In the code below the Subroutine ONECHAR takes 1 character from the user (keyboard) and saves it into the memory. The assembly code uses ONECHAR in a loop 6 times to input 6 characters and saves it to the memory. Finally it prints the string to the screen. a. Line 6 saves R0 before calling the subroutine ONECHAR. Briefly explain why this is necessary. b. What other register needs to be stored and restored inside the subroutine [Fill in lines 17, 19]. c. Once the subroutine is done, we have to restore registers. In line 8, fill in the register restored. What is pushed on the supervisor stack when an interrupt is serviced