27. Extract Bit Field from 16-bit Register

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

uint8_t extract_field(uint16_t reg) {
    const uint8_t start_pos = 4; 
    const uint8_t end_pos = 8;
    const uint8_t len = end_pos - start_pos + 1;

    uint16_t mask = (((1U << len) - 1U) << start_pos);

    uint8_t val = (uint8_t) ((reg & mask) >> start_pos); 

    return val;
}

int main() {
    uint16_t reg;
    scanf("%hx", &reg);
    printf("%u", extract_field(reg));
    return 0;
}