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) {
    if (len == 0 || pos >= 32) return reg;      // invalid
    if (len > 32 - pos) len = 32 - pos;         // clamp length

    uint32_t mask = ((1u << len) - 1) << pos;   // mask for field
    reg &= ~mask;                               // clear field
    reg |= (val & ((1u << len) - 1)) << pos;    // insert new value
    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;
}