A386DX 查看數據表（PDF） - Intel

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A386DX

32-BIT CHMOS MICROPROCESSOR WITH INTEGRATED MEMORY MANAGEMENT

Intel 

Intel386TM DX MICROPROCESSOR

Figure 2-9 Addressing Mode Calculations

231630 – 51

2 5 4 Differences Between 16 and 32

Bit Addresses

In order to provide software compatibility with the

80286 and the 8086 the Intel386 DX can execute

16-bit instructions in Real and Protected Modes The

processor determines the size of the instructions it is

executing by examining the D bit in the CS segment

Descriptor If the D bit is 0 then all operand lengths

and effective addresses are assumed to be 16 bits

long If the D bit is 1 then the default length for oper-

ands and addresses is 32 bits In Real Mode the

default size for operands and addresses is 16-bits

Regardless of the default precision of the operands

or addresses the Intel386 DX is able to execute ei-

ther 16 or 32-bit instructions This is specified via the

use of override prefixes Two prefixes the Operand

Size Prefix and the Address Length Prefix over-

ride the value of the D bit on an individual instruction

basis These prefixes are automatically added by In-

tel assemblers

Example The processor is executing in Real Mode

and the programmer needs to access the EAX regis-

ters The assembler code for this might be MOV

EAX 32-bit MEMORYOP ASM386 Macro Assem-

bler automatically determines that an Operand Size

Prefix is needed and generates it

Example The D bit is 0 and the programmer wishes

to use Scaled Index addressing mode to access an

array The Address Length Prefix allows the use of

MOV DX TABLE ESI 2 The assembler uses an

Address Length Prefix since with De0 the default

addressing mode is 16-bits

Example The D bit is 1 and the program wants to

store a 16-bit quantity The Operand Length Prefix is

used to specify only a 16-bit value MOV MEM16

DX

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