// Basic: b.sam
//
// int main() {
//    int x,y;
//    x := 10;
//    y := 20;
//    return x+y;
// }

PUSHIMM 0    // space for return value of program (result of main())
PUSHFBR      // save current FBR (starts at 0)
PUSHIMM 1    // need to push 1 for FBR
POPFBR       // set FBR to 1
JSR main     // invoke main and push return address on the stack
POPFBR       // reinstall old FBR
STOP         // end program
main:        // label of first command is function name
ADDSP 2      // allocate local variables
PUSHIMM 10   // push 10
STOREOFF 2   // store x  (why 2? see below)
PUSHIMM 20   // push 20
STOREOFF 3   // store y  (why 3? see below) <======= see diagram for SaM
PUSHOFF 2    // retrieve x                           up to this point
PUSHOFF 3    // retrieve y
ADD          // x+y
STOREOFF -1  // store x+y to return
ADDSP -2     // pop off local variables
JUMPIND      // return to caller

Stack for "see diagram" portion:

  +----+            Must account for local var positions in stack!
7 |    | <-- SP     To store x, use STOREOFF 2:
  +----+            --> STOREOFF params + 1 ra slot + 1 saved FBR slot + FBR
6 | 20 |            --> STOREOFF 0 + 1 + 1 + 1
  +----+            --> "store V[top] in V[3]"
5 | 10 |            
  +----+
4 | y  |
  +----+
3 | x  |
  +----+
2 | 5  | return address                                           new frame
  +----+                                                             ^
1 | 0  | <-- FBR (FBR is 1, which means it points to address 1)   ___|___
  +----+                                                             |
0 | 0  | return value                                                V
  +----+                                                          old frame