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

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 
PUSHIMM 20   // push 20
STOREOFF 3   // store y
//-----------------------//
// set up frame for add  //
//-----------------------//

// top of old frame:
PUSHIMM 0    // space for rv

// build new frame:
PUSHFBR      // store current FBR (saved FBR)
PUSHOFF 2    // push parameter x (value of x)
PUSHOFF 3    // push parameter y (value of y)
PUSHSP       // push SP ---------
PUSHIMM 3    // # of params+1    \___________ set up FBR 
SUB          // SP - (params+1)  /           
POPFBR       // store new FBR ---

// execute callee code:
JSR add      // go to add function

// finish execution:
ADDSP -2     // pop off params
POPFBR       // set FBR to previous, or saved, value (meaning: restore FBR)
STOREOFF -1  // save returned value
ADDSP -2     // pop off local vars
JUMPIND      // return from main (pops off ra)

// add function:
add:
PUSHOFF 1    // push value of a
PUSHOFF 2    // push value of b
ADD
STOREOFF -1  // store x+y to return
JUMPIND      // return to caller (pops off ra)