The ATmega8A stores four different calibration values for the internal RC Oscillator. These bytes resides in the signature row High byte of the addresses 0x0000, 0x0001, 0x0002, and 0x0003 for 1, 2, 4, and 8MHz respectively. During Reset, the 1MHz value is automatically loaded into the OSCCAL Register. If other frequencies are used, the calibration value has to be loaded manually, see OSCCAL – Oscillator Calibration Register for details.