160. Register Overlay byte and bitfield

#include <iostream>
#include <cstdint>
using namespace std;

union Reg {
    uint8_t raw;
    struct {
        uint8_t en : 1;
        uint8_t mode : 3;
        uint8_t reserved : 4;
    } bits;
};

int main() {
    unsigned int input;
    cin >> input;

    uint8_t val = static_cast<uint8_t>(input);

    Reg r;
    r.raw = val;

    cout << "en=" << static_cast<int>(r.bits.en)
         << " mode=" << static_cast<int>(r.bits.mode);

    return 0;
}

Explanation & Logic Summary:

The union Reg overlays a single 8-bit memory location with two views:

• raw allows full-register access
• bits allows named access to individual bit fields

Writing to raw updates the bitfields automatically.
The explicit integer casts ensure that uint8_t bitfields are printed as numbers, not ASCII characters.

Firmware Relevance & Real-World Context:

This pattern directly mirrors how firmware interacts with:

• Memory-mapped peripheral registers
• Control and status registers
• Configuration bytes defined in datasheets

Using unions with bitfields improves readability over manual masking and shifting while preserving low-level control—an essential embedded C++ skill.