Number Systems! Why Bits (Binary Digits)?!

Transcription

1 Number Systems Why Bits (Binary Digits)? Computers are built using digital circuits Inputs and outputs can have only two values True (high voltage) or false (low voltage) Represented as and Can represent many kinds of information Boolean (true or false) Numbers (23, 79, ) Characters ( a, z, ) Pixels, sounds Internet addresses Can manipulate in many ways Read and write Logical operations Arithmetic 2

2 Base and Base 2 Decimal (base ) Each digit represents a power of 473 = 4 x 3 + x x + 3 x Binary (base 2) Each bit represents a power of 2 = x x x x 2 + x 2 = 22 Decimal to binary conversion: Divide repeatedly by 2 and keep remainders 2/2 = 6 R = 6/2 = 3 R = 3/2 = R = /2 = R = Result = 3 Writing Bits is Tedious for People Octal (base 8) Digits,,, 7 Hexadecimal (base 6) Digits,,, 9, A, B, C, D, E, F = = 8 = = 9 = 2 = A = 3 = B = 4 = C = 5 = D = 6 = E = 7 = F Thus the 6-bit binary number converted to hex is B2A9 4 2

3 Representing Colors: RGB Three primary colors Red Green Blue Strength 8-bit number for each color (e.g., two hex digits) So, 24 bits to specify a color In HTML, e.g. course Schedule Web page Red: <span style="color:#ff">de-comment Assignment Due</ span> Blue: <span style="color:#ff">reading Period</span> Same thing in digital cameras Each pixel is a mixture of red, green, and blue 5 Finite Representation of Integers Fixed number of bits in memory Usually 8, 6, or 32 bits (, 2, or 4 bytes) Unsigned integer No sign bit Always or a positive number All arithmetic is modulo 2 n Examples of unsigned integers è è 5 è 6 è 33 è

4 Adding Two Integers From right to left, we add each pair of digits We write the sum, and add the carry to the next column Base Base Sum Carry Sum Carry 7 Binary Sums and Carries a b Sum a b Carry XOR ( exclusive OR ) AND

5 Modulo Arithmetic Consider only numbers in a range E.g., five-digit car odometer:,,, E.g., eight-bit numbers,,, 255 Roll-over when you run out of space E.g., car odometer goes from to,, E.g., eight-bit number goes from 255 to,, Adding 2 n doesn t change the answer For eight-bit number, n=8 and 2 n =256 E.g., ( ) mod 256 is simply 37 This can help us do subtraction by changing it to addition Suppose you want to compute a b Note that this equals a b = a + (256 b) How to compute 256 b? 9 One s and Two s Complement What s easy is computing 255 b (in 8 bits) Because it s b, so just flip every bit of b E.g., if b is (i.e., 69 in decimal) 255 b - b 255 b = 88 This is the one s complement of b; 2 n - - b; just flip all the bits of b But I want 2 n - b Two s complement Add to the one s complement E.g., = (255 69) + è 5

6 Putting it All Together Computing a b Same as a b (in 8-bit representation) Same as a + (255 b) + Same as a + onescomplement(b) + Same as a + twoscomplement(b) Example: The original number 69: One s complement of 69: Two s complement of 69: Add to the number 72: The sum comes to: Equals: 3 in decimal + Signed Integers How to represent negative as well as positive numbers Sign-magnitude representation Use one bit to store the sign, (n-) for magnitude Sign bit is for positive number, for negative number Examples E.g., è 44 E.g., è -44 Hard to do arithmetic this way, so rarely used Complement representation One s complement Flip every bit: E.g., è -44 Two s complement Flip every bit, then add : E.g., è

7 Overflow: Running Out of Room Adding two large integers together Sum might be too large to store in the number of bits available What happens? Unsigned integers All arithmetic is modulo arithmetic Sum would just wrap around End up with sum modulo 2 n Signed integers Can get nonsense values Example with 6-bit integers Sum: +2+3 Result: Bitwise Operators: AND and OR Bitwise AND (&) & Mod on the cheap E.g., 53 % 6 is same as 53 & 5; 53 & 5 Bitwise OR ( ) 5 4 7

8 Bitwise Operators: Not and XOR Not or One s complement (~) Turns s to s, and s to s E.g., set last three bits to x = x & ~7; XOR (^) if both bits are the same if the two bits are different ^ 5 Bitwise Operators: Shift Left/Right Shift left (<<): Multiply by powers of 2 Shift some # of bits to the left, filling the blanks with 53 53<<2 Shift right (>>): Divide by powers of 2 Shift some # of bits to the right For unsigned integer, fill in blanks with What about signed negative integers? Can vary from one machine to another 53 53>>2 6 8

9 Example: Counting the s How many bits in a number? E.g., how many bits in the binary representation of 53? Four bits How to count them? Look at one bit at a time Check if that bit is a Increment counter How to look at one bit at a time? To look at the value of the last bit: n & To check if it is a : (n & ) ==, or simply (n & ) 7 Counting the Number of Bits #include <stdio.h> #include <stdlib.h> int main(void) { unsigned int n; unsigned int count; printf("number: "); if (scanf("%u", &n) = ) { fprintf(stderr, "Error: Expect unsigned int.\n"); exit(exit_failure); } for (count = ; n > ; n >>= ) count += (n & ); printf("number of bits: %u\n", count); return ; } 8 9

10 Summary Computer represents everything in binary Integers, floating-point numbers, characters, addresses, Pixels, sounds, colors, etc. Binary arithmetic through logic operations Sum (XOR) and Carry (AND) Two s complement for subtraction Bitwise operators AND, OR, NOT, and XOR Shift left and shift right Useful for efficient and concise code, though sometimes cryptic 9