9.3.4 Fetching of Initial Instructions
After Reset has been released, the CPU starts fetching PC and SP values from the Reset address, 0x00000000. This points to the first executable address in the internal Flash memory. The code read from the internal Flash can be used to configure the clock system and clock sources. Refer to the ARM Architecture Reference Manual for more information on CPU startup (www.arm.com).
References:
GCLK - Generic Clock Controller