# COMP2611: Computer Organization. Data Representation

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1 COMP2611: Computer Organization Comp2611 Fall 2015

2 2 1. Binary numbers and 2 s Complement

3 Numbers 3 Bits: are the basis for binary number representation in digital computers What you will learn here: How to represent negative integer numbers? How to represent fractions and real numbers? What is a representable range of numbers in a computer? How to handle numbers that go beyond the representable range? To be covered in Computer Arithmetic: Arithmetic operations: How to add, subtract, multiply, divide binary numbers How to build the hardware that takes care of arithmetic operations

4 Base, Representation and Value 4 Numbers can be represented in any base Human: decimal (base 10, has 10 digits 0,1,,9) Computer: binary (base 2, has 2 digits, 0,1) Positional Notation: value of the ith digit d is d x Base i = (1 x 2 3 ) + ( 1 x 2 2 ) + ( 0 x 2 1 ) + ( 1 x 2 0 ) 10 = Bits are grouped and numbered 0, 1, 2, 3 from right to the left: Byte: a group of 8 bits Word: a group of 32 or 64 bits most significant bit (MSb) least significant bit (LSb) Value of the 32-bit integer binary numbers = (b 31 x 2 31 ) + (b 30 x 2 30 ) + + (b 1 x 2 1 ) + (b 0 x 2 0 )

5 2's Complement for signed integer numbers 5 How can we represent negative integer values in binary? All computers use 2's complement representation for signed numbers The most significant bit is called the sign bit: When it is 0 the number is non-negative When it is 1 the number is negative The positive half uses the same representation as before The negative half uses the conversion from the positive value illustrated below: Ex: What is the representation of -6 in 2 s complement on 4 bits? i) Start from the representation of = 6 10 ii) Invert bits to get 1 s complement = iii) Add 1 to get 2 s complement = -6 10

6 Examples 6 Ex: What is the representation of -6 in 2 s complement on 8 bits? i) Representation of +6 ii) Invert: iii) Add = = = Ex: What is the representation of -6 in 2 s complement on 32 bits? i) Start from the representation of = 6 10 ii) Invert bits to get 1 s complement = iii) Add 1 to get 2 s complement = -6 10

7 More on 2 s Complement 7 1 s complement MSb as in sign Invert all the other bits Given a positive number, negate all bits to get negative equivalent We don t need 2 representations for 0 2 s complement = 1 s complement + 1

8 Why invert and add 1 works? 8 In One s Complement we have: if x = 0 then x =1 x + x = In 2 s complement = -1, therefore x x x 1 x

9 Representable Range 9 Largest integer represented by a 32 bit word: = (2 31 1) 10 = 2,147,483, Smallest integer represented by a 32 bit word: = = -2,147,483, Example: what is largest and smallest integer represented by 8 bits (16 bits) Largest integer = 0x7F = 127 = = = 0x7FFF = = = Smallest integer Invert and add 1: = = 2 7 = 128 => Invert add 1: = 0x8000 = 2 15 = =>

10 Overflow and Underflow in Signed Integer 10 Given the number of bits used in representing a signed integer Overflow (signed integer) The value is bigger than the largest integer that can be represented Underflow (signed integer) The value is smaller than the smallest integer that can be represented Large negative zero Large positive Underflow Representable range Representable range Overflow

11 Signed and Unsigned Numbers 11 Signed numbers negative or non-negative integers, e.g. int in C/C++ Unsigned numbers non-negative integers, e.g. unsigned int in C/C++ Ranges for signed and unsigned numbers 32 bit words signed: from = (2 31 1) 10 = 2,147,483, to = = -2,147,483, bit words unsigned: from = 0 10 to = (2 32 1) 10 = 4,294,967,295 10

12 Sign Extension 12 Consider using a cast in C/C++ on a 32 bit machine int i; /* signed integer represented on 32 bits */ char a; /* Character represented on 8 bits */ i = (int) a; What are the values of upper 24 bits in i? Similar things happen in hardware when an instruction loads a 16 bit number into a 32 bit register (hardware variable) Bits 0~15 of the register will contain the 16-bit value What should be put in the remaining 16 bits (16~31) of the register? Zero extension fills missing bits with 0 Bitwise logical operations (e.g. bitwise AND, bitwise OR) Casting unsigned numbers to larger width Sign extension is a way to extend signed integer to more bits

13 Sign Extension 13 Bits 0~15 of the register will contain the 16bit value What should be put in the remaining 16 bits (16~31) of the register? ???? Depends on the sign of the 16 bit number If sign is 0 then fill with 0 If sign is 1 then fill with 1 For example: 2 (16 bits -> 32 bits): > (16 bits -> 32 bits): > Does sign extension preserve the same value?

14 14 2. Floating Point Numbers

15 Why Floating Point? 15 In addition to signed and unsigned integers, we also need to represent Numbers with fractions (called real numbers in mathematics) e.g Very small numbers e.g., Very large numbers e.g., x (a number a 32-bit integer can t represent) In decimal representation, we have decimal point In binary representation, we call it binary point = (1 x 2 2 ) + ( 0 x 2 1 ) + ( 1 x 2 0 ) + ( 1 x 2-1 ) + ( 1 x 2-2 ) 10 = Such numbers are called floating point in computer arithmetic Because the binary point is not fixed in the representation

16 Scientific Notation & Normalized Scientific Notation 16 Scientific notation A single digit to the left of the decimal point e.g x 10-3, 0.5 x 10 5 Normalized scientific notation Scientific notation with no leading 0 s e.g x 10-3, 5.0 x 10 4 Binary numbers can also be represented in scientific notation All normalized binary numbers always start with a 1 1. xxx xxx two 2 yyy yyy two Example =

17 Single-Precision Floating-Point Representation 17 Single-precision uses 32 bits Sign-and-magnitude representation: s exponent Significand/Mantissa 1 bit 8 bits 23 bits Interpretation S = sign; F = significand; E = exponent Value represented = (-1) s x F x 2 E Roughly gives 7 decimal digits in precision Exponent scale of about to Compromise between sizes of exponent and significand fields: Increase size of exponent increase representable range Increase size of significand increase accuracy

18 Double-Precision Floating-Point Representation 18 Double-precision floating-point uses 64 bits In 32 bit architectures like MIPS, each double-precision number requires two MIPS words 11 bits for exponent, 52 bits for significand s Exponent Significand/Mantissa 1 bit 11 bits 20 bits Significand/Mantissa (continued) 32 bits Provides precision of about 16 decimals Exponent scale from to

19 IEEE 754 Floating-Point Standard 19 Most computers use this standard for both single and double precision Why use a standard floating-point representation? Simplify porting floating-point programs across different computers To pack even more bits into the significand This standard makes the leading 1 bit (in 1.xx xxx) implicit Interpretation: (-1) s x (1 + 0.significand) x 2 E Effective number of bits used for representing the significand: 24 (i.e., ) for single precision 53 (i.e., ) for double precision Special case: Since 0 has no leading 1, it is given the reserved exponent value 0 so that the hardware does not attach a leading 1 to it

20 IEEE 754 Floating-Point Standard 20 Computation of significand: Significand The significand bits are denoted as s 1, s 2, s 3,..., from left to right To allow quick comparisons in hardware implementation: The sign is in the most significant bit The exponent is placed before the significand (Comparisons mean less than, greater than, equal to zero ) How to represent a NEGATIVE exponent? Biased exponent: a bias is implicitly added to the exponent 1 (-1) s x (1 + 0.significand) x 2 (E-bias) bias = 127 for single precision, bias = 1023 for double precision The most negative exponent = 0 2, the most positive = s1 2 s2 2 s3 2...

21 Example 21 What decimal number is represented by this word (single precision)? Answer: ( 1) ( 1) s (1 Significan d) 2 (1 0.25) 2 2 ( ) ( E Bias)

22 Example 22 Give the binary representation of in single & double precisions Answer = * 2 = 1.50, S1 = * 2 = 1.00, S2 = 1; stop because the fraction is 0.00 Scientific notation: x 2 0 Normalized scientific notation: x 2-1 Sign = 1 (negative), exponent = -1 Single precision: S = 1, E = , significand = (23 bits) = , (127 is the bias) Double precision: S = 1, E = , significand = (52 bits) = , (1023 is the bias)

23 IEEE 754 Standard for Floating Point Arithmetic 23 Single precision: Denormalized Normalized Significand Exponent ( 1) s (0. F) (2) 126 ( 1) s (1. F) (2) E 127 ( 1) s ( ) non-numbers e.g. 0/0, 1 Double precision: Significand Exponent ( 1) s (0. F) (2) 1022 ( 1) s (1. F) (2) E 1023 ( 1) s ( ) non-numbers e.g. 0/0, 1

24 Example = = = = = = = = = = = = infinity - infinity NaN (Not a Number) NaN (1.0 2 ) (2) (1.101 ) (2) (1.101 ) (2) (1.0 ) (2) (2) (0.1 ) (2) 126 (2) (2) 23 (2) 126 (2) 149

25 Overflow and Underflow in Floating Point 25 Overflow (floating-point) A positive exponent becomes too large to fit in the exponent field Underflow (floating-point) A negative exponent becomes too large to fit in the exponent field Large negative close to zero Large positive Overflow Representable range Representable range Overflow underflow

26 Communicating with People 26 How to represent Characters Characters are unsigned bytes e.g., in C++ Char Usually follow the ASCII standard Uses 8 bits unsigned to represent a character

27 Exercises 27 What does the following 32 bit pattern represent: 0x If it were a 2 s complement integer the MSb is 0 therefore this is a positive number evaluation left as an exercise An unsigned number Same value as above A sequence of ASCII encoded bytes: 2611 Checking the ascii table gives: 0x32 = code for character 2 0x36 = code for character 6 0x32 = code for character 1 0x32 = code for character 1 A 32 bit IEEE 754 floating point number s= 0, E = , S = This is a normalized number so E is biased.

28 Exercises 28 Consider building a floating point number system like the IEEE754 standard on 8 bit only, with 3 bits being reserved for the exponent. What is the value of the bias? 3 What is the representation of 0? What is the representation of -4? -4 = x 2 2 S=1, F= 0 and the biased exponent must be E 3 = 2 or E = +5 So - 4 = What is the next value representable after 4? = so we can see that 4 bits for the significand is not accurate enough What does the byte represent? - NAN What is the representation of -?

29 Key Concepts to Remember 29 2's complement representation for signed numbers Floating-point numbers Representation follows closely the scientific notation Almost all computers, including MIPS, follow IEEE 754 standard Single-precision floating-point representation takes 32 bits Double-precision floating-point representation takes 64 bits Overflow and underflow in signed integer and floating number

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