Intel 80286, 80287 Pointer Validation, Dynamic Segment Relocation and Expansion of Segment limit

ADVANCED TOPICS

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Figure 11-2. Dynamic Segment Relocation and Expansion of Segment limit

11.3 POINTER VALIDATION

Pointer validation is an important part of locating programming errors. Pointer validation is necessary for maintaining isolation between the privilege levels. Pointer validation consists of the following steps:

1.Check if the supplier of the pointer is entitled to access the segment.

2.Check if the segment type is appropriate to its intended use.

3.Check if the pointer violates the segment limit.

The 80286 hardware automatically performs checks 2 and 3 during instruction execution, while software must assist in performing the first check. This point is discussed in section 11.3.2. Software can explic- itly perform steps 2 and 3 to check for potential violations (rather than causing an exception). The unprivileged instructions LSL, LAR, VERR, and VERW are provided for this purpose.

The load access rights (LAR) instruction obtains the access rights byte of a descriptor pointed to by the selector used in the instruction. If that selector is visible at the CPL, the instruction loads the access byte into the specified destination register as the higher byte (the low byte is zero) and the zero flag is set. Once loaded, the access bits can be tested. System segments such as a task state segment or a descriptor table cannot be read or modified. This instruction is used to verify that a pointer refers toa segment of the proper privilege level and type. If the RPL or CPL is greater than DPL, or the selector is outside. the table limit, no access value is returned and the zero flag is cleared. Conforming code segmentSinay be accessed from any RPL or CPL.

Additional parameter checking can be performed via the load segment limit (LSL) instruction. If the descriptor denoted by the given selector (in memory or a register) is visible at the CPL, LSL loads the specified register with a .word that consists of the limit field of that descriptor. This can only be done for segments, task state segments, and local descriptor tables (i.e., words from control descriptors are inaccessiblc). Interpreting the limit is a function of the segment type. For example, downward expand- able data segments treat the limit differently than code segments do.

11-3