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) {
    // truncate value to requested length
    uint32_t mask = ((1 << len) - 1);
    val &= mask;
    // clear reg bits
    uint32_t shifted_mask = mask << pos;
    reg &= ~shifted_mask;
    // set bits new in reg
    reg |= val << pos;

    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;
}