OVERVIEW OF THE 80C186 FAMILY ARCHITECTURE
2-30
The BX or BP register can be specified as the base register for an effective address calculation.
Similarly, either the SI or the DI register can be specified as the index register. The displacement
value is a constant. The contents of the base and index registers can change during execution. This
allows one instruction to access different memory locations depending upon the current values in
the base or base and index registers. The default base register for effective address calculations
with the BP register is SS, although DS or ES can be specified.
Direct addressing is the simplest memory addressing mode (see Figure 2-13). No registers are in-
volved, and the effective address is taken directly from the displacement of the instruction. Pro-
grammers typically use direct addressing to access scalar variables.
With register indirect addressing, the effective address of a memory operand can be taken directly
from one of the base or index registers (see Figure 2-14). One instruction can operate on various
memory locations if the base or index register is updated accordingly. Any 16-bit general register
can be used for register indirect addressing with the JMP or CALL instructions.
In based addressing, the effective address is the sum of a displacement value and the contents of
the BX or BP register (see Figure 2-15). Specifying the BP register as a base register directs the
Bus Interface Unit to obtain the operand from the current stack segment (unless a segment over-
ride prefix is present). This makes based addressing with the BP register a convenient way to ac-
cess stack data.
Figure 2-13. Direct Addressing
Opcode
Mod R/M
EA
Displacement
A1016-0A
Содержание 80C186XL
Страница 1: ...80C186XL 80C188XL Microprocessor User s Manual...
Страница 2: ...80C186XL 80C188XL Microprocessor User s Manual 1995...
Страница 18: ...1 Introduction...
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Страница 28: ...2 Overview of the 80C186 Family Architecture...
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Страница 80: ...3 Bus Interface Unit...
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Страница 128: ...4 Peripheral Control Block...
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Страница 138: ...5 ClockGenerationand Power Management...
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Страница 154: ...6 Chip Select Unit...
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Страница 178: ...7 Refresh Control Unit...
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Страница 194: ...8 Interrupt Control Unit...
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Страница 228: ...9 Timer Counter Unit...
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Страница 254: ...10 Direct Memory Access Unit...
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Страница 284: ...11 Math Coprocessing...
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Страница 302: ...12 ONCE Mode...
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Страница 306: ...A 80C186 Instruction Set Additions and Extensions...
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Страница 318: ...B Input Synchronization...
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Страница 322: ...C Instruction Set Descriptions...
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Страница 372: ...D Instruction Set Opcodes and Clock Cycles...
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Страница 396: ...Index...
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