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INTEL386

Intel386 SX MICROPROCESSOR

Intel 

Intel386TM SX MICROPROCESSOR

Figure 2 7 Debug Registers

240187 – 8

3 0 REAL MODE ARCHITECTURE

When the processor is reset or powered up it is ini-

tialized in Real Mode Real Mode has the same base

architecture as the 8086 but allows access to the

32-bit register set of the Intel386 SX Microproces-

sor The addressing mechanism memory size and

interrupt handling are all identical to the Real Mode

on the 80286

The default operand size in Real Mode is 16 bits as

in the 8086 In order to use the 32-bit registers and

addressing modes override prefixes must be used

In addition the segment size on the Intel386 SX Mi-

croprocessor in Real Mode is 64K bytes so 32-bit

addresses must have a value less then 0000FFFFH

The primary purpose of Real Mode is to set up the

processor for Protected Mode operation

3 1 Memory Addressing

In Real Mode the linear addresses are the same as

physical addresses (paging is not allowed) Physical

addresses are formed in Real Mode by adding the

contents of the appropriate segment register which

is shifted left by four bits to an effective address

This addition results in a 20-bit physical address or a

1 megabyte address space Since segment registers

are shifted left by 4 bits Real Mode segments al-

ways start on 16-byte boundaries

All segments in Real Mode are exactly 64K bytes

long and may be read written or executed The

Intel386 SX Microprocessor will generate an excep-

tion 13 if a data operand or instruction fetch occurs

past the end of a segment

22

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