User's Manual

CHAPTER 6
MEMORY MANAGEMENT AND VIRTUAL ADDRESSING
In Protected Virtual Address Mode, the 80286 provides an advanced architecture that retains substan-
tial compatibility with the
8086 and other processors in the 8086 family. In many respects, the baseline
architecture of the processor remains constant regardless of the mode of operation. Application
programmers continue to
use the same set of instructions, addressing modes, and data types in Protected
Mode as
in Real Address Mode.
The major difference between the
two modes of operation is that the Protected Mode provides system
programmers with additional architectural features, supplementary to the baseline architecture, that
can be used to good advantage
in the design and implementation of advanced systems. Especially
noteworthy are the mechanisms provided for memory management, protection, and multitasking.
This chapter focuses
on the memory management mechanisms of Protected Mode; the concept of a
virtual address and the process of virtual-to-physical address translation are described
in detail in this
chapter. Subsequent chapters deal with other key aspects of Protected Mode operation. Chapter 7
discusses the issue of protection and the integrated mechanisms that support a system-wide protection
policy. Chapter 8 discusses the notion of a task and.its central role
in the 80286 architecture. Chapters
9 through
11 discuss certain additional topics-interrupt handling, special instructions, system initial-
ization,
etc.-that complete the system programmer's view of 80286 Protected Mode.
6.1 MEMORY MANAGEMENT OVERVIEW
A memory management scheme interposes a mapping operation between logical addresses (Le., addresses
as they are viewed by programs) and physical addresses (i.e., actual addresses in real memory). Since
the logical address spaces are independent of physical memory (dynamically relocatable), the mapping
(the assignment of real address space to virtual address space)
is transparent to software. This allows
the program development tools (for static systems) or the system software (for reprogrammable systems)
to control the allocation of space in real memory without regard to the specifics of individual programs.
Application programs may be translated and loaded independently since they deal strictly with virtual
addresses. Any program can be relocated to use any available segments of physical memory.
The
80286, when operated in Protected Mode, provides an efficient on-chip memory management
architecture. Moreover,
as described in Chapter 11, the 80286 also supports the implementation of
virtual memory
systems-that is, systems that dynamically swap chunks of code and data between real
memory and secondary storage devices (e.g., a disk) independent of and transparent to the executing
application programs. Thus, a program-visible address
is more aptly termed a virtual address rather
than a logical address since it may actually refer to a location not currently present
in real memory.
Memory management, then, consists of a mechanism for mapping the virtual addresses that are visible
to the program onto the physical addresses of real memory. With the
80286, segmentation is the key
to virtual memory addressing. Virtual. memory
is partitioned into a number of individual segments,
which are the units of memory that are mapped into physical memory and swapped
to and from
secondary storage devices. Most of this chapter
is devoted to a detailed discussion of the mapping and
virtual memory mechanisms of the
80286.
The concept of a task also plays a significant role in memory management since distinct memory
mappings may be assigned
to the different tasks in a multitask or multi-user environment. A complete
discussion of tasks
is deferred until Chapter 8, "Tasks and State Transition." For present purposes, it
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