376 datasheet(88/95 Pages) INTEL

88 / 95 page

376 EMBEDDED PROCESSOR

16-Bit Extensions of the

Instruction Set

Two prefixes the operand size prefix (66H) and the

effective address size prefix (67H) allow overriding

individually the default selection of operand size and

effective address size These prefixes may precede

any opcode bytes and affect only the instruction

they precede If necessary one or both of the prefix-

es may be placed before the opcode bytes The

presence of the operand size prefix (66H) and the

effective address prefix will allow 16-bit data opera-

tion and 16-bit effective address calculations

For instructions with more than one prefix the order

of prefixes is unimportant

Unless specified otherwise instructions with 8-bit

and 16-bit operands do not affect the contents of

the high-order bits of the extended registers

Encoding of Instruction Fields

Within the instruction are several fields indicating

ENCODING OF OPERAND LENGTH (w) FIELD

For any given instruction performing a data opera-

tion the instruction will execute as a 32-bit opera-

tion Within the constraints of the operation size the

w field encodes the operand size as either one byte

or the full operation size as shown in the table be-

low

w Field

Operand Size

Normal

with 66H Prefix

Operand Size

8 Bits

16 Bits

32 Bits

ENCODING OF THE GENERAL

The general register is specified by the reg field

which may appear in the primary opcode bytes or as

the reg field of the ‘‘mod rm’’ byte or as the rm

field of the ‘‘mod rm’’ byte

Encoding of reg Field When w Field

is not Present in Instruction

reg Field

with 66H Prefix

During 32-Bit

Data Operations

000

EAX

001

ECX

010

EDX

011

EBX

100

ESP

101

EBP

110

ESI

111

EDI

Encoding of reg Field When w Field

is Present in Instruction

with 66H Prefix

reg

Function of w Field

(when w e 0)

(when w e 1)

000

001

010

011

100

101

110

111

without 66H Prefix

reg

Function of w Field

(when w e 0)

(when w e 1)

000

EAX

001

ECX

010

EDX

011

EBX

100

ESP

101

EBP

110

ESI

111

EDI

376 Datasheet(PDF) 88 Page - Intel Corporation

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