Returns the size of the previous chunk, masking off what would be the flag bits if it were in the actual size
        field. Performs NO CHECKING to determine whether the previous chunk size is valid (for example, when the
        previous chunk is not free, its size cannot be determined).

def prev_size(self):
        """
        Returns the size of the previous chunk, masking off what would be the flag bits if it were in the actual size
        field. Performs NO CHECKING to determine whether the previous chunk size is valid (for example, when the
        previous chunk is not free, its size cannot be determined).
        """
        return self.state.memory.load(self.base, self._chunk_size_t_size) & ~CHUNK_FLAGS_MASK