Friday, November 20, 2009 1:04 AM,

I received this email last Tuesday the night before the exam.  It is 
important, but not something that I wanted to distract you with when you were 
preparing for the exam.  So, I pushed it on the stack temporarily.

Now the exam is over, so it is perfectly appropriate to share it with you.

A student writes:



	 Hello Dr Patt,
	            I just had something on my mind since the last lecture 
	 and it has nothing to do with the exam so when you get time feel free 
	 to respond. We went over interrupts and handling interrupts according 
	 to priority and privilege etc.  Well, when you return from an 
	 interrupt you use the rti which you said does not have any operands. 
	 Now wouldn’t the supervisor stack be considered an operand since it is 
	 an abstract data type? Or are the lc3's operands strictly restricted 
	 to 16 bit 2’s compliment integers? Those are the operands it supports 
	 for all the other instructions in the isa but shouldn’t it include the 
	 supervisor stack as well since a part of that isa is working on the 
	 supervisor stack during the rti instruction?
	 Thanks,
	 <<name withheld to protect the student who needs the term operand 
	 clarified>>



Operands usually refer to what is EXPLICITLY part of the instruction.
Sure, the supervisor stack is used.  However, which stack is used is not part 
of the instruction.  Which stack is used is determined by PSR[15], the bit 
identifying the privilege level of the code that is running.  Also, you can 
not execute RTI unless you are running with privilege.

Make sense?

Good luck with the rest of the rest of the course.  You are now on the home 
stretch.

Yale Patt