to change it.

Programmable ROM (PROM)

PROM can be programmed only once by a special device and it cannot be

modified or erased. It is not expensive and has several different applications,

including cell phones, video game consoles, medical devices, and other

electronics (Tyson n.d.).

Figure (15): PROM chip

Erasable Programmable ROM (EPROM)

EPROM overcomes the problem of PROM, which is the chip can be

erased by exposing it to Ultra-Violet light for duration of 40 minutes and more.

To rewrite the EPROM, the chip must be first erased and the eraser is not

selective, the entire EPROM chip should be erased.

There are several different applications for the EPROM, to store computer

BIOS which is used in order to bootstrap the operating system of a computer.

EPROM is also often found in the development of video game cartridges

(Tyson n.d.).

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Figure (16): EPROM chip

Electrically Erasable Programmable ROM (EEPROM)

EEPROM overcomes some of the shortages in the EPROM, which are:

The chip can be erased electrically and it can be programmed and erased

ten thousand times.

The erasing and programming take about 4 to 10 millisecond.

The chip can be erased one byte at a time, rather than erasing the entire

chip.

The chip does not have to be removed to be rewritten.

There are several different applications for the EEPROM, including Flash

drive, potentiometers, digital clocks and digital temperature sensors (Tyson

n.d.).

Figure (17): EEPROM chip

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Random Access Memory (RAM) Memory

RAM memory (also known as memory, main memory, system memory)

refers to the fact that data that is stored anywhere on the RAM memory and it

can be accessed directly regardless of its (random) location. This is in

contrast with other types of data storage such as hard drives where they have

to spin to the data's location first before being able to access it. It is small,

both in physical size and in the amount of data it can hold. It's much smaller

than your hard drive (What is RAM and How RAM Works).

RAM memory is a volatile memory; the data in RAM memory stays there

only as long as the computer is running. When the computer is turned on

again, the operating system and other files are once again loaded into RAM

memory, usually from your hard disk.

RAM memory is more expensive than hard drives. It will always cost more

per gigabyte. Today's computers come with 4 to 16 gigabyte (GB) of RAM

memory. Most computers are designed to allow adding additional RAM

memory modules up to a certain limit. Having more RAM memory in the

computer reduces the number of times that the CPU processor has to read

data in from the hard drive.

RAM memory is super fast compared to hard drives, but is slower than a

CPU processor's level-1 cache memory, level-2 cache memory or level-

3 cache memory. RAM memory access time is expressed in nanoseconds;

hard drive access time is expressed in milliseconds.

What is RAM memory used for in computers? Whenever someone runs a

program such as operating system, applications or open a file (e.g. videos,

images, music, documents), it is loaded temporarily from the hard drive into

the RAM memory. Once loaded into RAM memory, a user will be able to

access it smoothly with minimal delays.

There are different types of ROM and each type has unique characteristics

and applications:

http://searchstorage.techtarget.com/definition/cache-memory

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Dynamic RAM memory (DRAM)

DRAM stores data electrically in a memory cell, which uses a transistor

and a capacitor. The capacitor holds the bit of data (0 or 1), and the transistor

acts as a switch that lets memory chip read the capacitor or change its state.

It refreshes the memory cell every few milliseconds. It is very cheap. There

are several different applications for the DRAM including battery used in

operating systems, RAM memory used workstations and laptop computers, as

well as some video game consoles (Tyson and Coustan n.d.).

Figure (18): DRAM chip

Static RAM memory (SRAM)

SRAM uses a completely different technology, which is a flip-flop circuit for

a memory cell, takes four or six transistors along with some wiring, but never

has to be refreshed. It is very fast access speed, much faster than DRAM, but

it is more expensive and requires four times the amount of space for a given

amount of data compared to dynamic RAM.

There are several different applications for the SRAM, It can be found in

the cache memory of a computer or as part of the RAM digital to analog

converter on a video card. Static RAM is also used for high-speed registers,

caches and small memory banks like a frame buffer on a display adapter.

Several scientific and industrial subsystems, modern appliances, automotive

electronics, electronic toys, mobile phones, synthesizers and digital cameras

also use SRAM. It is also highly recommended for use in PCs, peripheral

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equipment, printers, LCD screens, hard disk buffers, router buffers and

buffers in CDROM / CDRW drives (Tyson and Coustan n.d.).

Figure (19): Static RAM chip

The following are some common types of RAM (Tyson and Coustan n.d.):

# Name Stand up for

1 FPM DRAM Fast page mode dynamic random access

memory

2 EDO DRAM Extended data-out dynamic random

access memory

3 SDRAM Synchronous dynamic random access

memory

4 DDR SDRAM Double data rate synchronous dynamic RAM

5 RDRAM Rambus dynamic random access

memory

6 Credit Card

Memory

7 PCMCIA Memory

Card

8 CMOS RAM

9 VRAM VideoRAM

Primary Memory, why do computers need it?

In conclusion, the primary memory is a very important component for the

computer. Besides all its features and benefits, it offers two vital roles that

without them the computer will not function properly or not work at all.

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The first role, it provides a very fast memory that can cope with the speed

of the CPU processor through the use of cache memory and RAM memory.

Data on primary memory is accessed about a million times faster than data on

secondary memory. The access time to read or write any particular byte can

range from a few nanoseconds (a thousand millionth of a second) up to 100

nanoseconds (a thousand millionth of a second).

The second role, it uses the ROM which plays a critical part in starting up

the computer and performs diagnostics, which also referred to as booting up

the computer. When someone turns on the computer, the BIOS wakes up and

performs what is called the power-on self-test (POST), which checks the

various components of the computer to make sure they are all present and

functioning properly. Once this test is completed, the CPU processor takes

over and starts launching the operating system.

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Reference and Resources

1. Agrawal, Sakash 2012, "Computer Memory? Different Types Of Memory in

Computer With Examples", available at

http://successnotes4u.blogspot.com/2012/02/computer-memory-different-

types-of.html

2. "Introduction to Information Technology", available at

http://vcampus.uom.ac.mu/cse1010e/index.htm

3. Mounika, Raghavaraph, "What are the differences between Primary and

Secondary Memory", available at http://readanddigest.com/what-are-the-

differences-between-primary-and-secondary-memory/

4. "How Computers Work: the CPU and Memory", available at

http://homepage.cs.uri.edu/book/cpu_memory/cpu_memory.htm

5. Tyson, Jeff, "How Computer Memory Works", available at

http://computer.howstuffworks.com/computer-memory.htm

6. "CPU Registers", available at http://www.sciencehq.com/computing-

technology/cpu-registers.html

7. "Computer Registers", available at

http://www.eazynotes.com/pages/computer-system-architecture/computer-

registers.html

8. Hruska, Joel 2014, "How L1 and L2 CPU caches work, and why they're an

essential part of modern chips", available

http://www.extremetech.com/extreme/188776-how-l1-and-l2-cpu-caches-

work-and-why-theyre-an-essential-part-of-modern-chips

9. " Computer Fundamentals Computer - Memory ", available at

http://www.tutorialspoint.com/computer_fundamentals/computer_memory.

htm

10. Davesh, "How Cache Memory Works", available at

http://www.engineersgarage.com/mygarage/how-cache-memory-works

11. Zandbergen, Paul, "Read-only Memory (ROM): Definition, Type & Quiz",

available at http://education-portal.com/academy/lesson/read-only-

memory-rom-definition-types-quiz.html

12. Tyson, Jeff, "How ROM Works", available at

http://computer.howstuffworks.com/rom.htm

http://successnotes4u.blogspot.com/2012/02/computer-memory-different-types-of.htmlhttp://successnotes4u.blogspot.com/2012/02/computer-memory-different-types-of.htmlhttp://vcampus.uom.ac.mu/cse1010e/index.htmhttp://readanddigest.com/what-are-the-differences-between-primary-and-secondary-memory/http://readanddigest.com/what-are-the-differences-between-primary-and-secondary-memory/http://homepage.cs.uri.edu/book/cpu_memory/cpu_memory.htmhttp://computer.howstuffworks.com/computer-memory.htmhttp://www.sciencehq.com/computing-technology/cpu-registers.htmlhttp://www.sciencehq.com/computing-technology/cpu-registers.htmlhttp://www.eazynotes.com/pages/computer-system-architecture/computer-registers.htmlhttp://www.eazynotes.com/pages/computer-system-architecture/computer-registers.htmlhttp://www.extremetech.com/extreme/188776-how-l1-and-l2-cpu-caches-work-and-why-theyre-an-essential-part-of-modern-chipshttp://www.extremetech.com/extreme/188776-how-l1-and-l2-cpu-caches-work-and-why-theyre-an-essential-part-of-modern-chipshttp://www.tutorialspoint.com/computer_fundamentals/computer_memory.htmhttp://www.tutorialspoint.com/computer_fundamentals/computer_memory.htmhttp://www.engineersgarage.com/mygarage/how-cache-memory-workshttp://education-portal.com/academy/lesson/read-only-memory-rom-definition-types-quiz.htmlhttp://education-portal.com/academy/lesson/read-only-memory-rom-definition-types-quiz.htmlhttp://computer.howstuffworks.com/rom.htm

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13. " What is RAM and How RAM Works ", available at

http://www.buildcomputers.net/what-is-ram.html

14. Tyson, Jeff, Coustan, Dave, "How RAM Works", available at

http://computer.howstuffworks.com/ram.htm

http://www.buildcomputers.net/what-is-ram.htmlhttp://computer.howstuffworks.com/ram.htm