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.
9.1.5.3 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 the base fields in the s-i-b byte.
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.
Table 9-15. mod base Field Encoding
mod Field
within
mode/rm
Byte
base Field
within
s-i-b
Byte
(bits 2:0)
Table 9-13. ss Field Encoding
32-Bit Address Mode
with mod r/m and s-i-b
Bytes Present
ss Field
Scale Factor
(bits 7:6)
00
01
01
11
x1
x2
x4
x8
00
00
00
00
00
00
00
00
000
001
010
011
100
101
110
111
DS:[EAX+(scaled index)]
DS:[ECX+(scaled index)]
DS:[EDX+(scaled index)]
DS:[EBX+(scaled index)]
SS:[ESP+(scaled index)]
DS:[d32+(scaled index)]
DS:[ESI+(scaled index)]
DS:[EDI+(scaled index)]
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.
01
01
01
01
01
01
01
01
000
001
010
011
100
101
110
111
DS:[EAX+(scaled index)+d8]
DS:[ECX+(scaled index)+d8]
DS:[EDX+(scaled index)+d8]
DS:[EBX+(scaled index)+d8]
SS:[ESP+(scaled index)+d8]
SS:[EBP+(scaled index)+d8]
DS:[ESI+(scaled index)+d8]
DS:[EDI+(scaled index)+d8]
Table 9-14. index Field Encoding
Index Field
Index Register
000
001
010
011
100
101
110
111
EAX
ECX
EDX
EBX
none
EBP
ESI
10
10
10
10
10
10
10
10
000
001
010
011
100
101
110
111
DS:[EAX+(scaled index)+d32]
DS:[ECX+(scaled index)+d32]
DS:[EDX+(scaled index)+d32]
DS:[EBX+(scaled index)+d32]
SS:[ESP+(scaled index)+d32]
SS:[EBP+(scaled index)+d32]
DS:[ESI+(scaled index)+d32]
DS:[EDI+(scaled index)+d32]
EDI
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