GXM datasheet(207/244 Pages) NSC

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Instruction Set (Continued)

9.1.5

s-i-b Byte (Scale, Indexing, Base)

The s-i-b fields provide scale factor, indexing and a base

field for address selection. The ss, index and base fields

are described next.

9.1.5.1

ss Field (Scale Selection)

The ss field (Table 9-13) specifies the scale factor used in

the offset mechanism for address calculation. The scale

factor multiplies the index value to provide one of the com-

ponents used to calculate the offset address.

9.1.5.2

index Field (Index Selection)

The index field (Table 9-14) specifies the index register

used by the offset mechanism for offset address calcula-

tion. When no index register is used (index field = 100),

the ss value must be 00 or the effective address is unde-

fined.

9.1.5.3

Base Field (s-i-b Present)

In Table 9-9 on page 205, the note “s-i-b present” for cer-

tain entries forces the use of the mod and base field as

listed in Table 9-15. The first two digits in the first column

of Table 9-15 identify the mod bits in the mod r/m byte.

The last three digits in the first column of this table identi-

fies thebasefields inthe s-i-bbyte.

Table 9-13. ss Field Encoding

ss Field

Scale Factor

Table 9-14. index Field Encoding

Index Field

Index Register

000

EAX

001

ECX

010

EDX

011

EBX

100

none

101

EBP

110

ESI

111

EDI

Table 9-15. mod base Field Encoding

mod Field

within

mode/rm

Byte

(bits 7:6)

base Field

within

s-i-b

Byte

(bits 2:0)

32-Bit Address Mode

with mod r/m and s-i-b

Bytes Present

000

DS:[EAX+(scaled index)]

001

DS:[ECX+(scaled index)]

010

DS:[EDX+(scaled index)]

011

DS:[EBX+(scaled index)]

100

SS:[ESP+(scaled index)]

101

DS:[d32+(scaled index)]

110

DS:[ESI+(scaled index)]

111

DS:[EDI+(scaled index)]

000

DS:[EAX+(scaled index)+d8]

001

DS:[ECX+(scaled index)+d8]

010

DS:[EDX+(scaled index)+d8]

011

DS:[EBX+(scaled index)+d8]

100

SS:[ESP+(scaled index)+d8]

101

SS:[EBP+(scaled index)+d8]

110

DS:[ESI+(scaled index)+d8]

111

DS:[EDI+(scaled index)+d8]

000

DS:[EAX+(scaled index)+d32]

001

DS:[ECX+(scaled index)+d32]

010

DS:[EDX+(scaled index)+d32]

011

DS:[EBX+(scaled index)+d32]

100

SS:[ESP+(scaled index)+d32]

101

SS:[EBP+(scaled index)+d32]

110

DS:[ESI+(scaled index)+d32]

111

DS:[EDI+(scaled index)+d32]

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