Run Time Stack - Ch 7 - Part 2 of 2

s0 for globals
s1 for display level 1 (d[1])
s2 for display level 2 (d[2])
s3 for display level 3 (d[3])
top_sp     Top of Stack (TOS)
-----------------------------------------------------------
This is a continuation from lecture1.rts providing another example.

                                       ---------------
                                       |  space      |
                                       |   for       |
                                       |   vars      |
                                       ---------------
                                       | old level ? | control
Activation record (AR) for a procedure | return addr |  info
                                       ---------------
*** AR(1) refers to AR for P1; AR(2) refers to AR for P2; AR(3) for P3 
and AR(4) for P4 (since each proc with have similar control information, 
and different space for local variables
***

  The size of the activation record is known at compile time,
  so we can set put the appropriate pushes/pops

In lecture 1,  we  used $s0 for globals, $s1 for level 1, $s2 for level 2
                  $s3 for level 3 and $top_sp for top of stack (TOS)

The code outline looks like:

main
   proc P1
       proc P2
            proc P3
            end 3;

            proc P4
              invokes proc P3 or proc P4 (recursively)
            end 4;
       invokes proc P3
       invokes proc P4
       end 2;
   invokes proc P2
   end 1;

invokes proc P1
end main

Assume:

Main calls P1 calls P2 calls P3 and P3 returns
                       calls P4 calls P4' calls P4'' calls P4''' calls P3

Set up for the first part of this calling sequence:

Main calls P1 calls P2 calls P3

The Display:

---------------------------
| calls    P1   P2   P3   |
|--------------------------
| d[3]     ?    ?    P3   |
---------------------------
| d[2]     ?    P2   P2   |
---------------------------
| d[1]     P1   P1   P1   |
---------------------------
| saved:   ?    ?    ?    |
---------------------------


The Display Picture of the RTS
-------------------------------

     $top_sp -> ------------------
                |local vars (P3) |
                |     : old d[3] |----> (old $s3 never set)
                |AR(3): return   | 
         $s3 -> ------------------
                |local vars (P2) |
                |     : old d[2] |----> (old $s2 never set)
                |AR(2): return   |
         $s2 -> ------------------
                |local vars (P1) |
                |     : old d[1] |----> (old $s1 never set)
                |AR(1): return   |
         $s1 -> ------------------
                |                |
                |  global vars   |
         $s0 -> ------------------
                |Code P1, P2, P3 |
                |                |
                |   code main    |
                ------------------


and now we continue, therefore P3 has returned.

Main calls P1 calls P2  calls P4 calls P4(') calls P4('') calls P4(''')
calls P3


---------------------- ------------------------------
| calls    P1   P2       P4   P4'  P4'' P4''' P3    |
|--------------------- ------------------------------
| d[3]     ?    ?        P4   P4'  P4'' P4''' P3    |
---------------------- ------------------------------
| d[2]     ?    P2       P2   P2   P2   P2    P2    |
---------------------- ------------------------------
| d[1]     P1   P1       P1   P1   P1   P1    P1    |
---------------------- ------------------------------
| saved:   ?    ?        ?    P4   P4'  P4''  P4''' |
----------------------  -----------------------------

     $top_sp -> ------------------
                |local vars (P3) |
                |     : old d[3] |----------------------------------*
                |AR(3): return   |                                  |
         $s3 -> ------------------                                  |
                |local vars (P4) |                                  |
                |     : old d[3] |--------------------------------* |
                |AR(4): return   |                                | |
                ------------------ <<-----------------------------|-*
                |local vars (P4) |                                |
                |     : old d[3] |-----------------------------*  |
                |AR(4): return   |                             |  |
                ------------------ <<--------------------------|--*
                |local vars (P4) |                             |
                |     : old d[3] |--------------------------*  |
                |AR(4): return   |                          |  |
                ------------------ <<-----------------------|--*
                |local vars (P4) |                          |
                |     : old d[3] |----> (old $s3 never set) |
                |AR(4): return   |                          |
                ------------------ <<-----------------------*
                |local vars (P2) |
                |     : old d[2] |----> (old $s2 never set)
                |AR(2): return   |
         $s2 -> ------------------
                |local vars (P1) |
                |     : old d[1] |----> (old $s1 never set)
                |AR(1): return   |
         $s1 -> ------------------
                |                |
                |  global vars   |
         $s0 -> ------------------
                |Code P1, P2, P3 |
                |                |
                |   code main    |
                ------------------