Which of the following statements are true. Select all that apply.

A. MAR register needs to be saved during context switch.

B. A system uses a timer register to store the current real-world time in the precision of a second. It is updated by the physical clock when the

time advances a second. This timer register needs to be saved during a context switch.

C. The user code of a root user can run in kernel mode due to its root privilege.

D. A root user can rewrite the OS to allow her program to switch to kernel mode to run her own user code (not the system trap functions).

E. A kernel trap does not require context switch even though the OS code, not the user code, is being executed.

F. The process control block (PCB) should have a pointer pointing to the list of all waiting processes.