8-11
PROGRAMMING WITH THE INTEL MMX TECHNOLOGY
8.1.2.MMX Data Types
The MMX technology defines the following new 64-bit data types (refer to Figure 8-2):
Packed bytes
Eight bytes packed into one 64-bit quantity.
Packed words
Four (16-bit) words packed into one 64-bit quantity.
Packed doublewordsTwo (32-bit) doublewords packed into one 64-bit quantity.
Quadword
One 64-bit quantity.
The bytes in the packed bytes data type are numbered 0 through 7, with byte 0 being contained
in the least significant bits of the data type (bits 0 through 7) and byte 7 being contained in the
most significant bits (bits 56 through 63). The words in the packed words data type are
numbered 0 through 4, with word 0 being contained in the bits 0 through 15 of the data type and
word 4 being contained in bits 48 through 63. The doublewords in a packed doublewords data
type are numbered 0 and 1, with doubleword 0 being contained in bits 0 through 31 and double-
word 1 being contained in bits 32 through 63.
The MMX instructions move the packed data types (packed bytes, packed words, or packed
doublewords) and the quadword data type to-and-from memory or to-and-from the IA general-
purpose registers in 64-bit blocks. However, when performing arithmetic or logical operations
on the packed data types, the MMX instructions operate in parallel on the individual bytes,
Figure 8-2. MMX Data Types
3006002
63
Packed bytes (8x8 bits)
56 5548 4740 3932 3124 2316 158 70
63
Packed word (4x16 bits)
48 47
32 31
16 15
0
63
Packed doublewords (2x32 bits)
32 31
0
63
Quadword (64 bits)
0
