Bit operation ==c language 2

One 、 Bit operators

1、 Bit and &

• Be careful ： Bit and &, Logic &&.
  give an example 0xAA&0xF0=0xA0,0xAA && 0xF0=1

2、 Bit or |

• Be careful ： Bit or |, Logical or ||

3、 Bit inversion ~

• Be careful ：C The inversion of language median is ~, Logical negation is ！.

4、 Bit exclusive or ^

• 0 or 1 And 1 Bit exclusive or will be reversed ,0 or 1 And 0 Different or unchanged 、

5、 Bit shift left 、 Shift right

• For unsigned numbers ：
  • When you move to the left, you make up the right 0, It's equivalent to logical shift .
  • When you move to the right, make up the left 0, It's equivalent to logical shift .
• For signed Numbers ：
  • When you move to the left, you make up the right 0, It's called arithmetic shift , It's equivalent to logical shift .
  • When there is a right shift, the left side is filled with the sign bit , Positive complement 0 Negative numbers make up 1, It's called arithmetic shift .
• The shift used in embedded system is unsigned number shift .

Two 、&,|,^ Special function when operating registers

1、 Register operation requirements

• ARM It's memory and IO Uniformly prepared （x86 It is not ）, Reading and writing registers is to manipulate hardware .
• How to set a special position without affecting other bits ？
  The answer is ： read - Change - Write Trilogy .

2、 Special positioning and zeroing &

3、 Specific location 1 use |

4、 Special positioning reverse use ^

3、 ... and 、 How to use bit operation to construct specific binary numbers

1、 Register operations often require a specific location to a specific value （“ Change ” In the process of ）

• solution 1： Use tool software or your own brain to calculate , Give directly 32 Bit specific number
• solution 2： Write your own code and use bit operation symbols to build

2、 Use shift to get a specific location as 1 Binary number of

• For example, you need one bit3~bit7 by 1 Binary number of ：(0x1f << 3).
• bit3 ~ bit7 by 1, meanwhile bit23 ~ 25 Also for the 1：((0x1f << 3) | (7 << 23)).

3、 Combining negation to obtain a specific location is 0 Binary number

• obtain bit4 ~ bit10 by 0, Others are 1 Number of numbers ：~(0x7f << 4)

4、 summary ：“ Change ” The process of using &、|、^ It can be completed by combining specific binary numbers

Four 、 Practice of bit operation

1、 Given an integer number a, Take out a Of bit3~bit8

• First step ：a &= (0x3f << 3);
• The second step ：a >>= 3;

2、 Given an integer number a, to a Of bit7~bit17 assignment 937

• First step ：a &= ~(0x7ff << 7); // Specific location 0
• The second step ：a |= (937 << 7); // Specific positioning assignment

3、 Given an integer number a, to a Of bit7~bit17 Add... To the value in 17

• First step ：b = a & (0x7ff << 7); b >>= 7; // Take out the value
• The second step ：b += 17; // add 17
• The third step ：a &= ~(0x7ff << 7); // Specific bit clearance 0
• Step four ：a |= (b << 7); // Specific positioning assignment

5、 ... and 、 Technology upgrade ： Use macro definition to complete bit operation

1、 Set directly with macro definitions 、 Reset

• #define SET_NTH_BIT(x,n) ((x) |= ((1U)<<(n)))
• #define CLEAR_NTH_BIT(x,n) ((x) &=~((1U)<<(n)))

2、 Use macro definition to intercept partial continuous bits of variables ,

• For example, intercepting variables 0x88(10001000b) Of bit2~bit4
  • #define GRTBITS(x,n,m) ( ( (x) & ~( ~(0U) << ((m) – (n) + 1)) ) << (n) ) >> (n) )