anchor.diff:       (td-buffer-pointer msg-td) buf-phys-addr
        (td-next-td msg-td) (array->phys-addr dummy-td)
        (td-buffer-end msg-td) (+ buf-phys-addr buf-size -1)
  +     (td-info-buf msg-td-info) buf
        ;; update queue tail pointer to new dummy td
        (ed-tdq-tail ed) (array->phys-addr dummy-td))
       dummy-td))
  
positive.code:     (if (= buf-pointer 0) buf-size (- buf-pointer buf-start))))
  
  ;;======================================================================
  ;; HCCA (Host Controller Communication Area) - defined in section 4.4
  ;;======================================================================
  
  
negative.code:                       index
                        length)
  
        (setf (gethash msg-td (td->info ohci)) td-info
              (ed-tdq-tail ed) (array->phys-addr dummy-td))
  
        (with-trace-level (3)
  