In Atmel’s AVR crack case, security fuses are physically located on the chip die. This could mean a separate memory cell located next to the main memory array, or even far from it. For example, the security is not very high as the fuses can be easily found and disabled by one or another method. Meantime, some methods require very expensive equipment and even if the avr MCU attacker knows where the fuse is, he will not be able to reset it until he gets access to such equipment and learns how to use it.