cs app lab1 深入理解计算机系统

#include <iostream>
// Rating: 1  
/*  
 * bitAnd - x&y using only ~ and |  用~和|表示& 
 *   Example: bitAnd(6, 5) = 4  0110 0101
 *   Legal ops: ~ | 
 *   Max ops: 8 
 *   Rating: 1 
 */  
int bitAnd(int x, int y) {  
  return ~(~x|~y);  
}  
/*  
 * bitXor - x^y using only ~ and &  用~和&表示^ 
 *   Example: bitXor(4, 5) = 1 
 *   Legal ops: ~ & 
 *   Max ops: 14 
 *   Rating: 1 
 */  
int bitXor(int x, int y) {  
  return ~(x&y)&~(~x&~y);  
}  
/*  
 * thirdBits - return word with every third bit (starting from the LSB) set to 1 
 * 从左到右每三个位有一个1 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 8 
 *   Rating: 1 
 */  
int thirdBits(void) { 
int result = 0x24;
 result = (result << 8) + 0x92;
 result = (result << 8) + 0x49;
 result = (result << 8) + 0x24;
 return result;
  return 2;  
}  
// Rating: 2  
/*  
 * fitsBits - return 1 if x can be represented as an  
 *  n-bit, two's complement integer. 如果x 能用n位二进制补码表示，返回1 
 *   1 <= n <= 32 
 *   Examples: fitsBits(5,3) = 0, fitsBits(-4,3) = 1 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 15 
 *   Rating: 2 
 */  
int fitsBits(int x, int n) {  
  int mask = x >> 31;
  return !(((~x & mask) + (x & ~mask)) >> (n+~0));;  // 实在想不出，抄答案了= = 
}  
/*  
 * sign - return 1 if positive, 0 if zero, and -1 if negative 抄的 
 *  Examples: sign(130) = 1 
 *            sign(-23) = -1 
 *  Legal ops: ! ~ & ^ | + << >> 
 *  Max ops: 10 
 *  Rating: 2 
 */  
int sign(int x) {  
    int s = x >> 31;
  int sig = (s & (~0)) + ((!s) & (!!x));
return sig;
}  
/*  
 * getByte - Extract byte n from word x 
 *   Bytes numbered from 0 (LSB) to 3 (MSB) 
 *   Examples: getByte(0x12345678,1) = 0x56 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 6 
 *   Rating: 2 
 */  
int getByte(int x, int n) {  
  return (x>>(n<<3))&0xff;  
}  
// Rating: 3  
/*  
 * logicalShift - shift x to the right by n, using a logical shift 逻辑右移
 *   Can assume that 0 <= n <= 31 
 *   Examples: logicalShift(0x87654321,4) = 0x08765432 
 *   Legal ops: ~ & ^ | + << >> 
 *   Max ops: 20 
 *   Rating: 3  
 */  
int logicalShift(int x, int n) {  
  return ((~(1<<31))>>(n+~0))&(x>>n);  
}  
/*  
 * addOK - Determine if can compute x+y without overflow  
 *   Example: addOK(0x80000000,0x80000000) = 0, 
 *            addOK(0x80000000,0x70000000) = 1,  
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 20 
 *   Rating: 3 
 */  
int addOK(int x, int y) {
int a =  x&1;
int b =  y&1;
int c =  a&b;//若最低位都为1，则相加会不丢失 
  return !(((x+y)>>1)^((x>>1)+(y>>1)+c)); //只这样会丢失右边第一位 
}  
/* invert - Return x with the n bits that begin at position p inverted  
 *          (i.e., turn 0 into 1 and vice versa) and the rest left  
 *          unchanged. Consider the indices of x to begin with the low-order 
 *          bit numbered as 0. 
 *   Example: invert(0x80000000, 0, 1) = 0x80000001, 10001110
 *            invert(0x0000008e, 3, 3) = 0x000000b6, 10110110 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 20 
 *   Rating: 3   
 */  
int invert(int x, int p, int n) {
int an = ~(-1<<n); //n位1 
int bn = an<<p; 
  return x^bn;  
}  
// Rating: 4  
/*  
 * bang - Compute !x without using ! 抄的 
 *   Examples: bang(3) = 0, bang(0) = 1 
 *   Legal ops: ~ & ^ | + << >> 
 *   Max ops: 12 
 *   Rating: 4  
 */  
int bang(int x) {  
  return (~((x >> 31) | (((~x) + 1) >> 31))) & 1;  
}  
// Extra Credit: Rating: 3  
/*  
 * conditional - same as x ? y : z  抄 
 *   Example: conditional(2,4,5) = 4 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 16 
 *   Rating: 3 
 */  
int conditional(int x, int y, int z) { 
  return (((!!x << 31) >> 31) & y) +((((!x) << 31) >> 31) & z);  
}  
// Extra Credit: Rating: 4  
/* 
 * isPower2 - returns 1 if x is a power of 2, and 0 otherwise 
 *   Examples: isPower2(5) = 0, isPower2(8) = 1, isPower2(0) = 0 
 *   Note that no negative number is a power of 2. 
 *   Legal ops: ! ~ & ^ | + << >> 
 *   Max ops: 20 
 *   Rating: 4 
 */  
int isPower2(int x) {  
  //判断是否为2的幂 抄的 
 //要点：x+mask 为 x-1  x&(x-1) 相当于得到x去掉了最低的为1的那位后的数，2的次幂数对应2进制应该只有1位1 同时还要保证x不为0和负数
  int mask = ~0;
  return (!(x & (x + mask)) & (!(x >> 31)) & (~(!x)));
} 
int main(int argc, char** argv) {
printf("%x %x %x %x %d %x %x\n",
bitAnd(6, 5),bitXor(4, 5),thirdBits(),fitsBits(5,3),sign(150),getByte(0x12345678,1),
logicalShift(0x87654321,4));
printf("%x %x  %d %x %x\n",addOK(0x8000000f,0x7000000f),invert(0x0000008e, 3, 3),
bang(0),conditional(2,4,5),isPower2(0));
}