Intel IA-32 - Manual (Page 81)

Vol. 3A 3-1
CHAPTER 3
PROTECTED-MODE MEMORY MANAGEMENT
This chapter describes the IA-32 architecture’s protected-mode memory management facilities,
including the physical memory requirements, segmentation mechanism, and paging mechanism.
See also: Chapter 4, “Protection” (for a description of the processor’s protection mechanism)
and Chapter 15, “8086 Emulation” (for a description of memory addressing protection in real-
address and virtual-8086 modes).
3.1 MEMORY MANAGEMENT OVERVIEW
The memory management facilities of the IA-32 architecture are divided into two parts: segmen-
tation and paging. Segmentation provides a mechanism of isolating individual code, data, and
stack modules so that multiple programs (or tasks) can run on the same processor without inter-
fering with one another. Paging provides a mechanism for implementing a conventional
demand-paged, virtual-memory system where sections of a program’s execution environment
are mapped into physical memory as needed. Paging can also be used to provide isolation
between multiple tasks. When operating in protected mode, some form of segmentation must be
used. There is no mode bit to disable segmentation. The use of paging, however, is optional.
These two mechanisms (segmentation and paging) can be configured to support simple single-
program (or single-task) systems, multitasking systems, or multiple-processor systems that used
shared memory.
As shown in Figure 3-1, segmentation provides a mechanism for dividing the processor’s
addressable memory space (called the linear address space) into smaller protected address
spaces called segments. Segments can be used to hold the code, data, and stack for a program
or to hold system data structures (such as a TSS or LDT). If more than one program (or task) is
running on a processor, each program can be assigned its own set of segments. The processor
then enforces the boundaries between these segments and insures that one program does not
interfere with the execution of another program by writing into the other program’s segments.
The segmentation mechanism also allows typing of segments so that the operations that may be
performed on a particular type of segment can be restricted.
All the segments in a system are contained in the processor’s linear address space. To locate a
byte in a particular segment, a logical address (also called a far pointer) must be provided. A
logical address consists of a segment selector and an offset. The segment selector is a unique
identifier for a segment. Among other things it provides an offset into a descriptor table (such
as the global descriptor table, GDT) to a data structure called a segment descriptor. Each
segment has a segment descriptor, which specifies the size of the segment, the access rights and
privilege level for the segment, the segment type, and the location of the first byte of the segment
in the linear address space (called the base address of the segment). The offset part of the logical
address is added to the base address for the segment to locate a byte within the segment. The
base address plus the offset thus forms a linear address in the processor’s linear address space.