GXM Datasheet, PDF (207/244 Pages) National Semiconductor (TI) – Geode™ GXm Processor Integrated x86 Solution with MMX Support

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.
Table 9-13. ss Field Encoding
ss Field
00
01
01
11
Scale Factor
x1
x2
x4
x8
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.
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
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.
Table 9-15. mod base Field Encoding
mod Field
within
mode/rm
Byte
(bits 7:6)
00
00
00
00
00
00
00
00
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)]
01
000
DS:[EAX+(scaled index)+d8]
01
001
DS:[ECX+(scaled index)+d8]
01
010
DS:[EDX+(scaled index)+d8]
01
011
DS:[EBX+(scaled index)+d8]
01
100
SS:[ESP+(scaled index)+d8]
01
101
SS:[EBP+(scaled index)+d8]
01
110
DS:[ESI+(scaled index)+d8]
01
111
DS:[EDI+(scaled index)+d8]
10
000
DS:[EAX+(scaled index)+d32]
10
001
DS:[ECX+(scaled index)+d32]
10
010
DS:[EDX+(scaled index)+d32]
10
011
DS:[EBX+(scaled index)+d32]
10
100
SS:[ESP+(scaled index)+d32]
10
101
SS:[EBP+(scaled index)+d32]
10
110
DS:[ESI+(scaled index)+d32]
10
111
DS:[EDI+(scaled index)+d32]
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