Common Techniques for Register Manipulation in C Language

When assigning values to registers using the C language, bit manipulation methods of the C language are often required.

Clearing a Specific Bit of a Register

Assume a represents the register, which already has a value. If we want to clear a specific bit while keeping other bits unchanged, the code is as follows.

// Define a variable a = 1001 1111 b (binary number)unsigned char a = 0x9f;// Clear bit2a &= ~(1<<2);// The 1 in parentheses is left shifted by two bits, (1<<2) gives binary number: 0000 0100 b// Bitwise NOT, ~(1<<2) gives 1111 1011 b// If the original value of a is in binary: a = 1001 1111 b// The result number and a perform a bitwise AND operation, a = (1001 1111 b)&(1111 1011 b),// After the operation, the value of a becomes a=1001 1011 b// The bit2 of a is cleared, while other bits remain unchanged.

Clearing Several Consecutive Bits of a Register

Since there may be several consecutive register bits used to control a certain function, assume we need to clear several consecutive bits of the register while keeping other bits unchanged, the code is as follows.

// If we divide the binary bits of a into groups of 2// that is, bit0, bit1 is the 0th group, bit2, bit3 is the 1st group,// bit4, bit5 is the 2nd group, bit6, bit7 is the 3rd group// To clear bit2 and bit3 of the 1st groupa &= ~(3<<2*1);// The 3 in parentheses is left shifted by two bits, (3<<2*1) gives binary number: 0000 1100 b// Bitwise NOT, ~(3<<2*1) gives 1111 0011 b// If the original value of a is in binary: a = 1001 1111 b// The result number and a perform a bitwise AND operation, a = (1001 1111 b)&(1111 0011 b),// After the operation, the value of a becomes a=1001 0011 b// The bit2 and bit3 of the 1st group of a are cleared, while other bits remain unchanged.// In the above (~(3<<2*1)), (1) is the group number; to clear the 3rd group bit6, bit7 should be 3 here// (2) is the number of bits per group, there are 2 binary bits per group; if divided into 4 bits per group, it should be 4 here// (3) is the value when all bits in the group are 1; if divided into 4 bits per group, this should be the binary number "1111 b"// For example, to clear bit4 and bit5 of the 2nd groupa &= ~(3<<2*2);

Assigning Values to Specific Bits of a Register

After clearing the bits of the register, it is convenient to write the necessary values to specific bits. The specific code is as follows.

// a = 1000 0011 b// Now set the cleared bit4 and bit5 of the 2nd group to the binary number "01 b" a |= (1<<2*2);// a = 1001 0011 b, successfully set the value of the 2nd group, while other groups remain unchanged

Inverting a Specific Bit of a Register

To perform an inversion operation on a specific bit of the register, i.e., changing 1 to 0 and 0 to 1, this can be done directly using the following operation.

// a = 1001 0011 b// Invert bit6, while keeping other bits unchanged a ^=(1<<6);// a = 1101 0011 b

Reference Format: Feng Dan, Luo Ling, Li Yu, et al. Exploration of Program Design Courses Integrated with Ideological and Political Education[J]. Computer Education, 2022(9): 72—76.

(Editing by WeChat: Shi Zhiwei)

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