# Integer Representation

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1 Integer Representation Representation of integers: unsigned and signed Modular arithmetic and overflow Sign extension Shifting and arithmetic Multiplication Casting 1

2 Fixed- width integer encodings Unsigned non- negative integers only Signed both negative and non- negative integers n bits offer only 2 n distinct values. Terminology: Most- significant bit(s) or high- order bit(s) MSB Least- significant bit(s) or low- order bit(s) LSB 2

3 Unsigned integer representation Example in 4- bit unsigned representation = 1 x x x x 2 0 weight position n- bit unsigned numbers: minimum = maximum = 3

4 Unsigned modular arithmetic, overflow Examples in 4- bit unsigned representation = = x + y in N- bit unsigned arithmetic is (x + y) mod 2 N in math unsigned overflow= "wrong" answer = wrap- around = carry 1 out of MSB = math answer too big to fit 4

5 Unsigned overflow Addition overflows if and only if a carry bit is dropped Modular Arithmetic 5

6 Sign- Magnitude representation?!!! Most- significant bit (MSB) is sign bit 0 means non- negative 1 means negative Remaining bits are an unsigned magnitude 8- bit sign- and- magnitude: 0x00 = represents ex 0x7F = represents 0x85 = represents 0x80 = represents Max and min for n bits? Anything weird here? 6

7 Sign- Magnitude Negatives!!! Cumbersome arithmetic. Example: 4-3!= 4 + (- 3) What about zero? Sign- magnitude is not such a good idea

8 Two s complement representation for signed integers n- bit representation _ _ - 2 (n- 1) 2 i weight n- 1 i position Positional representation, but most significant position has negative weight. 8

9 8- bit representations ex n- bit two's complement numbers: minimum = maximum = 9

10 4- bit unsigned vs. 4- bit two s complement x x x x x x x x difference = =

11 Two s complement addition Just Works Modular Arithmetic 11

12 Two s complement overflow Addition overflows if and only if the arguments have the same sign but the result does not. if and only if the carry in and out of the sign bit differ Modular Arithmetic Some CPUs/languages raise exceptions on overflow. C and Java cruise along silently... Oops? 12

13 Reliability Ariane 5 Rocket, 1996 Exploded due to cast of 64- bit floating- point number to 16- bit signed number. Overflow. Boeing 787, 2015 "... a Model 787 airplane can lose all alternating current (AC) electrical power caused by a software counter internal to the GCUs that will overflow after 248 days of continuous power. We are issuing this AD to prevent loss of all AC electrical power, which could result in loss of control of the airplane." - - FAA, April 2015

14 A few reasons two s complement is awesome Same exact addition algorithm as for unsigned numbers. Easy: x + -x == 0 Subtraction is just addition: 4-3 == 4 + (-3) Simple hardware! MSB is sign: negatives start with 1, non- negatives start with 0 Negative one is Complement rules: x + ~x == -1 ~x + 1 == -x

15 Another derivation ex How should we represent 8- bit negatives? For all positive integers x, the representations of x and x must sum to zero. Use the standard addition algorithm Find a rule to represent x where that works 15

16 Convert small two's complement representation to a larger representation? bit bit 2 How should these bits be filled? bit - 4???????? bit - 4 ex 17

17 Sign extension Fill new bits with copies of the sign bit bit bit bit bit - 4 Casting from smaller to larger signed type does sign extension. 18

18 How are shifting and arithmetic related? ex unsigned x = 27; y = x << 2; y == x*2 n mod 2 w logical shift left: shift in zeros from right x/2 n logical shift right: shift in zeros from left unsigned x = 237; y = x >> 2; y == 59 19

19 How are shifting and arithmetic related? ex signed x = - 101; y = x << 2; y == logical shift left: x*2 n mod 2 w shift in zeros from the right x/2 n arithmetic shift right: shift in copies of MSB from left signed x = - 19; y = x >> 2; y ==

20 Multiplication Compute answer in 2x bits. Most languages drop high order half. 2 x x x x Modular Arithmetic 21

21 Multiplication Compute answer in 2x bits. Most languages drop high order half. 5 x x x x Modular Arithmetic 22

22 Multiplication Compute answer in 2x bits. Most languages drop high order half. 5 x x x x Modular Arithmetic 23

23 Multiplication by shift- and- add ex Available operations x << k implements x * 2 k x + y Implement y = x * 24 using only <<, +, and integer literals 24

24 What does this function compute? ex unsigned puzzle(unsigned x, unsigned y) { } unsigned result = 0; for (unsigned i = 0; i < 32; i++){ } if (y & (1 << i)) { result = result + (x << i); } return result; 25

25 Casting Integers in C!!! Number literals: 37 is signed, 37U is unsigned Integer Casting: bits unchanged, just reinterpreted. Explicit casting: int tx = (int) 73U; // still 73 unsigned uy = (unsigned) -4; // big positive # Implicit casting: Actually does tx = ux; // tx = (int)ux; uy = ty; // uy = (unsigned)ty; void foo(int z) {... } foo(ux); // foo((int)ux); if (tx < ux)... // if ((unsigned)tx < ux)... 26

26 More Implicit Casting in C!!! If you mix unsigned and signed in a single expression, then signed values are implicitly cast to unsigned. Includes comparisons (<, >, ==, <=, >=) How are the argument bits interpreted? Argument 1 Op Argument 2 Type Result 0 == 0U unsigned 1-1 < 0 signed 1-1 < 0U unsigned < U < < -2 (unsigned)-1 < < U < (int) u Note: T min = - 2,147,483,648 T max = 2,147,483,647 27

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