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Memory Layout and Access Chapter Four
Chapter One discussed the basic format for data in memory. Chapter Three covered
how a computer system physically organizes that data. This chapter discusses how the
80x86 CPUs access data in memory.
4.0 Chapter Overview
This chapter forms an important bridge between sections one and two (Machine
Organization and Basic Assembly Language, respectively). From the point of view of
machine organization, this chapter discusses memory addressing, memory organization,
CPU addressing modes, and data representation in memory. From the assembly language
programming point of view, this chapter discusses the 80x86 register sets, the 80x86 mem-
ory addressing modes, and composite data types. This is a pivotal chapter. If you do not
understand the material in this chapter, you will have difficulty understanding the chap-
ters that follow. Therefore, you should study this chapter carefully before proceeding.
This chapter begins by discussing the registers on the 80x86 processors. These proces-
sors provide a set of general purpose registers, segment registers, and some special pur-
pose registers. Certain members of the family provide additional registers, although
typical application do not use them.
After presenting the registers, this chapter describes memory organization and seg-
mentation on the 80x86. Segmentation is a difficult concept to many beginning 80x86
assembly language programmers. Indeed, this text tends to avoid using segmented
addressing throughout the introductory chapters. Nevertheless, segmentation is a power-
ful concept that you must become comfortable with if you intend to write non-trivial
80x86 programs.
80x86 memory addressing modes are, perhaps, the most important topic in this chap-
ter. Unless you completely master the use of these addressing modes, you will not be able
to write reasonable assembly language programs. Do not progress beyond this section of
the text until you are comfortable with the 8086 addressing modes. This chapter also dis-
cusses the 80386 (and later) extended addressing modes. Knowing these addressing
modes is not that important for now, but if you do learn them you can use them to save
some time when writing code for 80386 and later processors.
This chapter also introduces a handful of 80x86 instructions. Although the five or so
instructions this chapter uses are insufficient for writing real assembly language pro-
grams, they do provide a sufficient set of instructions to let you manipulate variables and
data structures – the subject of the next chapter.
4.1 The 80x86 CPUs:A Programmer’s View
Now it’s time to discuss some real processors: the 8088/8086, 80188/80186, 80286, and
80386/80486/80586/Pentium. Chapter Three dealt with many hardware aspects of a com-
puter system. While these hardware components affect the way you should write soft-
ware, there is more to a CPU than bus cycles and pipelines. It’s time to look at those
components of the CPU which are most visible to you, the assembly language program-
mer.
The most visible component of the CPU is the register set. Like our hypothetical pro-
cessors, the 80x86 chips have a set of on-board registers. The register set for each processor
in the 80x86 family is a superset of those in the preceding CPUs. The best place to start is
with the register set for the 8088, 8086, 80188, and 80186 since these four processors have
the same registers. In the discussion which follows, the term “8086” will imply any of
these four CPUs.
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