number systems
TRANSCRIPT
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Problem-solving using “computers”
• Computers solve “computable” problems
A ProblemA ProblemDescribing
The Problemin Math.
Describing The Problem
in Math.
“Computing”The
CorrespondingMath.
Problem
“Computing”The
CorrespondingMath.
Problem
ReturningThe Result
ReturningThe Result
Solution To The Problem
Solution To The Problem
Human problem-solving v.s. computer-based problem-solving
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Transformation
input
think
act
input
compute
act
Binary system
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The Number System
• A number system is to enumerate the “states” of something– For example, money, days, month, year,
minutes, hours, …
• In human’s world, we have very good tools
…which counts 10 states
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The Needs of Number Systems
• However, there are some states are not Decimal. For example,– Days and weeks– Minutes and hours– Months and year
• We need convenient representations for different “nature states”
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Egyptian numerals (10-based)
Source: http://www-gap.dcs.st-and.ac.uk/~history
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Babylonians numerals (60-based)
Source: http://www-gap.dcs.st-and.ac.uk/~history
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1*603 + 57*602 + 46*60 + 40*600 = 424000
Source: http://www-gap.dcs.st-and.ac.uk/~history
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Chinese numerals (10-based)
4359
45698
Source: http://www-gap.dcs.st-and.ac.uk/~history
壹貳參肆伍陸柒捌玖拾佰仟萬億兆京…
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The Number Systems
• A number system specifies how/what “numbers” represent and operate.
• A K-based number system uses N different symbols to represent N different entities or “states”
• Each symbol is a “digit”
• Multiple digits are used for describing M>N states
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K-based Number System
• 2-based: ON/OFF; Yes/No; T/F; 1/0; etc.
• 3-based: A/B/C; True/False/Unknown; 0/1/2; etc.
• …
• 7-based: Mon/Tue/Wes/Thu/Fri/Sat/Sun; 0/1/2/3/4/5/6; etc.
• 8-based: ; 0/1/2/3/4/5/6/7
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K-based Number System
• 10-based: /////////; 0/1/2/3/4/5/6/7/8/9; etc.
• 16-based: ///////////; 0/1/2/3/4/5/6/7/8/9/A/B/C/D/E/F; etc.
State-1State-2
State-3State-4
State-5State-6
State-7State-8
State-9State-10
State-11State-12
State-13State-14
State-15State-16
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K-based Number SystemIn the 16-based number system:
0/1/2/3/4/5/6/7/8/9/A/B/C/D/E/F; etc.
Why not “10” 2 digits
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More States
In a 6-based number system : state 1 (empty state) (0) : state 2 (1st) : state 3 (2nd) : state 4 (3rd) : state 5 (4th) : state 6 (5th)
: state 7 (6th) : state 8 (7th) : state 9 (8th) : state 10 (9th) : state 11 (10th) : state 12 (13th)
• …
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More States
In a 6-based number system
• 0: state 1 (empty state) (0)
• 1: state 2 (1st)
• 2: state 3 (2nd)
• 3: state 4 (3rd)
• 4: state 5 (4th)
• 5: state 6 (5th)
• 10: state 7 (6th)
• 11: state 8 (7th)
• 12: state 9 (8th)
• 13: state 10 (9th)
• 14: state 11 (10th)
• 15: state 12 (13th)
• 20: state 13 (14th)
• …
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The Number of Cases
• Generally, in a K-based number system,– 1 digit describes K1 cases– 2 digits describes K2 cases– 3 digits describes K3 cases– N digits describes KN cases
• For example,– 2 3-based (0,1,2) digits: 00, 01, 02, 10, 11, 12, 20, 21,
22 (32=9 cases)– 3 10-based (0,1,2,..,9) digits: 000, 001, 002, 003, 004,
005, 006,007, 008, 009, 010, 011,…,998, 999 (103=1000 cases)
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K-based vs The Decimal system (10-based)
• In general, any K-based number N can be expressed as
• Usually, Nk is denoted as – (Ap-1Ap-2….A1A0.A-1A-2….A-q)k – Ap-1:Most Significant Digit, MSD– A-q :Least Significant Digit, LSD
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112/04/12 Copyright: [email protected] 19Source: 計算機概論 , 王孝熙著 , 東華書局 .
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Using octal and hex numbers
• Computers use binary, but the numbers are too long and confusing for people
• Translation between binary and octal or hex is easy
– One octal digit equals three binary digits 101101011100101000001011 5 5 3 4 5 0 1 3– One hexadecimal digit equals four binary digits 101101011100101000001011 B 5 C A 0 B
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112/04/12 Copyright: [email protected] 21Source: 計算機概論 , 王孝熙著 , 東華書局 .
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112/04/12 Copyright: [email protected] 22Source: 計算機概論 , 王孝熙著 , 東華書局 .
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112/04/12 Copyright: [email protected] 23Source: 計算機概論 , 王孝熙著 , 東華書局 .
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112/04/12 Copyright: [email protected] 24Source: 計算機概論 , 王孝熙著 , 東華書局 .
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112/04/12 Copyright: [email protected] 25Source: 計算機概論 , 王孝熙著 , 東華書局 .
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112/04/12 Copyright: [email protected] 26Source: 計算機概論 , 王孝熙著 , 東華書局 .
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The binary number system
• In modem computers, the binary number system is easier to be implemented.
• Currently, they are implemented in silicon chips (VLSI)– Circuits for computation
01111
0001110010
15
3+10
+2 18
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The binary number system
• The binary (base 2 or 2-based) number system uses two “binary digits, ” (abbreviation: bits) -- 0 and 1
• A bit is a single two-valued quantity: yes or no, true or false, on or off, high or low, good or bad
• One bit can distinguish between two cases: T, F (True/False; Yes/No)
• Two bits can distinguish between four cases: TT, TF, FT, FF• Three bits can distinguish between eight cases: TTT, TTF,
TFT, TFF, FTT, FTF, FFT, FFF• In general, n bits can distinguish between 2n cases• A byte is 8 bits, therefore 28 = 256 cases
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Data Processing in Computers with the Binary System
• Data processing in binary computers– Numeric data: +2, -5.5, 50000.235698, etc.– Alphanumeric data: characters, name, address,
telephone number, etc.
• Two main tasks: – All data are converted into proper binary
formats (bits)– Rules for counting and computing these bits
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Part I: Numeric Data• Numeric data:
– integers, real – positive, negative
• Representation of numeric data– Sign-magnitude– 1’s Complement – 2’s Complement
• Negative/Positive in a n-bit integer I
I=(An-1An-2….A1A0)2
– An-1= “0”, I is positive– An-1=“1”, I is negative
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Sign-Magnitude• An n-bit integer I=
– Min: 00000…0 (n-1 0’s)=0– Max: 11111…1 (n-1 1’s)=2n-1-1– +0= (0000)2 vs –0=(1000)2
– +3= (0011)2 vs –3=(1011)2
• Disadvantages– 2 different 0’s (+0/-0 )– Not easy to be implemented in simple circuits (usually ad
der)
(An-1An-2….A1A0)2
sign:0(+), 1(-) magnitude
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1’s Complement• An n-bit integer I=
– For positive integers: the same as that in sign-magnitude
– For negative integers: complement their positive representations
– For example: 4-bit integers: • +3=(0011)2, -3=(1100)2
• +0=(0000)2, -0=(1111)2
• Disadvantages– 2 different 0’s (+0/-0)– Easy to be implemented in simple circuits (usually adder) but not
as efficient as 2’s complement
(An-1An-2….A1A0)2
sign:0(+), 1(-) magnitude
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2’s Complement• An n-bit integer I=
– For positive integers: the same as that in sign-magnitude
– For negative integers: find their 1’s complement + 1• For example: 4-bit integers:
• +3 = (0011)2 , -3 = (1100)2+1 = (1101)2
• +0 = (0000)2, -0 = (1111)2 + 1 = (0000)2
• There is only one “0”
(An-1An-2….A1A0)2
sign:0(+), 1(-) magnitude
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Range of integersSuppose that we use a byte (8-bit) to represent an integer
Max. (+) Max. (-) +0 -0
Sign-Magnitude
(01111111)2=+127 (11111111) 2=-127 (00000000) 2 (10000000) 2
1‘s Complement
(01111111)2=+127 (10000000) 2=-127 (00000000) 2 (11111111) 2
2‘s complement
(01111111) 2=+127 (10000000) 2=-128 (00000000) 2 (00000000) 2
Positive numbers Negative numbers
Sign-Magnitude 0~2n-1-1 -(2n-1-1)~01‘s Complement 0~2n-1-1 -(2n-1-1)~02‘s complement 0~2n-1-1 -(2n-1)~0
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Other Binary Codes for Decimal Numbers
• BCD code
• 2421 code
• Excess-3 code
• 84-2-1 code
• 4-bit for a number (0~9)
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8421 BCD Code• In the 8421 Binary Coded Decimal (BCD) representation
each decimal digit is converted to its 4-bit pure binary equivalent.
• Each decimal number maps to four bits and is weighted by its bit-position (each bit represents a number, 1, 2, 4, 8)
• BCD code is also called as 8421 code
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4221 BCD Code
• In 4221 BCD code, each bit is weighted by 4, 2, 2 and 1 respectively.
• Unlike BCD coding there are no invalid representations.
Decimal 42211's
complement0 0000 11111 0001 11102 0010 11013 0011 11004 1000 01115 0111 10006 1100 00117 1101 00108 1110 00019 1111 0000
advantages
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Excess-3 Code• Add 3 for each binary number
• Examples– Excess-3 code of “210” = (0010)2 +(0011)2
=(0101)2
– Excess-3 code of “510” = (0101)2+(0011)2=(1000)2
• Advantages
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84-2-1 Code• 4-bit for each number (0~9) weighted (left to righ
t) as 8, 4, -2, and -1 。• Example
– 84-2-1 code of “3” = 0101 (0+4+0+(-1)=3)
– 84-2-1 code of “5” =1011 (8+0+(-2)+(-1)=5)
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Gray Codes
• In pure binary coding or 8421 BCD, counting from 7 (0111) to 8 (1000) requires 4 bits to be changed simultaneously. If this does not happen then various numbers could be momentarily generated during the transition so creating spurious numbers which could be read.
• In gray coding, only one bit changes between subsequent numbers.
• Generating gray codes: start with all 0s and then proceed by changing the least significant bit (LSB) which will bring about a new state.
• Advantages: fast, relatively free from errors.
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Gray Codes
• Gray code is not unique, there are many possibilities to generate gray codes
• For example, – G1={0=00, 1=01, 2=11, 3=10}
– G2={00=10, 1=11, 2=01, 3=00}
• Unique gray code: reflected Gray code
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Floating-Point Representation
• Integers are fixed-point numbers in binary computers• Floating-point literals are written with a decimal point:
8.5 -7.923 5.000 • Real numbers are represented as “floating-point” numbers
in binary system (all computers)• The representation of floating-point numbers are various in
different CPU• In Intel 80486 CPU
– Single Precision: 32 bits– Double Precision: 64 bits– Extended Precision: 80bits
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Floating-point literals
• Floating-point numbers may also be written in “scientific notation”– times a power of 10
• We use E to represent “times 10 to the”• Example: 4.32E5 means 4.32 x 105
http://www.nuvisionmiami.com/books/asm/workbook/floating_tut.htm
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Example
Binary ValueBiased
Exponent Sign, Exponent, Mantissa
- 1.11 127 1 01111111 11000000000000000000000
+1101.101 130 0 10000010 10110100000000000000000
- .00101 124 1 01111100 01000000000000000000000
+100111.0 132 0 10000100 00111000000000000000000
+.0000001101011 120 0 01111000 10101100000000000000000
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Binary Arithmetic on Numeric Data
• Binary addition
• Binary subtraction
• Binary multiplication
• Binary division
To be discussed in Unit 5
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Part II: Alphanumeric Data• Alphanumeric data: character, letter, symbol, digit)• Not for calculation, but for representation some “mea
nings”• Coding
– ASCII(as-kee):America Standard Code for Information Interchange
– EBCDIC(eb-ce-dick):Extended Binary Coded Decimal Interchange Code (used by IBM, UNIVAC mainframes)
– BIG-5: for Chinese characters
– Uni-code
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ASCII Code• 7-bit for each character, 27=128 combinations for 128 characters
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Extended ACSII8-bit for each character, 28=256 combinations for 256 characters 。
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Examples
N: 4E=00101110 a: 61=01100001t: 74=01110100i: 69=01101001o: 6F=01101111 n: 6E=01101110a: 61=01100001l: 6C=01101010
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EBCDIC
• 8-bit for each character – 4-bit Zone bits: identifying the code is for char
acter, (un)signed number, or symbols– 4-bit Digit bits: for numbers 0~9 。
http://www.natural-innovations.com/computing/asciiebcdic.html
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Big5 Code
• Used for Chinese characters
• 1 character = 2 bytes (16 bits)
• Example
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Unicode
• ASCII was very simplistic, and so was extended by adding 'extended' sets by various manufacturers. Apart from being confusing this was still restricted to 256 characters.
• Now computers are more widely established around the world the need to show other characters such as Japanese and Chinese languages along with various symbols became more important.
• 2 bytes (16-bit) for each unicode
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Binary Operations on Alphanumeric Data
• Binary addition?
• Binary subtraction?
• Binary multiplication?
• Binary division?
To be discussed in Unit 5
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Part III: Extensions
• Everything in the computer is stored as a pattern of bits– Binary distinctions are easy for hardware to work with
• Numbers are stored as a pattern of bits– Computers use the binary number system
• Characters are stored as a pattern of bits– One byte (8 bits) can represent one of 256 characters
• So, is everything in the computer stored as a number?– No it isn’t, it’s stored as a bit pattern
– There are many ways to interpret a bit pattern
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Extension: Image representation
• Nature creations are analog, not digital• Digitizing (sampling)• Resolution: 800*600, 1026*768
Original 7*7 digitizing 14*14 digitizing
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For Black/White Images
Original 7*7 digitizing 14*14 digitizing
00000001111100…
0000000000000000000000000000011111111100000111111111100001111111111110…
00000001111100…
0000000000000000000000000000011111111100000111111111100001111111111110…
(01F0..)2
(00000007FA1..)2
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Extension: Sampling
Max. AMP/16
t1 t2 t3 t4 …
16=24
t1: 8=(1000)2
t2: 9=(1001)2
t3: 7=(0111)2
t4: 2=(0010)2
t1: 2=(0010)2
…
Quality vs sampling rate