The previous section listed five ways in which Pop-11 programs implicitly use the stack. We can now combine points 1, 2, 3, 4, and 5, and look closely at the implicit stack instructions when Pop-11 runs the procedure perim, defined in Chapter 1.
First a reminder of its definition in chapter 1:
define perim(len, breadth) -> result;
(len + breadth) * 2 -> result
enddefine;
Assuming the definition has already been compiled, we can step through
what happens when it is obeyed in the following instruction (declare the
variable num, compute the perimeter of a 3 by 4 room and assign the
result to num):
vars num;
perim(3, 4) -> num;
This implicitly "expands" into the following Pop-11 virtual machine
instructions:
STEP What happens VM instruction
(a) put 3 on the stack (i.e. PUSH 3),
(b) put 4 on the stack (i.e. PUSH 4),
(c) run perim (i.e. CALL perim),
this will do various things and leave a result on the stack
(d) move the top of stack to num (i.e. POP num)
We can look more closely at step (c), i.e. what happens when the
procedure runs. Look at the definition, from chapter 1:
define perim(len, breadth) -> result;
(len + breadth) * 2 -> result
enddefine;
When this runs, with 3 and 4 on the stack, as in step (c) above it does
the following:
STEP What happens VM instruction
(ca) move top of stack (i.e. 4) to breadth (POP breadth)
(cb) move top of stack (i.e. 3) to len (POP len)
(cc) copy value of len (i.e. 3) to stack (PUSH len)
(cd) copy value of breadth (i.e. 4) to stack (PUSH breadth)
(ce) run + (CALL +)
(which takes 3 and 4 off the stack and puts back 7)
(cf) put 2 on the stack (PUSH 2)
(cg) Run the multiplication procedure (CALL *)
(this takes 7 and 2 off and puts back 14)
(ch) Move top of stack (14) to result (POP result)
(ci) Finally, because result is an output
local, its value is put on the stack (PUSH result)
It is not absolutely essential to understand such details of implicit
stack manipulation on order to develop Pop-11 programs, but it will make
many things clearer and will help with more advanced programming. For
practice try the following exercises: