Method: (INITIALIZE-BLOCK T T MARK-AND-SWEEP-HEAP)

Source

(defmethod initialize-block (block block-size (heap mark-and-sweep-heap))
  ;; This is called by a free list heap while creating free blocks.
  ;; Write the block size (as a negative number) in the start of the
  ;; block (just behind the header) to indicate that this is a free
  ;; block.  This is necessary for the sweep phase of a mark-and-sweep
  ;; collector to distinguish it from a block that contains an object.
  (file-position (heap-stream heap) (+ block (block-header-size heap)))
  (serialize (- block-size) (heap-stream heap)))