Qualcomm interview questions

what's the different between Class and Structure in Objective C

Given a uni processor architecture. A process X has code at memory location 10,000 , the PC(program counter) is pointing to it and executing instruction from there. A process Y has code at memory location 20,000. The OS(kernel) has its own code at some memory location. How does the PC change from process from code at location 10,000 or 10,050(after some time of execution),to the kernel location during a context switch to Process B ? Please let me know if you have the answer. Bamboozled by this question,don't know if I get onsite :(