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) {
    uint8_t len = end-start+1; //Move it 1 past
    uint8_t ones = (1<<len) - 1; // This makes a mask: e.g 1000, then subtracting 1 == 0111
    reg |= (ones<<start); //Set the bits
    return reg;
}

int main() {
    uint8_t reg, start, end;
    scanf("%hhu %hhu %hhu", &reg, &start, &end);
    printf("%u", set_range(reg, start, end));
    return 0;
}