3 MemoryMemory is a collection of cells, each with a unique physical address for random (direct) accessmemory is divided into fixed-length units or wordsInformation that is stored in memory cells is in binary coded format:Instructions that make up programsData: text symbols, numbers, images, etc.
4 Information Representation The Binary System: Using On/Off Electrical States to Represent Data & InstructionsThe binary system has only two digits--0 and 1.Bit - binary digitByte - group of 8 bits used to represent one character, digit, or other value
5 Representing Information with Bit Combinations To encode entities (e.g., symbols), we need to assign a unique number to each entity (e.g., social security number). Binary encoding means that we assign a unique combinations of bits to each object.One bit can be either 0 or 1. Therefore, one bit can represent only two things.To represent more than two things, we need multiple bits. Two bits can represent four things because there are four combinations of 0 and 1 that can be made from two bits: 00, 01, 10,11.If we want to represent more than four things, we need more than two bits. In general, 2n bits can represent 2n things because there are 2n combinations of 0 and 1 that can be made from n bits.Q: how many bits do we need to encode all the 37 people in the class?
6 Information Representation Kilobyte approx bytes (actually 210 = 1024 bytes)Megabyte approx. 1,000,000 bytes (one million)Gigabyte approx. 1,000,000,000 bytes (one billion)Terabyte approx. 1 trillion bytesPetabyte approx. 1 quadrillion bytesFACTOIDs:The prefix “mega” in “megabyte” comes from the Greek word “megas” meaning “mighty” or “great.”The prefix “giga” in “gigabyte” comes from a Greek word meaning “giant.”The prefix “tera” in “terabyte” comes from a Greek word meaning “monster.”You might think that the largest unit of storage capacity is a petabyte, but in fact, there are also exabytes, zetabytes, and yottabytes.
7 Representing Text and Symbols To represent a text document in digital form, we simply need to be able to represent every possible character that may appear.There are finite number of characters to represent. So the general approach for representing characters is to list them all and assign each a number (represented in binary).An encoding scheme is simply a list of characters and the codes used to represent each one.To represent symbols, computers must use a standard encoding scheme, so that the same symbols have the same codes across different computers.Discussion question: How many possible different characters can Unicode represent and how is that derived?Answer: 2 to the 16th power=65,526 character combinations
8 ASCII Encoding SchemeASCII stands for American Standard Code for Information Interchange. The ASCII character set originally uses 8 bits to represent each character, allowing for 256 (or 28) unique characters.Discussion question: How many possible different characters can Unicode represent and how is that derived?Answer: 2 to the 16th power=65,526 character combinations
9 Representing Text and Symbols ASCII - the binary code most widely used with microcomputersEBCDIC - used with large computersUnicode - uses two bytes for each character rather than oneDiscussion question: How many possible different characters can Unicode represent and how is that derived?Answer: 2 to the 16th power=65,526 character combinations
10 The Parity BitParity bit - an extra bit attached to the end of a byte for purposes of checking for accuracyEven parity - sum of bits must come out evenEx: given code , the extended code is:Ex: given code , the extended code is:Odd parity - sum of bits must come out oddEven parity scheme
11 Representing Numbers The binary number system Decimal is base 10: 0,1,2,3,4,5,6,7,8,9Binary is base 2: 0,1Any decimal number can be converted to binary by doing base conversion from base 10 to base 2.Any binary number can be converted to decimal by doing base conversion from base 2 to base 10.
12 Number base 10 - decimal The Decimal Number 101 102 101 100 100’s 10’s 1’sx 1 =x =x =101
13 Number base 2 - binary The Binary Number 101 22 21 20 4’s 2’s 1’s 4’s 2’s 1’sx 1 =x =x =5
15 Binary Conversion - Examples = 866432168421Easier way to remember:Just add the values for each position where there is a 112864321684211= 181
16 Hexadecimal Representation Hexadecimal (Hex) = Base 16Hex digits: 0, 1, 2, …, 9, A, B, C, D, E, FDecimalHexBinary000010001200103001140100501016011070111DecimalHexBinary810009100110A101011B101112C110013D110114E111015F1111
17 Hexadecimal Representation Hex can be used as a short hand for long binary stringsUse one Hex digit to represent every group of 4 bitsStart from the right and an go left grouping 4 bit sequencesAdd leading 0’s if the last group has less then 4 bitsAD65B
18 Hexadecimal Representation What is Hex 4C8F in binary?4 C F0100110010001111
19 Representing Images as Bit maps Image is collection of dots (pixels)Pixel = “picture element”Black & white: one bit per pixelColor: each pixel represented by combination of green, red, blue in varying intensity, to form all colors. Three bytes per pixel: one byte (8 bits) for each color intensity, valueUsually each byte is represented in HexD4 7F 59 red (D4), green (7F), blue (59)For example, D4 is binary which is decimal value 212Bit maps are not efficient3 byte/pixel, for 1280 x 1024 pixels = several megabytesImage cannot be enlarged, since pixels get bigger and image gets grainy or “blocky”.GIF and .JPEG formats compress images
20 Image Formats GIF JPEG (JPG) Graphics Interchange Format Developed by Compuserve (ISP)Stores only 256 colorsLoses some picture quality but is simple and fastCommon in computer action gamesJPEG (JPG)Joint Photographic Experts GroupStores differences between adjacent pixels, not absolute valuesUses variable-length data (values take a minimum number of bits to store), uses only 5% of the space of bitmaps
21 Image Formats Vector Images Pixels are not mapped Equations for the lines and curves making up the image are storedImage is stored as the instructions for drawing the imageImages are easily scaledModern type fonts are vector imagesUsed in computer aided design (CAD) systems for “blueprint” drawingsGood for three-dimensional drawingsWindows metafile (.wmf) or Visio (.vsd)Cannot produce photographic images