Intel 80287, 80286 manual Descriptor Tables

MEMORY MANAGEMENT AND VIRTUAL ADDRESSING

Within each of the two regions addressable by a program-either the global address space or a partic- ular local address space-as many as 8,192 (213 ) distinct segments may be defined. The INDEX field of the segment selector allows for a unique specification of each of these segments. This 13-bit quantity acts as an index into a memory-resident table, called a descriptor table, that records the mapping between segment address and the physical locations allocated to each distinct segment. (These descrip- tor tables, and their role in virtual-to-physical address translation, are described in the sections that follow.)

In summary, a Protected Mode virtual address is a 32-bit pointer to a particular byte location within a one-gigabyte virtual address space. Each such pointer consists of a 16-bit selector component and a 16-bit offset component. The selector component, in turn, comprises a 13-bit table index, a I-bit table indicator (local versus global), and a 2-bit RPL field; all but this last field serve to select a particular segment from among the 16K segments in a task's virtual address space. The offset component of a full pointer is an unsigned 16-bit integer that specifies the desired byte location within the selected segment.

6.3 DESCRIPTOR TABLES

A descriptor table is a memory-resident table either defined by program development tools in a static system or controlled by operating system software in systems that are reprogrammable. The descriptor table contents govern the interpretation of virtual addresses. Whenever the 80286 decodes a virtual address, translating a full 32-bit pointer into a corresponding 24-bit physical address, it implicitly refer- ences one of these tables.

Within a Protected Mode system, there are ordinarily several descriptor tables resident in memory. One of these is the global descriptor table (GDT); this table provides a complete description of the global address space. In addition, there may be one or more local descriptor tables (LDTs), each describing the local address space of one or more tasks.

For each task in the system, a pair of descriptor tables-consisting of the GDT (shared by all tasks) and a particular LDT (private to the task or to a group of closely related tasks)-provides a complete description of that task's virtual address space. The protection mechanism described in Chapter 7, "Protection," ensures that a task is granted access only to its own virtual address space. In the simplest of system configurations, tasks can reside entirely within the GDT without the use of local descriptor tables. This will simplify system software by only requiring maintenance of one table (the GDT) at the expense of no isolation between tasks. The point is: the 80286 memory management scheme is flexible enough to accommodate a variety of implementations and does not require use of all possible facilities when implementing a system.

The descriptor tables consist of a sequence of 8-byte entries called descriptors. A descriptor table may contain from 1 to 8192 entries.

Within a descriptor table, two main classes of descriptors are recognized by the 80286 architecture. The most important of these, from the standpoint oi memory managemellL, an; .:;alled segmeiit descrip- tors; these determine the set of segments that are included within a given address space. The other class are special-purpose control descriptors-such as call gates and task descriptors-to implement protection (described in succeeding chapters) and special system data segments.

Figure 6-3 shows the format of a segment descriptor. Note that it provides information about the physical-memory base address and size of a segment, as well as certain access information. If a partic- ular segment is to be included within a virtual address space, then a segment descriptor that describes that segment must be included within the appropriate descriptor table. Thus, within the GDT, there

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