anchor.diff:             condition-code)))
   
         (setf dummy-td msg-td ;; re-use previous td as "dummy" td
  -            msg-td (phys-addr->array (ed-tdq-head ed))
  +            msg-td (phys-addr->array (ed-tdq-head-tdq-addr ed))
               msg-xfer-info (gethash msg-td (td->xfer-info ohci))
   
               ;; clean up dummy-td
  
positive.code:     (let* ((endpoint (aref (usb-device-endpoints device) 0))
             (event-type (ohci-endpoint-event-type endpoint))
             (ed (ohci-endpoint-ed endpoint))
             (msg-td (phys-addr->array (ed-tdq-head ed)))
             (msg-xfer-info (gethash msg-td (td->xfer-info ohci)))
             (msg-buf (alloc-buffer/8 (buf-pool ohci) 8))
             (dummy-td (alloc-td ohci))
  
negative.code: (defun ed-tdq-head (ed)
    (aref ed +endpt-tdq-head+))
  
  (defun ed-tdq-head-tdq-addr (ed)
    (logandc2 (aref ed +endpt-tdq-head+) #x0F))
  
  (defun (setf ed-tdq-head) (phys-addr ed)
    ;; clears the toggle carry and halted bits.
    (validate-address/16 tdq-head phys-addr)
  