Computer Science 1000

1. Computer Science 1000 Terminology II

2. 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?

3. 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

4. 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?

5. 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)

6. 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

7. 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

8. 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:

9. 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

10. 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

11. 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

12. 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

13. example: kev.png

14. 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)

15. 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

16. 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)

17. 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

18. 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

19. 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

20. Other Interesting Example http://en.wikipedia.org/wiki/Binary_prefix

21. 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

22. 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

23. 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)

24. 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

25. CPU – RAM

26. 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

27. 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

28. 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

29. 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

30. 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

32. 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)

33. RAM – Example • 8 GB = $50-$60  $6.25/GB - $7.50/GB

34. Hard Drive - Example • 1 TB = $70-$75  $0.07/GB - $0.075/GB

35. 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?

36. Hard Drive vs RAM • answer: speed!! • RAM is fast compared to HD • performance measured in two ways • access time • transfer rate

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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