// Pops the a value from the appropriate stack, decrements sp, and returns
// the popped value.
private short popValue(int stackID) throws ProgramException {
short newSP = (short) (getSP() - 1);
short value;
if (stackID == MAIN_STACK) value = stackSegment.getValueAt(newSP);
else value = workingStackSegment.getValueAt(newSP);
checkSP(newSP);
setSP(newSP);
return value;
}
// Returns the element at the top of the stack and decrements sp by 1.
private short popAndReturn() throws ProgramException {
short newSP = (short) (getSP() - 1);
checkSP(newSP);
setSP(newSP);
return stackSegment.getValueAt(newSP);
}
/** Returns the value of the function to the top of the stack. */
public void returnFromFunction() throws ProgramException {
// make sure that there's somewhere to return to (old local <> 0)
if (stackSegment.getValueAt(Definitions.LOCAL_POINTER_ADDRESS) == 0)
throw new ProgramException(
"Nowhere to return to in "
+ getCallStack().getTopFunction()
+ "."
+ getCurrentInstruction().getIndexInFunction());
// done in order to clear the method stack's contents
workingStackSegment.setStartAddress(getSP());
bus.send(ram, Definitions.LOCAL_POINTER_ADDRESS, ram, Definitions.R13_ADDRESS); // R13 = lcl
bus.send(
stackSegment,
stackSegment.getValueAt(Definitions.LOCAL_POINTER_ADDRESS) - 5,
ram,
Definitions.R14_ADDRESS); // R14 = return address
bus.send(
stackSegment,
getSP() - 1,
stackSegment,
ram.getValueAt(Definitions.ARG_POINTER_ADDRESS)); // *arg = return value
setSP((short) (ram.getValueAt(Definitions.ARG_POINTER_ADDRESS) + 1)); // SP = arg + 1
bus.send(
stackSegment,
ram.getValueAt(Definitions.R13_ADDRESS) - 1,
ram,
Definitions.THAT_POINTER_ADDRESS); // that = *(R13 - 1)
bus.send(
stackSegment,
ram.getValueAt(Definitions.R13_ADDRESS) - 2,
ram,
Definitions.THIS_POINTER_ADDRESS); // this = *(R13 - 2)
bus.send(
stackSegment,
ram.getValueAt(Definitions.R13_ADDRESS) - 3,
ram,
Definitions.ARG_POINTER_ADDRESS); // arg = *(R13 - 3)
bus.send(
stackSegment,
ram.getValueAt(Definitions.R13_ADDRESS) - 4,
ram,
Definitions.LOCAL_POINTER_ADDRESS); // lcl = *(R13 - 4)
// removes the top function from the call stack
callStack.popFunction();
// check whether there is a "calling frame"
if (stackFrames.size() > 0) {
// retrieve stack frame address of old function
int frameAddress = ((Integer) stackFrames.lastElement()).intValue();
stackFrames.removeElementAt(stackFrames.size() - 1);
workingStackSegment.setStartAddress(frameAddress);
// disable non relevant range of the local segment - enable only the locals
// of the function that we returned to.
localSegment.setEnabledRange(
Math.max(localSegment.getStartAddress(), Definitions.STACK_START_ADDRESS),
frameAddress - 1,
true);
// enable in the arg segment only the number of args that were sent to the function
// that we returned to.
argSegment.setEnabledRange(
argSegment.getStartAddress(), localSegment.getStartAddress() - 6, true);
// enable this, that according to their retrieved pointers
thisSegment.setEnabledRange(
Math.max(thisSegment.getStartAddress(), Definitions.HEAP_START_ADDRESS),
Definitions.HEAP_END_ADDRESS,
true);
thatSegment.setEnabledRange(
Math.max(thatSegment.getStartAddress(), Definitions.HEAP_START_ADDRESS),
Definitions.SCREEN_END_ADDRESS,
true);
} /* else {
error("Nowhere to return to");
} */
// Allow return if we previously had "function" even with no call -
// For the SimpleFunction test
short returnAddress = ram.getValueAt(Definitions.R14_ADDRESS);
if (returnAddress == VMProgram.BUILTIN_FUNCTION_ADDRESS) {
staticSegment.setEnabledRange(0, -1, true); // empty static segment
builtInFunctionsRunner.returnToBuiltInFunction(popValue(METHOD_STACK));
} else if (returnAddress >= 0 && returnAddress < program.getSize()) {
// sets the static segment range
if (stackFrames.size() > 0) {
setStaticRange(callStack.getTopFunction());
} else {
staticSegment.setStartAddress(Definitions.VAR_START_ADDRESS);
staticSegment.setEnabledRange(
Definitions.VAR_START_ADDRESS, Definitions.VAR_END_ADDRESS - 1, true);
}
program.setPC((short) (returnAddress - 1)); // set previousPC correctly
program.setPC(returnAddress); // pc = *sp
} else {
error("Illegal return address");
}
}