Question

Which of the following two approches is more efficient on an ATmega328P?

unsigned int value;
unsigned char char_high, char_low;

char_high = value>>8;
value = value<<8;
char_low = value>>8;

OR

unsigned int value;
unsigned char char_high, char_low;

char_high = value>>8;
char_low = value & 0xff;

Answer 1

You really should measure. I won’t answer your question (since you’d benefit more from measuring than I would), but I’ll give you a third option:

struct {
    union {
        uint16_t big;
        uint8_t small[2];
    };
} nums;

(be aware of the difference between big endian and little endian here)