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 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)
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 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
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
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 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
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) T (tera) - e.g. gigabyte (GB) - 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 2 10, and not 1000 hence, 1 KB = 1024 bytes, 3 KB = 3072 bytes... other multipliers: mega: 2 20 = 1048576 giga: 2 30 = 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 10 3, like you are used to mega : 10 6 giga: 10 9 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 2 30
Other Interesting Example http://en.wikipedia.org/wiki/binary_prefix
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
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 = 2 30 bytes, not 10 9 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 = 10 9 bytes, not 2 30
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)
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 price most consumer hard drives: 500GB 2TB of HD most consumer RAM: 2GB 16GB 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 RAM answer: speed!! RAM is fast compared to HD performance measured in two ways access time transfer 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 minimized otherwise, your processor would always be waiting although more expensive and less spacious, RAM makes your current computer experience possible
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
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 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