80C186EC/80C188EC Microprocessor User's Manual

OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
FFFFFH
A
B
Data:
DS:
B
C
Code:
CS:
E
Stack:
SS:
H
D
Extra:
ES:
J
E
F
G
H
J
I
K
0H
A1037-0A
Figure 2-7. Currently Addressable Segments
The segment register is automatically selected according to the rules in Table 2-2. All information
in one segment type generally shares the same logical attributes (e.g., code or data). This leads to
programs that are shorter, faster and better structured.
The Bus Interface Unit must obtain the logical address before generating the physical address.
The logical address of a memory location can come from different sources, depending on the type
of reference that is being made (see Table 2-2).
Segment registers always hold the segment base addresses. The Bus Interface Unit determines
which segment register contains the base address according to the type of memory reference
made. However, the programmer can explicitly direct the Bus Interface Unit to use any currently
addressable segment (except for the destination operand of a string instruction). In assembly language,
this is done by preceding an instruction with a segment override prefix.
2-11