Computer Science 1000

Terminology II

Storage a computer has two primary tasks

store data operate on data

a processor's primary job is to operate on data math operations move operations note that processors do have a very small amount of

storage

how the majority of data stored by the machine?

Storage there are a variety of storage media available for

computers: RAM hard drive removable media

these storage types are differentiated by: capacity price latency

first, we should determine what is being stored

Information Storage ask a non-computer person what their computer

stores programs/apps/games pictures songs/videos email documents (text)

what does it mean to store an object, like a piece of text, in a computer?

in other words, how is it represented?

Information Storageconsider a notebook (for

comparison)how is a piece of text

stored/represented? as a set of written symbols the set of available symbols

depends on your language individual symbols can be

combined into other objects (e.g. words, sentences)

Information Storage in a computer, information is

stored as a set of bits a bit is short for binary digit in simplest terms, a binary digit is

either 0 or 1 hence, information stored by a

computer is simply a set of 0s and 1s

Information Storage how does the computer store

other information? other information is encoded in

binary

the way that information is stored in binary depends on the information type

Information Storage numbers

people typically use numbers in decimal format represented by digits 0-9

any decimal number can be represented in binary form

for example, here are the first 16 integers in binary:

Dec Bin Dec Bin Dec Bin Dec Bin

0 0 4 100 8 1000 12 1100

1 1 5 101 9 1001 13 1101

2 10 6 110 10 1010 14 1110

3 11 7 111 11 1011 15 1111

Information Storage numbers – notes

the entire number is typically coded in binary, not each individual digit

e.g. 49 in binary is 110001, not 1001001

most numbers are stored as a fixed number of bits e.g. 32-bit numbers

each number stored as a 32-bit sequence smaller numbers are padded on left with zeroes (like decimal) e.g. 14 (1110) as 32-bit number:

00000000000000000000000000001110

Information Storage text

each character in a piece of text has a binary encoding

e.g. ASCII: 8-bit sequence each character has a unique 8-bit sequence

Information Storage image

a digital picture is made up of pixels (tiny squares)

each pixel stored as its coloureach colour has a unique binary encoding images will often indicate their colour depth

e.g. 24-bit colour uses 24 bits per colour example (RGB): pure red: 111111110000000000000000

Information Storagecontext

the previous representation of the colour red is also the binary representation of 16,711,680

so when we see that sequence, how do we determine what kind of data it is?

it's up to a program to interpret the number often, the file type is used as a hint different programs will interpret the same sequence

differently

example: kev.png

Information Storage representations

the previous was a brief introduction to how information is encoded, to facilitate understanding of memory and storage

later in the semester, we will consider an entire chapter on how information is stored, with topics like:

binary representation of negative numbers, and numbers with a decimal point (3.4)

other text representations (e.g. Unicode)

Information Storageunits and prefixes

byte: 8 bits (typically) most storage is measured in bytes, rather than bits hence, a 100 byte file would contain 800 bits

bits and bytes are typically abbreviated as b and B

hence, 80 B = 80 bytes = 640 b = 640 bits

Information Storage – Unit Prefixesbits and bytes are often abbreviated using

SI (metric) prefixes for example:

K (kilo) - e.g. kilobyte (KB)M (mega) - e.g. megabit (Mb)G (giga) - e.g. gigabyte (GB)T (tera) - e.g. terabyte (TB)

Information Storage – Units it is not always clear what the multiplier is when referring to main memory, we typically use powers of

two hence, the prefix kilo means multiply by 210 , and not 1000 hence, 1 KB = 1024 bytes, 3 KB = 3072 bytes ... other multipliers:

mega: 220 = 1048576 giga: 230 = 1073741824

when used in this context, known as binary prefixes

Information Storage – Units when referring to other storage types, we typically use powers

of 10 hence, the prefix kilo means multiply by 103, like you are used

to mega : 106

giga: 109

hence, 500 GB = 500,000,000,000 bytes when used in this context, known as decimal prefixes

Information Storage – Units the industry is not consistent

when you buy a 4 GB USB key, Windows will often report it as smaller, as it assumes that 4 GB = 4 x 230

Other Interesting Example

http://en.wikipedia.org/wiki/Binary_prefix

Storage Medianow that we know what is being stored, and

how to define it, let's consider different ways to store it

types we will consider:volatile storagepersistent storage

Volatile Storage typically referred to as memory defined as storage that requires a continuous power source to

maintain its state in other words, when its power source is disconnected, all

memory is erased and you lose your data

your CPU cache discussed previously would be considered volatile memory

however, RAM is the primary volatile storage on most computers

RAM Random Access Memory also referred to as main memory the location of your program and associated data

when your program is running example: consider a running web browser stores:

instructions (for your processor) images and text from the webpage things that you can't see (e.g. cookies, passwords)

RAM the most defining feature of a system's main

memory is its capacity the amount of information that it can store

modern consumer systems typically have 2-16 GB of RAM

4-8 GB is very common in 8 GB of RAM, you could store:

~4.2 million pages of text (~129 Encyclopaedia Britannica 2010 ed.) ~2000 songs

remember: for main memory, 1 GB = 230 bytes, not 109

http://pc.net/helpcenter/answers/how_much_text_in_one_megabyte

CPU – RAM

Why does RAM capacity affect performance? recall that RAM stores programs and data hence, the bigger the RAM, the more programs and

data it can store this means:

more programs can be loaded into memory at once* more data can be stored in main memory (important for

large media items like movies) certain programs (e.g. newer games) have minimum

memory requirements just to run

* this ignores a concept called virtual memory, discussed later

Why Random Access Memory? named because any location on RAM chip can be

accessed in (nearly) the same amount of time

compare this to sequential access memory example: magnetic tape storage items directly under the reader can be accessed quite

quickly feeding the tape to find other locations is extremely slow hence, RAM devices are typically much faster

Persistant Storage sometimes referred to as non-volatile memory defined as storage that maintains its state even

when no power source is connected in other words, state is maintained between power

interruptions although there are other potential forms of data corruption

many types of persistent storage hard drives optical drives key drives

Hard Drive also referred to as hard disk or simply disk the primary source of persistent storage on modern

machines like RAM

can store programs, documents, images, videos, etc

unlike RAM: items in persistent storage are typically not in use they are loaded into RAM from your hard drive in order to

be used

Hard Drive like RAM, the most defining feature of a hard drive

is its capacity typical consumer hard drives range in size from 500

GB to 4 TB consider 2 TB of hard disk space:

~1 billion pages of text (~30000 Encyclopaedia Britannica) ~500000 songs (mp3) remember: for persistent storage, 1 GB = 109 bytes, not

230

Hard Drive vs RAMRAM and hard drives store data in

fundamentally different waysdetails beyond scope of the class

one of the ways in which they differ is priceby price, let's consider $/GB (to be fair)note that certain things can affect this range

(e.g. laptop RAM is usually more expensive than desktop RAM)

RAM – Example

8 GB = $50-$60 $6.25/GB - $7.50/GB

Hard Drive - Example

1 TB = $70-$75 $0.07/GB - $0.075/GB

Hard Drive vs RAM persistence

hard drives are persistent, no data is lost when power is disrupted

RAM is volatile, loss of power = RAM is erased capacity

most consumer hard drives: 500GB – 2TB of HD most consumer RAM: 2GB – 16GB

price hard drives cost pennies per GB of storage RAM costs dollars (about a 100 times more)

what is the advantage of RAM over an HD?

Hard Drive vs RAManswer: speed!!RAM is fast compared to HDperformance measured in two ways

access timetransfer rate

Storage – Access Time time to retrieve a single random piece of data for modern RAM:

50 – 150 nanoseconds*

for modern hard drives: 5 – 15 milliseconds*

hence, RAM is the clear winner performance can vary depending on how data is

accessed**

*http://www.webopedia.com/TERM/A/access_time.html**http://queue.acm.org/detail.cfm?id=1563874

Storage – Transfer rate how much data can be transferred in a second for modern RAM:

6-17 GB/s

for modern hard drives: 50-120 MB/s*

again, RAM is the clear winner better technologies (e.g. SSD drives) improve HD

performance, but still much slower than RAM

*http://www.storagereview.com/ssd_vs_hdd

RAM vs. Hard Drive in summary, RAM has the ability to access

and transfer data much quicker for running programs, it is critical that data

latency be minimizedotherwise, your processor would always be

waitingalthough more expensive and less spacious,

RAM makes your current computer experience possible

Hard Drive – RPMsone other common feature listed with typical

hard drives is their RPMscommon values: 5400, 7200, 10000

RPMs stand for revolutions per minutebasically, more RPMs = better performance to understand why, we must how consider

how hard drives are constructed

Hard Drive – Construction data stored magnetically on platters,

which are just smooth round surfaces data is read/written by the head, which is

at the end of the arm mechanism that you see

these platters spin, and the arm moves to a particular location and reads the data that passes under it

hence, the faster it spins, the faster that data can be accessed

Hard Drive – SSDa newer technology than magnetic drivesno moving parts (quiet)considerable performance improvement

over magnetic hard drives throughput: 200-500 MB/s

considerably more expensiveover $1/GB