Let’s take a look at a flowchart for the Boot loader. At reset, you initialize and go through the Boot loader. You can then see if the reset was due to a watchdog timeout. If not, you can go down and check whether you should enter the ISP mode. Look for port 0.14 being low. If that port is low, then you will start executing ISP code. The other thing that the Boot loader does is if you don't have a watchdog reset, you can go and check the user code. The user code is valid if you have a valid signature at location 0x14 hex in the device. That is a checksum of the first 32 vectors in the vector table on the device. If that user code is valid, then you can go ahead and execute user code. If it is not valid, then you will also execute the ISP, which begins talking to the utility to allow downloading from the PC to the device, without the hex code that you want to program into the device. The remaining registers are available to the other modes, except for the registers that are colored in the graphic. The registers that are colored are unique to each of the specific modes.