INTEL386 Просмотр технического описания (PDF) - Intel

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INTEL386

Intel386 SX MICROPROCESSOR

Intel 

Intel386TM SX MICROPROCESSOR

Figure 2 4 Addressing Mode Calculations

240187 – 5

DIFFERENCES BETWEEN 16 AND 32 BIT

ADDRESSES

In order to provide software compatibility with the

8086 and the 80286 the Intel386 SX Microproces-

sor can execute 16-bit instructions in Real and Pro-

tected Modes The processor determines the size of

the instructions it is executing by examining the D bit

in a Segment Descriptor If the D bit is 0 then all

operand lengths and effective addresses are as-

sumed to be 16 bits long If the D bit is 1 then the

default length for operands and addresses is 32 bits

In Real Mode the default size for operands and ad-

dresses is 16 bits

Regardless of the default precision of the operands

or addresses the Intel386 SX Microprocessor is

able to execute either 16 or 32-bit instructions This

is specified through the use of override prefixes

Two prefixes the Operand Length Prefix and the

Address Length Prefix override the value of the D

bit on an individual instruction basis These prefixes

are automatically added by assemblers

The Operand Length and Address Length Prefixes

can be applied separately or in combination to any

instruction The Address Length Prefix does not al-

low addresses over 64K bytes to be accessed in

Real Mode A memory address which exceeds

0FFFFH will result in a General Protection Fault An

Address Length Prefix only allows the use of the ad-

ditional Intel386 SX Microprocessor addressing

modes

When executing 32-bit code the Intel386 SX Micro-

processor uses either 8 or 32-bit displacements and

any register can be used as base or index registers

When executing 16-bit code the displacements are

either 8 or 16-bits and the base and index register

conform to the 80286 model Table 2 4 illustrates

the differences

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