anchor.diff:                                 :drive-n (aref sector 36) ; Operating system specific
                                  :boot-code (subseq sector 62 510))))
       (when (check-signature (aref sector 38))
  -      (setf (fat-%signature fat12) (aref sector 38)
  +      (setf (fat-%n-root-entry fat12) (sys.int::ub16ref/le sector 17)
  +            (fat-%signature fat12) (aref sector 38)
               (fat-%volume-id fat12) (sys.int::ub32ref/le sector 39)
               (fat-%volume-label fat12) (map 'string #'code-char (subseq sector 43 54))
               (fat-%fat-type-label fat12) (map 'string #'code-char (subseq sector 54 60))))
  
positive.code:           (fat-%sectors-per-cluster ffs) (check-sectors-per-cluster (aref sector 13) bytes-per-sector)
            (fat-%n-reserved-sectors ffs) (sys.int::ub16ref/le sector 14)
            (fat-%n-fats ffs) n-fats
            (fat-%n-root-entry ffs) (sys.int::ub16ref/le sector 17)
            (fat-%n-sectors16 ffs) (sys.int::ub16ref/le sector 19)
            (fat-%media-type ffs) (check-media-type (aref sector 21))
            (fat-%sectors-per-track ffs) (sys.int::ub16ref/le sector 24)
  
negative.code:           (fat-%n-clusters ffs) (+ (compute-cluster-count sector) 2))))
  
  (defclass fat12 (fat-base)
    ((%n-root-entry :initarg :n-root-entry :accessor fat-%n-root-entry :type (unsigned-byte 16))))
  
  (defun check-signature (signature)
    (if (or (= signature #x28)
  