iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURES7 o 7 oRESERVED FOR iAPX 386 +6MUST BE ZEROP DPL 0 0 1 TYPE X X XWORD COUNTONLY FOR TYPE~OO+4DESTINATION SELECTOR X X +2DESTINATION OFFSETNOT USED FOR TYPE~01 or 11oXX -.NOT USED121960-49Figure 2-6. Gate DescriptorPRESENT BITSince a gate descriptor does not refer directly to a segment, the present bit in a gate descriptor doesnot necessarily indicate whether a segment is present. It can be used for other purposes, however. Referto Chapter 11 for an example of using the present bit to facilitate late binding.Control Transfer MechanismsTable 2-2 summarizes the mechanisms for each class of control flow transfer.Control transfers within a segment function similarly to intrasegment transfers on the iAPX 86,88,except that the processor checks that the destination address does not exceed the segment limit.Figure 2-7 illustrates a change in control flow between segments at the same privilege level. Any ofthe following instructions can effect such a transfer:J M P offset selectorCAL L offset selectorRET (offset and selector taken from stack)The selector selects a descriptor for an executable segment. The DPL in the target segment's descriptormust be the same as the privilege level under which the calling segment is running. A CALL or2-9 121960-001