After being reset, the CPU will execute instructions from the lowest address in the Flash program memory, 0x0000. The Program Counter (PC) addresses the next instruction to be fetched.
The CPU supports instructions that can change the program flow conditionally or unconditionally and are capable of addressing the whole address space directly. Most AVR instructions use a 16-bit word format, and a limited number use a 32-bit format.
During interrupts and subroutine calls, the return address PC is stored on the stack as a word pointer. The stack is allocated in the general data SRAM, and consequently, the stack size is only limited by the total SRAM size and the usage of the SRAM. After the Stack Pointer (SP) is reset, it points to the highest address in the internal SRAM. The SP is read/write accessible in the I/O memory space, enabling easy implementation of multiple stacks or stack areas. The data SRAM can easily be accessed through the five different Addressing modes supported by the AVR CPU.