iAPX 286 Operating System Writers Guide 1983

interDATA SHARING, ALIASING, AND SV!'IICHRONIZATIONDESCRIPTORTABLEJ---------t 1DESCRIPTORTABLEINDEXFROMSELECTORPOINTERTABLEr===:l--INDEXFROMSELECTORPOINTERPOINTERTABLE~11--------tPOINTERALIAS LISTS0--------0~------------------------------DHEADERD~-----------------------------,D 0---------0oHEADERr-----------------------------,IIIIIIIIIHEADERDD 0-,.-------0I L _____________________________Figure 5-3. Identifying Alias List121960-31Low-Level Mutual ExclusionThe most basic form of synchronization is control over critical sections. A critical section is a sequenceof instructions that operates on shared resources in such a way that errors could result if anothersequence of instructions operated on the same resources within the same time span. Any means thatprevents any two critical sections from overlapping in time would prevent such errors. In a singleprocessorsystem, only an interrupt can cause the operations of one critical section to interleave withthose of another. Disabling interrupts provides the necessary mutual exclusion.Procedures that disable interrupts to provide mutual exclusion must adhere to certain rules:Determine the maximum permissible delay for servicing an interrupt, and do not code a criticalsection that takes more time.• Avoid causing a fault that might keep interrupts disabled for a longer time than permissible_5-5 121960-001