18. Set Multiple Bits in 8-bit Register

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

uint8_t set_range(uint8_t reg, uint8_t start, uint8_t end) {
    // Assumes 0 <= start <= end <= 7
    uint8_t width = (uint8_t)(end - start + 1);

    uint16_t mask16;
    if (width >= 8) {
        // Only possible when start=0 and end=7 for an 8-bit register
        mask16 = 0xFFu;
    } else {
        // Build the mask in a wider type to avoid undefined shifts
        mask16 = ((1u << width) - 1u) << start;
    }

    return reg | (uint8_t)mask16;
}

int main(void) {
    uint8_t reg, start, end;
    if (scanf("%hhu %hhu %hhu", &reg, &start, &end) != 3) {
        return 1;
    }
    printf("%u", set_range(reg, start, end));
    return 0;
}