# Floating Point. The World is Not Just Integers. Programming languages support numbers with fraction

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1 1 Floating Point The World is Not Just Integers Programming languages support numbers with fraction Called floating-point numbers Examples: (π) (e) or (seconds in a nanosecond) 86,400,000,000,000 or (nanoseconds in a day) last number is a large integer that cannot fit in a 32-bit integer We use a scientific notation to represent Very small numbers (e.g ) Very large numbers (e.g ) Scientific notation: ± d. f 1 f 2 f 3 f 4 10 ± e 1 e 2 e 3

2 2 Floating-Point Numbers Examples of floating-point numbers in base , , , Examples of floating-point numbers in base , , , Exponents are kept in decimal for clarity binary point The binary number ( ) 2 = = Floating-point numbers should be normalized decimal point Exactly one non-zero digit should appear before the point In a decimal number, this digit can be from 1 to 9 In a binary number, this digit should be 1 Normalized FP Numbers: and NOT Normalized: and Floating-Point Representation A floating-point number is represented by the triple S is the Sign bit (0 is positive and 1 is negative) Representation is called sign and magnitude E is the Exponent field (signed) Very large numbers have large positive exponents Very small close-to-zero numbers have negative exponents More bits in exponent field increases range of values F is the Fraction field (fraction after binary point) More bits in fraction field improves the precision of FP numbers S Exponent Fraction Value of a floating-point number = (-1) S val(f) 2 val(e)

3 3 IEEE Floating-Point Standard Single Precision Floating Point Numbers (32 bits) 1-bit sign + 8-bit exponent + 23-bit fraction Double Precision Floating Point Numbers (64 bits) 1-bit sign + 11-bit exponent + 52-bit fraction S Exponent 8 Fraction 23 S Exponent 11 Fraction 52 (continued) Normalized Floating Point Numbers For a normalized floating point number (S, E, F) S E F = f 1 f 2 f 3 f 4 Significand is equal to (1.F) 2 = (1.f 1 f 2 f 3 f 4 ) 2 IEEE assumes hidden 1. (not stored) for normalized numbers Significand is 1 bit longer than fraction Value of a Normalized Floating Point Number is ( 1) S (1.F) 2 2 val(e) ( 1) S (1.f 1 f 2 f 3 f 4 ) 2 2 val(e) ( 1) S (1 + f f f f ) 2 2 val(e) ( 1) S is 1 when S is 0 (positive), and 1 when S is 1 (negative)

4 4 Biased Exponent Representation How to represent a signed exponent? Choices are Sign + magnitude representation for the exponent Two s complement representation Biased representation IEEE uses biased representation for the exponent Value of exponent = val(e) = E Bias (Bias is a constant) Recall that exponent field is 8 bits for single precision E can be in the range 0 to 255 E = 0 and E = 255 are reserved for special use E = 1 to 254 are used for normalized floating point numbers Bias = 127 (half of 254), val(e) = E 127 val(e=1) = 126, val(e=127) = 0, val(e=254) = 127 Biased Exponent Cont d For double precision, exponent field is 11 bits E can be in the range 0 to 2047 E = 0 and E = 2047 are reserved for special use E = 1 to 2046 are used for normalized floating point numbers Bias = 1023 (half of 2046), val(e) = E 1023 val(e=1) = 1022, val(e=1023) = 0, val(e=2046) = 1023 Value of a Normalized Floating Point Number is ( 1) S (1.F) 2 2 E Bias ( 1) S (1.f 1 f 2 f 3 f 4 ) 2 2 E Bias ( 1) S (1 + f f f f ) 2 2 E Bias

5 5 Examples of Single Precision Float What is the decimal value of this Single Precision float? Solution: Sign = 1 is negative Exponent = ( ) 2 = 124, E bias = = 3 Significand = ( ) 2 = = 1.25 (1. is implicit) Value in decimal = = What is the decimal value of? Solution: implicit Value in decimal = +( ) = ( ) = ( ) 2 = Examples of Double Precision Float What is the decimal value of this Double Precision float? Solution: Value of exponent = ( ) 2 Bias = = 6 Value of double float = ( ) (1. is implicit) = ( ) 2 = 74.5 What is the decimal value of? Do it yourself! (answer should be = )

6 6 Converting FP Decimal to Binary Convert to binary in single and double precision Solution: Fraction bits can be obtained using multiplication by = = = (0.1101) = = ½ + ¼ + 1/16 = 13/ = 1.0 Stop when fractional part is 0 Fraction = (0.1101) 2 = (1.101) (Normalized) Exponent = 1 + Bias = 126 (single precision) and 1022 (double) Single Precision Double Precision Floating Point Addition Example Consider adding: (1.111) (1.011) For simplicity, we assume 4 bits of precision (or 3 bits of fraction) Cannot add significands Why? Because exponents are not equal How to make exponents equal? Shift the significand of the lesser exponent right until its exponent matches the larger number (1.011) = (0.1011) = ( ) Difference between the two exponents = 1 ( 3) = 2 So, shift right by 2 bits Now, add the significands: Carry

7 7 Addition Example cont d So, (1.111) (1.011) = ( ) However, result ( ) is NOT normalized Normalize result: ( ) = ( ) In this example, we have a carry So, shift right by 1 bit and increment the exponent Round the significand to fit in appropriate number of bits We assumed 4 bits of precision or 3 bits of fraction Round to nearest: ( ) 2 (1.001) 2 Renormalize if rounding generates a carry Detect overflow / underflow If exponent becomes too large (overflow) or too small (underflow) + Floating Point Subtraction Example Consider: (1.000) (1.000) We assume again: 4 bits of precision (or 3 bits of fraction) Shift significand of the lesser exponent right Difference between the two exponents = 2 ( 3) = 5 Shift right by 5 bits: (1.000) = ( ) Convert subtraction into addition to 2's complement Sign 2 s Complement Since result is negative, convert result from 2's complement to sign-magnitude 2 s Complement

8 8 Subtraction Example cont d So, (1.000) (1.000) = Normalize result: = For subtraction, we can have leading zeros Count number z of leading zeros (in this case z = 1) Shift left and decrement exponent by z Round the significand to fit in appropriate number of bits We assumed 4 bits of precision or 3 bits of fraction Round to nearest: (1.1111) 2 (10.000) 2 Renormalize: rounding generated a carry = Result would have been accurate if more fraction bits are used + Floating Point Multiplication Example Consider multiplying: by As before, we assume 4 bits of precision (or 3 bits of fraction) Unlike addition, we add the exponents of the operands Result exponent value = ( 1) + ( 2) = 3 Using the biased representation: E Z = E X + E Y Bias E X = ( 1) = 126 (Bias = 127 for SP) E Y = ( 2) = 125 E Z = = 124 (value = 3) Now, multiply the significands: (1.010) 2 (1.110) 2 = ( ) 2 3-bit fraction 3-bit fraction 6-bit fraction

9 9 Multiplication Example cont d Since sign S X S Y, sign of product S Z = 1 (negative) So, = However, result: is NOT normalized Normalize: = Shift right by 1 bit and increment the exponent At most 1 bit can be shifted right Why? Round the significand to nearest: (3-bit fraction) Result (normalized) Detect overflow / underflow No overflow / underflow because exponent is within range + Extra Bits to Maintain Precision Floating-point numbers are approximations for Real numbers that they cannot represent Infinite variety of real numbers exist between 1.0 and 2.0 However, exactly 2 23 fractions can be represented in SP, and Exactly 2 52 fractions can be represented in DP (double precision) Extra bits are generated in intermediate results when Shifting and adding/subtracting a p-bit significand Multiplying two p-bit significands (product can be 2p bits) But when packing result fraction, extra bits are discarded We only need few extra bits in an intermediate result Minimizing hardware but without compromising precision

10 10 Guard Bit Guard bit: guards against loss of a significant bit Only one guard bit is needed to maintain accuracy of result Shifted left (if needed) during normalization as last fraction bit Example on the need of a guard bit: (subtraction) (shift right 7 bits) Guard bit do not discard (2's complement) (add significands) (normalized) Round and Sticky Bits Two extra bits are needed for rounding Just after normalizing a result significand Round bit: appears just after the normalized significand Sticky bit: appears after the round bit (OR of all additional bits) Reduce the hardware and still achieve accurate arithmetic As if result significand was computed exactly and rounded Consider the same example of previous slide: OR-reduce (2's complement) (sum) (normalized) Round bit Sticky bit

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