Computer science 1000
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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

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Computer Science 1000

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Computer science 1000

Computer Science 1000

Terminology II


Computer science 1000

  • 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?


Computer science 1000

  • 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


Computer science 1000

  • 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?


Computer science 1000

  • Information Storage

    • consider 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)


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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:


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • Information Storage

    • image

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

      • each pixel stored as its colour

      • each 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


Computer science 1000

  • Information Storage

    • context

      • 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


Computer science 1000

  • example: kev.png


Computer science 1000

  • 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)


Computer science 1000

  • Information Storage

    • units 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


Computer science 1000

  • Information Storage – Unit Prefixes

    • bits 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)


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • Other Interesting Example

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


Computer science 1000

  • Storage Media

    • now 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 storage

      • persistent storage


Computer science 1000

  • 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


Computer science 1000

  • 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)


Computer science 1000

  • 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


Computer science 1000

  • CPU – RAM


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • Hard Drive vs RAM

    • RAM and hard drives store data in fundamentally different ways

      • details beyond scope of the class

    • one of the ways in which they differ is price

      • by 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)


Computer science 1000

  • RAM – Example

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


Computer science 1000

  • Hard Drive - Example

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


Computer science 1000

  • 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?


Computer science 1000

  • Hard Drive vs RAM

    • answer: speed!!

    • RAM is fast compared to HD

    • performance measured in two ways

      • access time

      • transfer rate


Computer science 1000

  • 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


Computer science 1000

  • 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


Computer science 1000

  • 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 minimized

      • otherwise, your processor would always be waiting

      • although more expensive and less spacious, RAM makes your current computer experience possible


Computer science 1000

  • Hard Drive – RPMs

    • one other common feature listed with typical hard drives is their RPMs

      • common values: 5400, 7200, 10000

    • RPMs stand for revolutions per minute

    • basically, more RPMs = better performance

    • to understand why, we must how consider how hard drives are constructed


Computer science 1000

  • 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


Computer science 1000

  • Hard Drive – SSD

    • a newer technology than magnetic drives

    • no moving parts (quiet)

    • considerable performance improvement over magnetic hard drives

      • throughput: 200-500 MB/s

    • considerably more expensive

      • over $1/GB


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