B-20
INSTRUCTION FORMATS AND ENCODINGS
If an MMX instruction operates on a general-purpose register (reg), the register is encoded
in the R/M field of the ModR/M byte. Table B-13 shows the encoding of general-purpose reg-
isters when used in MMX instructions.
B.3.3.MMX Instruction Formats and Encodings Table
Table B-14 shows the formats and encodings for MMX instructions for the data types sup-
portedpacked byte (B), packed word (W), packed doubleword (D), and quadword (Q). Fig-
ure B-2 describes the nomenclature used in columns (3 through 6) of the table.
Table B-13. Encoding of the General-Purpose Register Field (reg)
When Used in MMX Instructions.
reg Field Encoding
Register Selected
000
EAX
001
ECX
010
EDX
011
EBX
100
ESP
101
EBP
110
ESI
111
EDI
Code
Meaning
Y
Supported
N
Not supported
O
Output
I
Input
n/a
Not Applicable
Figure B-2. Key to Codes for MMX Data Type Cross-Reference