C Program to reverse bits in a 32-bit unsigned word

C Program

#include <stdio.h>
#include <stdint.h>

// Function to reverse bits of a 32-bit unsigned integer
uint32_t reverseBits(uint32_t n) {
    uint32_t result = 0;
    for (int i = 0; i < 32; i++) {
        result <<= 1;         // Shift result left
        result |= (n & 1);    // Copy LSB of n into result
        n >>= 1;              // Shift n right
    }
    return result;
}

int main() {
    uint32_t num, reversed;

    printf("Enter a 32-bit unsigned integer: ");
    scanf("%u", &num);

    reversed = reverseBits(num);

    printf("Original number (decimal): %u\n", num);
    printf("Reversed bits (decimal): %u\n", reversed);

    printf("Original number (hex): 0x%08X\n", num);
    printf("Reversed bits (hex): 0x%08X\n", reversed);

    return 0;
}

Code Walkthrough

The function reverseBits iterates 32 times (for each bit).

In each step:

• The result is shifted left to make space.
• The least significant bit (LSB) of n is appended to result.
• Then n is shifted right to process the next bit.

Note for ARM Cortex-M Developers: While this O(N) iterative implementation is highly portable and excellent for interviews, ARM Cortex-M3/M4/M7/M33 microcontrollers possess a dedicated hardware instruction for bit reversal. This can be executed in a single clock cycle using the CMSIS compiler intrinsic __RBIT(n) (e.g. reversed = __RBIT(num);), which dramatically improves performance and efficiency in real-time DSP or communication drivers.