Computer Organization: A Programmer's Perspective

Transcription

1 A Programmer's Perspective Representing Numbers Gal A. Kaminka

2 Fractional Binary Numbers 2 i 2 i b i b i 1 b 2 b 1 b 0. b 1 b 2 b 3 b j 1/2 1/4 1/8 Representation Bits to right of binary point represent fractional powers of 2 2 j Represents rational number: b k 2 k A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 2 i k j

3 Frac. Binary Number Examples Value 5+3/ Representation 2+7/ / Observations Divide by 2 by shifting right Multiply by 2 by shifting left Numbers of form just below 1.0 1/2 + 1/4 + 1/ /2 i Use notation 1.0 A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 3

4 Representable Numbers Limitation Can only exactly represent numbers of the form x/2 k Other numbers have repeating bit representations Value Representation 1/ [01] 2 1/ [0011] 2 1/ [0011] 2 Solution: Use floating point representation A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 4

5 IEEE Floating Point IEEE Standard 754 Established in 1985 as uniform standard for floating point arithmetic Before that, many idiosyncratic formats Supported by all major CPUs Driven by Numerical Concerns Nice standards for rounding, overflow, underflow Hard to make go fast Numerical analysts predominated in defining standard A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 5

6 Floating Point Representation Numerical Form: 1 s M 2 E Sign bit s determines whether number is negative or positive Significand M normally a fractional value in range [1.0,2.0). Exponent E weights value by power of two Encoding MSB is sign bit exp field encodes E frac field encodes M s exp frac A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 6

7 Floating Point Precisions Encoding MSB is sign bit exp field encodes E frac field encodes M Sizes s exp frac Single precision (32 bits): 8 exp bits, 23 frac bits Double precision (64 bits): 11 exp bits, 52 frac bits Extended precision (79 bits): 15 exp bits, 63 frac bits Only found in Intel-compatible machines Stored in 80 bits (1 bit wasted) A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 7

8 Normalized Numeric Values Condition: exp and exp Exponent coded as biased value: Exp = E + Bias Exp : unsigned value denoted by exp Bias : Bias value»single precision: 127 (Exp: 1 254, E: )»Double precision: 1023 (Exp: , E: )»in general: Bias = 2 e - 1-1, where e is number of exponent bits Significand coded with implied leading 1 M = 1.xxx x 2 xxx x: bits of frac Minimum when (M = 1.0) Maximum when (M = 2.0 ) Get extra leading bit for free A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 8

9 Normalized Encoding Example Value Float F = ; = = X 2 13 Significand M = frac= Exponent E = 13 Bias = 127 Exp = 140 = Floating Point Representation (Class 02): Hex: D B Binary: : : A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 9

10 Denormalized Values Condition: exp = Value Exponent value E = Bias + 1 Significand value M = 0.xxx x 2 (xxx x: bits of frac) Cases exp = 000 0, frac = Represents value 0 Note that have distinct values +0 and 0 exp = 000 0, frac Numbers very close to 0.0 Lose precision as get smaller Gradual underflow A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 10

11 Special Values Condition: exp = Cases exp = 111 1, frac = Represents value(infinity) Operation that overflows Both positive and negative E.g., 1.0/0.0 = 1.0/0.0 = +, 1.0/0.0 = exp = 111 1, frac Not-a-Number (NaN) Represents case when no numeric value can be determined E.g., sqrt( 1), A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 11

12 Summary of Floating Point Real Number Encodings Normalized Denorm +Denorm +Normalized + NaN 0 +0 NaN A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 12

13 Tiny Floating Point Example 8-bit Floating Point Representation the sign bit is in the most significant bit. the next four bits are the exponent, with a bias of 7. the last three bits are the frac Same General Form as IEEE Format normalized, denormalized representation of 0, NaN, infinity s exp frac 0 A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 13

14 Values Related to the Exponent Exp exp E 2 E /64 (denorms) / / / / / / n/a (inf, NaN) A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 14

15 Dynamic Range s exp frac E Value Denormalized numbers Normalized numbers /8*1/64 = 1/ /8*1/64 = 2/ /8*1/64 = 6/ /8*1/64 = 7/ /8*1/64 = 8/ /8*1/64 = 9/ /8*1/2 = 14/ /8*1/2 = 15/ /8*1 = /8*1 = 9/ /8*1 = 10/ /8*128 = /8*128 = n/a inf closest to zero largest denorm smallest norm closest to 1 below closest to 1 above largest norm A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 15

16 Distribution of Values 6-bit IEEE-like format e = 3 exponent bits f = 2 fraction bits Bias is 3 Notice how the distribution gets denser toward zero Denormalized Normalized Infinity A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 16

17 Distribution of Values (close-up view) 6-bit IEEE-like format e = 3 exponent bits f = 2 fraction bits Bias is Denormalized Normalized Infinity A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 17

18 Interesting Numbers Description exp frac Numeric Value Zero Smallest Pos. Denorm {23,52} X 2 {126,1022} Single 1.4 X Double 4.9 X Largest Denormalized (1.0 ) X 2 {126,1022} Single 1.18 X Double 2.2 X Smallest Pos. Normalized X 2 {126,1022} Just larger than largest denormalized One Largest Normalized (2.0 ) X 2 {127,1023} Single 3.4 X Double 1.8 X A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 18

19 Special Properties of Encoding FP Zero Same as Integer Zero All bits = 0 Can (Almost) Use Unsigned Integer Comparison Must first compare sign bits Must consider -0 = 0 NaNs problematic Will be greater than any other values What should comparison yield? Otherwise OK Denorm vs. normalized Normalized vs. infinity A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 19

20 Floating Point in C C Guarantees two levels float double Conversions single precision double precision Casting int, float, and double changes numeric values double or float to int Truncates fractional part Like rounding toward zero Not defined when out of range Generally saturates to TMin or TMax int to double: exact, as long as int has 53 bit word size int to float: will round according to rounding mode A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 24

21 Ariane 5 Why Exploded 37 seconds after liftoff Cargo worth $500 million Computed horizontal velocity as floating point number Converted to 16-bit integer Worked OK for Ariane 4 Overflowed for Ariane 5 Used same software A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 25

22 Summary IEEE Floating Point Has Clear Mathematical Properties Represents numbers of form M X 2 E Can reason about operations independent of implementation As if computed with perfect precision and then rounded Not the same as real arithmetic Violates associativity/distributivity Makes life difficult for compilers & serious numerical applications programmers A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 26

23 Floating Point Puzzles (in 32bits) Assume neither d nor f is NAN x == (int)(float) x x == (int)(double) x f == (float)(double) f d == (float) d f == -(-f); 2/3 == 2/3.0 d < 0.0 ((d*2) < 0.0) d > f -f > -d d * d >= 0.0 (d+f)-d == f int x = ; float f = ; double d = ; A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 27

24 Floating Point Puzzles (in 32bits) Assume neither d nor f is NAN x == (int)(float) x No: 24 bit significand x == (int)(double) x Yes: 53 bit significand f == (float)(double) f Yes: increases precision d == (float) d No: loses precision f == -(-f); Yes: Just change sign bit 2/3 == 2/3.0 No: 2/3 == 0 d < 0.0 ((d*2) < 0.0) Yes! d > f -f > -d Yes! d * d >= 0.0 Yes! (d+f)-d == f No: Not associative A Programmer's Perspective Based on class notes by Bryant and O'Hallaron 28