// 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;
}