iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURESTable 2-2. Control Transfer MechanismsTransferTypeOperationDescriptorReferencedTableIntrasegment JMP, CALL, RET (none)Intersegment JMP, CALL, RET, IRET code segment GOT/LOT(*)CALL, JMP call gate GOT/LOT(same PL)INT instruction, trap or lOTexternal interrupt,interruptor exceptiongate (same PL)Interlevel CALL call gate GOT/lOTINT instruction, trap or lOTexternal interrupt,interruptor exceptiongateRET,IRET code segment GOT/lOTIntertask CALL, JMP, IRET task state GOTsegmentCALL, JMP task gate GOT/LOTINT instruction, task gate lOTexternal interrupt,exception* .Includescases in which the target segment is incidentally the same as the calling segment.JMP instruction may also reference a call gate. If the target executable segment is at the same privilegelevel, no level change occurs.For transfers of control between segments at different privilege levels (as illustrated in figure 2-8) thereare three differences:• Only the following instructions can be used:CAL L offset selectorRETJumps between privilege levels within a task are not allowed.• The selector does not select the descriptor of an executable segment but rather selects a gatedescriptor.• The offset operand must be present but is ignored.2-10 121960-001