iAPX 286 Operating System Writers Guide 1983

IntelU~ING HARDWARE PROTECTION FEATURESPL/M-286 COMPILER 960-505 date PAGE 2151617181920212223242526272829303132333412222222222212222222/*******************************************************//* Create a descriptor at a given slot to a given '*//* segment, and return selector that points *//* to that slot. */POINT AT: PROCEDURE (SLOT, RIGHTS, PHYS_ADDR_PTR, LIMIT)- PUBLIC REENTRANT;DECLARE SLOT SELECTOR,SLOTW WORD AT (@SLOT), /* Alternate type */RIGHTS BYTE,PHYS ADDR PTR POINTER,PHYS-ADDR- BASED PHYS ADDR PTR STRUCTURE(LO-WORD WORD, --DECLARE SLOTIDTA_SELDTHI-WORD WORD) ,LIMITWORD;WORD, /* Slot index */SELECTOR, /* To be set to eitherGOT alias or LOT alias. */BASED DTA SEL (DT SIZE)STRUCTURE- (DESC_STR);DTA'SEL = FIND DT ALIAS(SLOT);SLOTI = SHR(SLOTW;3); /* Expose index value. */DT(SLOTI) .LO BASE PHYS ADDR.LO WORD;DT(SLOTI) .HI-BASE LOW(PHYS ADDR.HI WORD);DT(SLOTI).LIMIT LIMIT; - -DT(SLOTI) .SW RESRVD 0;DT(SLOTI) .RIGHTS RIGHTS;RETURN;/*******************************************************//* Invalidate descriptor indexed by SLOT. */NULLIFY: PROCEDURE (SLOT) PUBLIC REENTRANT;DECLARE SLOTSLOTWDECLARE SLOTIDTA SELDTSELECTOR,WORD AT (@SLOT); /* Alternate type */WORD, /* Slot index */SELECTOR, /* To be set to eitherGDT alias or LDT alias. */BASED DTA SEL (DT SIZE)STRUCTURE-(DESC_STR);DTA SEL = FIND DT ALIAS(SLOT);SLOTI = SHR(SLOTW;3); /* Get index part of selector. */DT(SLOTI) .RIGHTS = 80H;/* This invalid value preventsuse of the descriptor. */RETURN;END NULLIFY;/*******************************************************/Figure 2-16. Descriptor Manipulation Example (Cont'd.)2-24 121960-001