CSC362 2011F, Class 39: The Run-Time Stack

Overview:
* Format of stack frames.
* Tracing the stack.
* Function/procedure initialization.
* Function/procedure cleanup.

Admin:
* Phase 6 of the project is due next Wednesday.
  * Don't need to do arrays.
* My primary plans for this coming weekend are to celebrate my wife's birthday and grade stuff from your class.  I hope to be caught up by Monday.
* Thoughts on Steganography CS extra?
* EC for TK recital Saturday at 2pm.
* EC for Erin Nichols events next Tuesday: Lunchtime talk and Tuesday extra.
* EC for swim meet (Friday 6-9 pm, Saturday all day; two hours suffice).
* Do you still want to ask questions on Phase 5?
  * Sam has a general observation.
  * Don't do everything at once within a phase.

Eventually, you need to turn offsets within the stack frame into things
that can be parameters to STAC instructions.

We'll want to write
  new_relative (SP->info.reg, offset)

----

Last thing: Function calls!

Let's review the layout of the stack and what happens to the stack
while the program is running.

* What goes on the stack for a function all?
  * Local variables
  * Parameters
  * Return address

Original layout

     |        |
1028 +--------+----------------------<- frame pointer
     |PREVFP  | CURRENT FRAME
1024 +--------+
     |RETADDR |   
1020 +--------+
     |PARAM0  |
1016 +--------+
     |PARAM1  |                           STACK GROWS DOWN
1012 +--------+                                 |
     |LOCAL0  |                                 v
1008 +--------+
     |LOCAL1  |
1008 +--------+   <- stack pointer
     |        |

Call a procedure;  What happens to the stack?
     |        |
1028 +--------+---------------------------------
     |PREVFP  | PARENT FRAME
1024 +--------+
     |RETADDR |   
1020 +--------+
     |PARAM0  |
1016 +--------+
     |PARAM1  |                           STACK GROWS DOWN
1012 +--------+                                 |
     |LOCAL0  |                                 v
1008 +--------+
     |LOCAL1  |
1004 +--------+--------------------------------- <--- FRAME POINTER
     |1028    | CURRENT FRAME
1000 +--------+
     |RETADDR |
 996 +--------+
     | PARAM0 |
     +--------+
     | LOCAL0 |
     +--------+<- stack pointer
     |        |

When we're done with the procedure, we want to pop the frame.

What are the STAC instructions to build up the stack for a procedure call?
* Push frame poiner
* Push return address
* Push parameters
* Decrement the stack pointer by the size of the activation record

What are the STAC instructions to restore after we're done with the call?
* Restore the stack pointer
  MOV %sp %fp  (aka %sp = %fp)
1004 +--------+--------------------------------- <--- FRAME POINTER stack pointer
     |1028    | CURRENT FRAME
1000 +--------+
     |RETADDR |
 996 +--------+
     | PARAM0 |
     +--------+
     | LOCAL0 |
     +--------+
     |        |

* "Pop" into the current frame pointer
  MOV %fp %sp(-4)
  OR
  MOV %fp %fp(-4)
* JUMP %sp(-8)

Note: poier == pointer == pointner == poitner 

We have left out a few things that go the stack ... 
* Return value!
* Temporaries!