21. Replace Bit Field in a 32-bit Register

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

uint32_t replace_field(uint32_t reg, uint32_t val, uint8_t pos, uint8_t len) {
    // reg = 0b1111 1111; val = 0b1001; pos = 4; len = 4
    uint32_t mask = ~(0xFFFFFFFF << len); //0..00001111
    mask <<= pos; //0..000011110000
    val  <<= pos; //0..000010010000
    mask = (~mask) | val; // //0..111110011111
    reg &= mask; //10011111 
    return reg;
}

int main() {
    uint32_t reg, val;
    uint8_t pos, len;
    scanf("%u %u %hhu %hhu", &reg, &val, &pos, &len);
    printf("%u", replace_field(reg, val, pos, len));
    return 0;
}