unsigned long millis() {
unsigned long m;
uint8_t oldSREG = SREG; //狀態寄存器(包括是否允許 Interrupt); 1clock
// disable interrupts while we read timer0_millis or we might get an
// inconsistent value (e.g. in the middle of a write to timer0_millis)
cli( ); // 禁止中斷; 1 clock
m = timer0_millis; // 讀取記憶體的全域變量 timer0_millis;8 clock
SREG = oldSREG; // 恢復狀態寄存器(注意不一定恢復中斷喔 !);1 clock
return m; // 6 clocks
} // millis( // total 17 clock cycles