How to set PC (Program Counter) register to another function (for scheduler)
This:
SP = (uint16_t)(&amp;dummy)

is changing the stack pointer. Not what you want. You want to instead
push the destination address to the stack. However, rather than
simulating a function return, it should be simpler to perform an
indirect jump:
asm volatile(&quot;ijmp&quot; :: &quot;z&quot;(dummy));

Don't forget that your naked ISR has to save all the call-used
registers, as well as SREG, r0 and r1, in its custom prologue. And
restore them in its epilogue. Check the AVR calling
conventions.