iAPX 286 Operating System Writers Guide 1983

TASK MANAGEMENTThe example in figure 4-5 illustrates the general case in which all the pertinent information is in separatesegments. This case uses five GDT slots. You can free one GDT slot by including the TSS within theTIB. The TIB descriptor can serve as the data-segment alias for the TSS. Figure 4-6 illustrates such aconfiguration.Speed of access to the TDB is critical in some applications. Figure 4-7 shows another configuration oftask information that helps improve access speed. Here the task's stack segment for PL 0 contains boththe TSS and the TIB. The advantage of this approach is that the TIB and the TSS can be addressedrelative to the base address that the processor loads into SS when transferring control to a PL-O operatingsystemprocedure. This eliminates the need for loading the DS or ES register to access the currenttask's TSS or TIB and also frees a segment register for other use. In such a case, the SP portion of theTSS initial stack values for PL 0 is set to an offset beyond the TIB. In many applications it is stilldesirable for the GDT to hold a descriptor for the TIB so as to facilitate access to TIBs for tasks otherthan the current one.ACCESS MODE 2When the scheduler is dispatching a different task, it needs quick access to the queues of waiting andready tasks. If the links that implement these queues thread through the many segments that containTIBs, the time needed to search and update the queues is extended by the time needed to load aLOTGOTbfTIBLOT ALIAS LOT TSS - ~.......... TSSTIB1\ ...... , I- LOT SELECTOR"'-'--Figure 4-6. Task Information Structure B121960-254-12 121960-001