iAPX 286 Operating System Writers Guide 1983

INTRODUCTION TO PROTECTED MULTITASKING ON THE iAPX 286Hardware system tables enable both the hardware and the operating system to distinguish between thephysical segments of individual tasks. Figure 1-1 shows how physical segments of one task are logicallyseparate from those of other tasks. Since references to physical segments are always relative to systemdescriptor tables, the actual locations of physical segments in physical memory are not significant tothe tasks and therefore are not illustrated.Descriptor tables serve not only to identify the segments that belong to a task but also to isolate theaddress space of one task from that of another, so that one task cannot inadvertently affecUhe operationsofanother.Multitasking works through close interaction of the operating system with hardware features. Whenthe executing task needs to wait for some event (such as the arrival of data from some I/O device), itnotifies the operating system. The operating system determines which other task should execute next,and then causes the processor to store the state of the current task, retrieve the state of the next task;and begin executing the next task at the point where its processing last halted. The processor thenexecutes that task until .the task needs to wait for some· event. (This is a somewhat oversimplifiedTASK APLlM·286 COMPILATIONTASKDMULTIPLANTASKBFORTRANANALYZERTASK CAEDIT'"Figure 1-1. Segregation of Segments by Tasks with Private LOTs121960·401-2121960-001