3.3.2.1 Runtime Startup Code
Pre-built object files, which contain the runtime startup code, are provided with the compiler. These form part of the device family packs, and can be found in <dfp>/avr/lib, under the relevant directory for your project's target device.
Should you require any special initialization to be performed immediately after Reset, you should write a powerup initialization routine (described later in 4.9.3 The Powerup Routine).