General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

eturn takes the current frame from the stack, jumps to previous program address.
(which is cached in the frame)
call and return jointly have the effect of replacing the arguments by the result of the
procedure.
c○: Michael Kohlhase 311
We will now extend our implementation of the virtual machine by the new instructions. The
central idea is that we have to realize call frames on the stack, so that they can be used to store
the data for managing the recursion.
Realizing Call Frames on the Stack
Problem: How do we know
what the current frame is?
(after all, return has to pop it)
Idea: Maintain another register: the
frame pointer (FP), and cache information
about the previous frame
and the number of arguments in the
frame.
frame pointer
return address 0
previous frame
argument number
first argument -1
last argument -n
Add two internal cells to the frame, that are hidden to the outside. The upper one is called
the anchor cell.
In the anchor cell we store the stack address of the anchor cell of the previous frame.
The frame pointer points to the anchor cell of the uppermost frame.
c○: Michael Kohlhase 312
With this memory architecture realizing the four new commands is relatively straightforward.
Realizing proc
proc a l jumps over the procedure with the help of the length l of the procedure.
label instruction effect comment
〈proc〉 MOVE IN1 ACC ACC : = VPC
STORE 0 P (0): = ACC cache VPC
LOADIN 1 2 ACC : = P (VPC + 2) load length
ADD 0 ACC : = ACC +P (0) compute new VPC value
MOVE ACC IN1 IN1: = ACC update VPC
JUMP 〈jt〉 jump back
c○: Michael Kohlhase 313
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