Representing Integers

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1 Representing Integers Jinkyu Jeong Computer Systems Laboratory Sungkyunkwan University SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong

2 Unsigned Integers Encoding unsigned integers B = [ bw- 1, bw-2,..., b0 ] x = D( B) = w - 1 åb i i= 0 2 i D(x) = = = 2003 What is the range for unsigned values with w bits? SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 2 2

3 Signed Integers (1) Encoding positive numbers Same as unsigned numbers Encoding negative numbers Sign-magnitude representation Ones complement representation Two s complement representation SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 3 3

4 Signed Integers (2) Sign-magnitude representation b w-1 b w-2 b 1 b 0 Sign bit S w b æ - = - çå w- B) ( 1) è i= ( 1 Magnitude 2 Two zeros [000 00], [ ] Used for floating-point numbers 2 0 b i i ö ø SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 4 4

5 Signed Integers (3) Ones complement representation b w-1 b w-2 b 1 b 0 Sign bit w æ - 2 w-1 O( B) = -b - + çå w -1(2 1) bi 2 è i= 0 Easy to find n Two zeros [ ], [ ] No longer used i ö ø SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 5 5

6 Signed Integers (4) Two s complement representation b w-1 b w-2 b 1 b 0 Sign bit 1101 w æ - 2 w-1 ö = - + çå i O( B) b w -1 2 bi è i= 0 ø 1011 Unique zero Easy for hardware leading 0 0 leading 1 < 0 Used by almost all modern machines SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 6 6

7 Signed Integers (5) Two s complement representation (cont d) Following holds for two s complement ~ x + 1 == x Complement Observation: ~ x + x == == 1 Increment ~ x + x == 1 ~ x + x + ( x + 1) == 1 + ( x + 1) ~ x + 1 == x SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 7 7

8 Numeric Ranges (1) Unsigned values UMin = 0 [000 00] UMax = 2 w 1 [111 11] Two s complement values TMin = 2 w-1 [100 00] TMax = 2 w-1 1 [011 11] Decimal Hex Binary Values for w = 16 UMax FF FF TMax F FF TMin FF FF SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 8 8

9 Numeric Ranges (2) Values for different word sizes w = 8 w = 16 w = 32 w = 64 UMax ,535 4,294,967,295 18,446,744,073,709,551,615 TMax ,767 2,147,483,647 9,223,372,036,854,775,807 TMin ,768-2,147,483,648-9,223,372,036,854,775,808 Observations TMin = TMax + 1 (Asymmetric range) UMax = 2 * TMax + 1 In C programming #include <limits.h> INT_MIN, INT_MAX, LONG_MIN, LONG_MAX, UINT_MAX, Values platform-specific SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 9 9

10 Type Conversion (1) Unsigned: w bits à w+k bits Zero extension: just fill k bits with 0 s unsigned short x = 2003; unsigned ix = (unsigned) x; x w = ix k = 16 w = 16 SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1010

11 Type Conversion (2) Signed: w bits à w+k bits Given w-bit signed integer x Convert it to w+k-bit integer with same value Sign extension Make k copies of sign bit X Sign bit w X k w SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1111

12 Type Conversion (3) Sign extension example Converting from smaller to larger integer type C automatically performs sign extension short int x = 2003; int ix = (int) x; short int y = -2003; int iy = (int) y; Decimal Hex Binary x D ix D y F8 2D iy FF FF F8 2D SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1212

13 Type Conversion (4) Unsigned & Signed: w+k bits à w bits Just truncate it to lower w bits Equivalent to computing x mod 2 w unsigned int x = 0xcafebabe; unsigned short ix = (unsigned short) x; int y = 0x2003beef; short iy = (short) y; Decimal Hex Binary x CA FE BA BE ix BA BE y BE EF iy BE EF SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1313

14 Type Conversion (5) Unsigned à Signed The same bit pattern is interpreted as a signed number U 2 w 2 w 1 0 w-1 2Tw ( x) = í w w-1 Unsigned ì x, îx - 2 Two s complement +2 w 1 0, x x 2 w 1 < ³ 2 2 unsigned short x = 2003; short ix = (short) x; unsigned short y = 0xbabe; short iy = (short) y; Decimal Hex Binary x D ix D y BA BE iy BA BE SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1414

15 Type Conversion (6) Signed à Unsigned Ordering inversion Negative à Big positive T 2U w Two s complement +2 w 1 ( x) = w ì x + 2, x < í î x, x ³ 2 w 2 w short x = 2003; unsigned short ix = (unsigned short) x; short y = -2003; unsigned short iy = (unsigned short) y; Decimal Hex Binary x D w 1 0 Unsigned ix D y F8 2D iy F8 2D SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1515

16 Type Casting in C Constants By default, considered to be signed integers Unsigned if they have U or u as suffix e.g. 0U, 12345U, 0x1A2Bu Type casting Explicit casting Implicit casting via Assignments Procedure calls int tx, ty; unsigned ux, uy; tx = (int) ux; uy= (unsigned) ty; int f(unsigned); tx = ux; f(ty); SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1616

17 Expression Evaluation in C If mix unsigned and signed in single expression, signed values implicitly cast to unsigned Including comparison operations <, >, ==, <=, >= Expression Type Evaluation 0 == 0U -1 < 0-1 < 0U -1 > -2 (unsigned) -1 > > U > > (int) U unsigned signed True True unsigned? signed? unsigned? signed? unsigned? signed? SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1717

18 Example 1 #include <stdio.h> int main () { unsigned i; for (i = 10; i >= 0; i--) printf ( %u\n, i); } SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1818

19 Example 2 #include <stdio.h> #define DELTA sizeof(int) int main () { int i; for (i = 10; i DELTA >= 0; i -= DELTA) printf ( %d\n, i); } SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 1919

20 Example 3 #include <string.h> int strlonger (char *s, char *t) { return (strlen(s) strlen(t)) > 0; } SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 2020

21 Example 4 int sum_array (int a[], unsigned len) { int i; int result = 0; for (i = 0; i <= len 1; i++) result += a[i]; } return result; SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 2121

22 Example 5 #include <stdio.h> int main () { unsigned char c; } while ((c = getchar())!= EOF) putchar (c); SSE2030: Introduction to Computer Systems, Spring 2018, Jinkyu Jeong (jinkyu@skku.edu) 2222

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