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

eturn interprets the top of the stack as the result,
it jumps to the return address,
deletes the current frame
and puts the result back on top of the remaining stack.
[proc 2 26,
con 0, arg 2, leq, cjp 5,
con 1, return,
con 1, arg 2, sub, arg 1,
call 0, arg 1, mul,
return,
con 2, con 10, call 0,
halt]
100
32 0
10 -1
2 -2
we push argument 1 (in this case 10), multiply the top two numbers, and push the result
to the stack
[proc 2 26,
con 0, arg 2, leq, cjp 5,
con 1, return,
con 1, arg 2, sub, arg 1,
call 0, arg 1, mul,
return,
con 2, con 10, call 0,
halt]
100
return interprets the top of the stack as the result,
it jumps to the return address (32 this time),
deletes the current frame
and puts the result back on top of the remaining stack (which is empty here).
[proc 2 26,
con 0, arg 2, leq, cjp 5,
con 1, return,
con 1, arg 2, sub, arg 1,
call 0, arg 1, mul,
return,
con 2, con 10, call 0,
halt]
we are finally done; the result is on the top of the stack. Note that the stack below has
not changed.
What have we seen?
100
c○: Michael Kohlhase 310
The four new VMP commands allow us to model recursive functions.
proc a l contains information about the number a of arguments and the length l of the
procedure
arg i pushes the i th argument from the current frame to the stack.
(Note that arguments are stored in reverse order on the stack)
call p pushes the current program address (opens a new frame), and jumps to the program
address p
177