anchor.diff:    (loop
        with td->xfer-info = (td->xfer-info ohci)
        with td-tail-phys-addr = (ed-tdq-tail ed)
  -     for td-phys-addr = (logandc2 (ed-tdq-head ed) #x0F) then next-td-phys-addr
  +     for td-phys-addr = (ed-tdq-head-tdq-addr ed) then next-td-phys-addr
        for td = (phys-addr->array td-phys-addr)
        for xfer-info = (gethash td td->xfer-info)
        for next-td-phys-addr = (td-next-td 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:            condition-code)))
  
        (setf dummy-td msg-td ;; re-use previous td as "dummy" td
              msg-td (phys-addr->array (ed-tdq-head-tdq-addr ed))
              msg-xfer-info (gethash msg-td (td->xfer-info ohci))
  
  
  