Intel 80286, 80287 manual Chapter Base Architecture, Memory Organization and Segmentation

CHAPTER 2

80286 BASE ARCHITECTURE

This chapter describes the 80286 application programming environment as seen by assembly language programmers. It is intended to introduce the programmer to those features of the 80286 architecture that directly affect the design and implementation of 80286 application programs.

2.1 MEMORY ORGANIZATION AND SEGMENTATION

The main memory of an 80286 system makes up its physical address space. This address space is organized as a sequence of 8-bit quantities, called bytes. Each byte is assigned a unique address ranging from 0 up to a maximum of 220 (1 megabyte) in Real Address Mode, and up to 224 (16 megabytes) in

Protected Mode. .

A virtual address space is the organization of memory as viewed by a program. Virtual address space is also organized in units of bytes. (Other addressable units such as words, strings, and BCD digits are described below in section 2.2, "Data Types.") In Real Address Mode, as with the 8086 itself, programs view physical memory directly, inasmuch as they manipulate pure physical addresses. Thus, the virtual address space is identical to the physical address space (1 megabyte).

In Protected Mode, however, programs have no direct access to physical addresses. Instead, memory is viewed as a much larger virtual address space of 230 bytes (1 gigabyte). This 1 gigabyte virtual address is mapped onto the Protected Mode's 16-megabyte physical address space by the address trans- lation mechanisms described in Chapter 6.

The programmer views the virtual address space on the 80286 as a collection of up to sixteen thousand linear subspaces, each with a specified size or length. Each of these linear address spaces is called a segment. A segment is a logical unit of contiguous memory. Segment sizes may range from one byte up to 64K (65,536) bytes.

80286 memory segmentation supports the logical structure of programs and data iq memory. Programs are not written as single linear sequences of instructions and data, but rather as modules of code and data. For example, program code may include a main routine and several separate procedures. Data may also be organized into various data structures, some private and some shared with other programs in the system. Run-time stacks constitute yet another data requirement. Each of these several modules of code and data, moreover, may be very different in size or vary dynamically with program execution.

Segmentation supports this logical structure (see figure 2-1). Each meaningful module of a program may be separately contained in individual segments. The degree of modularization, of course, depends on the requirements of a particular application. Use of segmentation benefits almost all applications. Programs execute faster and require less space. Segmentation also simplifies the design of structured software.

2.2DATA TYPES

Bytes and words are the fundamental units in which the 80286 manipulates data, i.e., the fundamental data types.

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