During power-up the oszillator starts up and internal logic needs to hold the microcontroller in a RESET-mode until the supply voltage is at the minimum level, the oszillator is running stable and internal registers are set to a defined state. Then the microcode excecution is started at a fixed address and performs the define start-up sequence and starts then execution of the macroinstruction(this is the programm you usually can do with the assembler).
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