Intel 210200-002 manual Cal address Fig

INTRODUCTION

memory (Fig. 1-11). The code segment regis- ter points to the base of the program currently running. The stack segment register points to the base of the 8088's stack, the data segment register points to the base of one data area, and the extra segment register points to the base of another area where data can be stored. Each segment register is 16-bits wide, and one of the four is used in the com- putation of every memory address that the 8088 generates.

How are Addresses Generated?

CODE

STACK

DATA

EXTRA

SEGMENT

REGISTERS

Every time the 8088 needs to generate a
memory address, one of the segment registers	CONTENTS OF 8088 SEGMENT REGISTERS
is automatically chosen and added to a logi-
	POINT TO THE BASE ADDRESS OF THE
cal address (Fig. 1-12).	CORRESPONDING AREAS IN MEMORY.
For an instruction fetch, the code segment	Figure 1-11. Segment Registers
register is automatically added to the logical
address (in this case the contents of the		IMPLICIT
instruction pointer) to compute the value of	SELECTION
	CODE	~-S-E-G-M-E-N-T~I
the instruction address.
For an operation referencing the 8088's stack,	STACK	+
	DATA
the stack segment register is automatically
added to the logical address (the SP register	EXTRA
contents) to compute the value of the stack	SEGMENT		20 BIT
address.	REGISTERS		PHYSICAL
			ADDRESS
For data reference operation, where either
the data or extra segment registers are chosen	Figure 1-12.	Howan Address is Buill

MODULE	MODULE
CODE	STACK

MODULE	SYSTEM
DATA	DATA

Figure 1-10. iAPX 88 Architecture Quick Access to Four Segment Types

1-6