M80C286 datasheet(6/60 Pages) INTEL

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M80C286

Table 3 Segment Register Selection Rules

Memory

Segment Register

Implicit Segment

Reference Needed

Used

Selection Rule

Instructions

Code (CS)

Automatic with instruction prefetch

Stack

Stack (SS)

All stack pushes and pops Any memory reference which uses BP

as a base register

Local Data

Data (DS)

All data references except when relative to stack or

string destination

External (Global) Data

Extra (ES)

Alternate data segment and destination of string operation

All instructions that address operands in memory

must specify the segment and the offset For speed

and compact instruction encoding segment selec-

tors are usually stored in the high speed segment

registers An instruction need specify only the de-

sired segment register and an offset in order to ad-

dress a memory operand

Most instructions need not explicitly specify which

segment register is used The correct segment reg-

ister is automatically chosen according to the rules

of Table 3 These rules follow the way programs are

written (see Figure 5) as independent modules that

require areas for code and data a stack and access

to external data areas

Special segment override instruction prefixes allow

the implicit segment register selection rules to be

overridden for special cases The stack data and

extra segments may coincide for simple programs

To access operands not residing in one of the four

immediately available segments a full 32-bit pointer

or a new segment selector must be loaded

Addressing Modes

The M80C286 provides a total of eight addressing

modes for instructions to specify operands Two ad-

dressing modes are provided for instructions that

operate on register or immediate operands

The operand is locat-

ed in one of the 8 or 16-bit general registers

Immediate Operand Mode

The operand is in-

cluded in the instruction

Six modes are provided to specify the location of an

operand in a memory segment A memory operand

address consists of two 16-bit components seg-

ment selector and offset The segment selector is

supplied by a segment register either implicitly cho-

sen by the addressing mode or explicitly chosen by

a segment override prefix The offset is calculated

by summing any combination of the following three

address elements

the displacement (an 8 or 16-bit immediate val-

ue contained in the instruction)

the base (contents of either the BX or BP base

registers)

271103 – 4

Figure 5 Segmented Memory Helps

Structure Software

the index (contents of either the SI or DI index

registers)

Any carry out from the 16-bit addition is ignored

Eight-bit displacements are sign extended to 16-bit

values

Combinations of these three address elements de-

fine the six memory addressing modes described

below

Direct Mode

The operand’s offset is contained in

the instruction as an 8 or 16-bit displacement ele-

ment

The operand’s offset is in

one of the registers SI DI BX or BP

Based Mode

The operand’s offset is the sum of an

8 or 16-bit displacement and the contents of a base

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