s02_01.htm
来自「Programmer s Reference Manual is an impr」· HTM 代码 · 共 93 行
HTM
93 行
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 2.1</TITLE></HEAD><BODY><B>up:</B> <A HREF="c02.htm">Chapter 2 -- Basic Programming Model</A><BR><B>prev:</B> <A HREF="c02.htm">Chapter 2 -- Basic Programming Model</A><BR><B>next:</B> <A HREF="s02_02.htm">2.2 Data Types</A><P><HR><P><H1>2.1 Memory Organization and Segmentation</H1>The physical memory of an 80386 system is organized as a sequence of 8-bitbytes. Each byte is assigned a unique address that ranges from zero to amaximum of 2^(32) -1 (4 gigabytes).<P>80386 programs, however, are independent of the physical address space.This means that programs can be written without knowledge of how muchphysical memory is available and without knowledge of exactly where inphysical memory the instructions and data are located.<P>The model of memory organization seen by applications programmers isdetermined by systems-software designers. The architecture of the 80386gives designers the freedom to choose a model for each task. The model ofmemory organization can range between the following extremes:<UL><LI> A "flat" address space consisting of a single array of up to 4gigabytes.<LI> A segmented address space consisting of a collection of up to 16,383linear address spaces of up to 4 gigabytes each.</UL><P>Both models can provide memory protection. Different tasks may employdifferent models of memory organization. The criteria that designers use todetermine a memory organization model and the means that systems programmersuse to implement that model are covered in Part -- Programming.<H2>2.1.1 The "Flat" Model</H2>In a "flat" model of memory organization, the applications programmer seesa single array of up to 2^(32) bytes (4 gigabytes). While the physicalmemory can contain up to 4 gigabytes, it is usually much smaller; theprocessor maps the 4 gigabyte flat space onto the physical address space bythe address translation mechanisms described in <A HREF="c05.htm">Chapter 5</A> . Applicationsprogrammers do not need to know the details of the mapping.<P>A pointer into this flat address space is a 32-bit ordinal number that mayrange from 0 to 2^(32) -1. Relocation of separately-compiled modules in thisspace must be performed by systems software (e.g., linkers, locators,binders, loaders).<H2>2.1.2 The Segmented Model</H2>In a segmented model of memory organization, the address space as viewed byan applications program (called the logical address space) is a much largerspace of up to 2^(46) bytes (64 terabytes). The processor maps the 64terabyte logical address space onto the physical address space (up to 4gigabytes ) by the address translation mechanisms described in <A HREF="c05.htm">Chapter 5</A> .Applications programmers do not need to know the details of this mapping.<P>Applications programmers view the logical address space of the 80386 as acollection of up to 16,383 one-dimensional subspaces, each with a specifiedlength. Each of these linear subspaces is called a segment. A segment is aunit of contiguous address space. Segment sizes may range from one byte upto a maximum of 2^(32) bytes (4 gigabytes).<P>A complete pointer in this address space consists of two parts (see <A HREF="s02_02.htm#fig2-1">Figure 2-1</A> ):<OL> <LI>A segment selector, which is a 16-bit field that identifies asegment.<LI>An offset, which is a 32-bit ordinal that addresses to the byte levelwithin a segment.</OL>During execution of a program, the processor associates with a segmentselector the physical address of the beginning of the segment. Separatelycompiled modules can be relocated at run time by changing the base addressof their segments. The size of a segment is variable; therefore, a segmentcan be exactly the size of the module it contains.<P><HR><P><B>up:</B> <A HREF="c02.htm">Chapter 2 -- Basic Programming Model</A><BR><B>prev:</B> <A HREF="c02.htm">Chapter 2 -- Basic Programming Model</A><BR><B>next:</B> <A HREF="s02_02.htm">2.2 Data Types</A></BODY>⌨️ 快捷键说明
复制代码Ctrl + C
搜索代码Ctrl + F
全屏模式F11
增大字号Ctrl + =
减小字号Ctrl + -
显示快捷键?