Intel 80286 manual Virtual Memory Management, Special Segment Attributes, Conforming Code Segments

CHAPTER 11

ADVANCED TOPICS

This chapter describes some of the advanced topics as virtual memory management, restartable instructions, special segment attributes, and the validation of descriptors and pointers.

11.1 VIRTUAL MEMORY MANAGEMENT

When access to a segment is requested and the access byte in its descriptor indicates the segment is not present in real memory, the not-present fault occurs (exception 11, or 12 for stacks). The handler for this fault can be set up to bring the absent segment into real memory (swapping or overwriting another segment if necessary), or to terminate execution of the requesting program if this is not possible.

The accessed bit (bit 0) of the access byte is provided in both executable and data segment descriptors to support segment usage profiling. Whenever the descriptor is accessed by the 80286 hardware, the A-bit will be set in memory. This applies to selector test instructions (described below) as well as to the loading of a segment register. The reading of the access byte and the restoration of it with the A~bit set is an indivisible operation, i.e., it is performed as a read-modify-write with bus lock. If an operating system develops a profile of segment usage over time, it can recognize segments of low or zero access and choose among these candidates for replacement.

When a not-present segment is brought into real memory, the task that requested access to it can continue its execution because all instructions that load a segment register are restartable.

Not-present exceptions occur only on segment register load operations, gate accesses, and task switches. The saved instruction pointer refers to the first byte of the violating instruction. All other aspects of the saved machine state are exactly as they were before execution of the violating instruction began. After the fault handler clears up the fault condition and performs an IRET, the program continues to execute. The only external indication of a segment swap is the additional execution time.

11.2 SPECIAL SEGMENT ATTRIBUTES

11.2.1 Conforming Code Segments

Code segments intended for use at potentially different privilege levels need an attribute that permits them to emulate the privilege level of the calling task. Such segments are termed "conforming" segments. Conforming segments are also useful for interrupt-driven error routines that need only be as privileged as the routine that caused the error.

A conforming code segment has bit 2 of its access byte set to 1. This means it can be referenced by a CALL or JMP instruction in a task of equal or lesser privilege, i.e., CPL of the task is numerically greater than or equal to DPL of this segment. CPL does not change when executing the conforming code segment. A conforming segment continues to use the stack from the CPL. This is the only case in which the DPL of a code segment can be numerically less than the CPL. If bit 2 is a 0, the segment is not conforming and can be referenced only by a task of CPL=DPL.

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