[C language foundation] 15 bit operation

One 、 Shift operation

Operator	meaning
<<	Move left
>>	Move right

1.1 << Move left

• i << j
  • take i All bits in move left j A place , The right to repair 0;
  • All less than int The type of , Shift to int The way to do it , The result is int type .
• x << 1 Equivalent to x *= 2
• x << n Equivalent to x *= xⁿ
• a number ** Every left shift 1 position , Equivalent to multiplying 2**

1.2 >> Move right

• i >> j
  • take i All bits in move left j A place , The right to repair 0;
  • All less than int The type of , Shift to int The way to do it , The result is int type .
  • about unsigned type , Left complement 0;
  • about signed type , Fill in the original highest digit on the left （0 or 1, Leave the sign unchanged ）
• x >> 1 Equivalent to x /= 2
• x >> n Equivalent to x /= xⁿ
• a number ** Every shift right 1 position , Equivalent to dividing 2**

int a = 0x80000000;
unsigned int b = 0x80000000;
printf(" a = %d\n", a);
printf(" b = %u\n", b);
printf("a>>16 = %d\n", a>>16);
printf("b>>16 = %u\n", b>>16);

Output results ：

1.3 Be careful

Do not set the number of bits shifted left or right to a negative number , Otherwise, it will result in undefined behavior .

Two 、 Bitwise operation

Bitwise operators ：

Operator	meaning
&	Bitwise AND
`	`
~	According to the not
^	Bitwise XOR

2.1 & Bitwise AND

• If (x)_i == 1 And (y)_i == 1, be (x & y)_i = 1. otherwise (x & y)_i = 0;
• Yes 0 Must be 0, The same is 1 Only then 1
• and 0 Meet each other Must be 0, and 1 Meet each other Is the same

give an example ：
0101 1010 & 10001 100 = 0000 1000（5A & 8C = 08）

Bitwise and common applications ：

• Let one or more bits be 0：x & 0xFE（ Give Way x The lowest is 0）
• Take a paragraph from a number ： x & 0xFF（ take x Lowest 1 Bytes , Others are 0）

give an example ：
1010 0101 & 0xFE = A5 & FE = A4
1010 1010 1010 1010 & 0xFF = AAAA & FF = AA（125252 & FF = 252）

2.2 | Press bit or

• If (x)_i == 1 And (y)_i == 1, be (x | y)_i = 1. otherwise (x | y)_i = 0;
• Yes 1 Must be 1, The same is 0 Only then 0
• and 1 Phase or Must be 1, and 0 Phase or Is the same

Bitwise and common applications ：

• Make one or more bits 1：x | 0x01（ Give Way x The lowest is 1）
• Put the two numbers together ：0x00FF | 0xFF00 = 0xFFFF

2.3 ~ According to the not

• ( ~x )_i = 1 - ( x )_i
• 01 swap .

give an example ：
~1010 1010 = 0101 0101（~AA = 55）

Bitwise negation is different from complement ：

• Bitwise inversion is to put the 01 Flip ;
  • ~1010 1010 = 0101 0101
• Ask for a complement ：
  • The complement of a positive number is equal to the original code ;
  • A negative complement , The sign bits remain the same , The other bits are reversed and then added 1.

2.4 ^ Bitwise XOR

• If ( x )_i == ( y )_i, that ( x ^ y )_i = 0, Otherwise 1;
• Same as 0, Dissimilarity is 1
• XOR a variable twice with the same value , It's like doing nothing . x ^ y ^ y = x

2.5 Logical operation and bitwise operation

Logical operation is different from bitwise operation , Don't confuse , But you can think of logical operations as all non 0 Value change 1, Then perform bitwise operation .

• 5 & 4 = 4   and 5 && 4 ——> 1 & 1 = 1
• 5 | 4 = 5   and 5 || 4 ——> 1 | 1 = 1
• ~4 = 3    and !4 ——> !1 = 0

3、 ... and 、 Examples of bit operations

3.1 Output a binary number of numbers

int number;
scanf("%d", &number);
unsigned mask = 1u<<31;		//unsigned Omit types between and variables , The default is unsigned int type 
							//1u Expressed as unsigned int Type of 1
							//1u<<31 = 10000...00(31 individual 0)
							
for(; mask; mask>>=1){
    		//mask Move right , There is always only one at a time 1
	printf("%d", number & mask ? 1 : 0);
}
printf("\n");

3.2 Control the specific position of a number

const unsigned int SBS = 1u<<2;		//SBS = 00...0100
const unsigned int PE = 1u<<3;		//PE = 00...01000

U0LCR |= SBS | PE;			// Give Way U0LCR Of the 2 Position and number 3 Position as 1, The remaining bits remain unchanged (SBS|PE = 00...01100)
U0LCR &= ~(SBS | PE);		// Give Way U0LCR Of the 2 Position and number 3 Position as 0, The remaining bits remain unchanged 

Four 、 Bit segment

4.1 explain

• Put one int Several bits of are combined into a structure , Specific bits form a bit segment , For example struct, Control specific positions through members in the structure .

struct{
    
	unsigned int leading : 3;	// Colon after member : The last number is the number of bits occupied by the member 
	unsigned int FLAG1 : 1;
	unsigned int FLAG2 : 1;
	int trailing : 11;
};

4.2 give an example

void prtBin(unsigned int number);
struct U0{
    
	unsigned int leading : 3;	// Colon after member : The last number is the number of bits occupied by the member , Starting from the low 
	unsigned int FLAG1 : 1;
	unsigned int FLAG2 : 1;
	int trailing : 27;			// Guarantee a total of 32 position ,unsigned int The number of digits 
};

int main(){
    
	struct U0 uu;
	uu.leading = 2;
	uu.FLAG1 = 0;
	uu.FLAG2 = 1;
	uu.trailing = 0;
	printf("sizeof(uu)=%lu\n", sizeof(uu));
	prtBin(*(int*)&uu);		// take uu The data in is forcibly converted to int type ( It was originally struct U0 type )
	
	return 0;
}

void prtBin(unsigned int number)
{
    
	unsigned mask = 1u<<31;
	for(; mask; mask >>=1){
    
		printf("%d", number & mask ? 1:0);
	}
	printf("\n");
}

Output results ：

explain ：

• Lowest 3 Position as uu.leading = 2, Next is uu.FLAG1 = 0,uu.FLAG2 = 1. Others are uu.trailing = 0.
• If in the structure definition trailing Change to occupy 28 position , Then the total number of structural digits exceeds 32 position , The compiler will add a int, Output sizeof(uu) = 8 .

4.3 Be careful

• After combining the special positioning into bit segments , It can be accessed directly by the member name of the bit segment . Specific shift 、 And 、 Or more convenient
• The editor will arrange the arrangement of bits , Not portable , For example, some editors start with the rightmost bit , Some start at the far left .
• When more than one bit is required int when , Multiple int.