The main feature of the Boot loader is that it is executed each time the part is powered up, or on reset. It also provides code protection for the 64- and 144-pin devices. On these devices, the Boot loader is 8 KB in size and resides in the top of Flash memory. The Boot loader provides an interface for Flash programming modes ISP and IAP, and checks for valid code at reset by looking at a memory location for a checksum. If the checksum is found valid, the boot loader transfers execution to user code.