Intel 64 and IA-32 Architectures Software Developers Manual Volume 2B, Instruction Set Reference, N-Z

4-208 Vol. 2B
INSTRUCTION SET REFERENCE, N-Z
The source operand can be an MMX technology register or a 32-bit memory location,
or it can be an XMM register or a 128-bit memory location. The destination operand
can be an MMX technology register or an XMM register. When the source data comes
from a 128-bit memory operand, an implementation may fetch only the appropriate
64 bits; however, alignment to a 16-byte boundary and normal segment checking
will still be enforced.
The PUNPCKLBW instruction interleaves the low-order bytes of the source and desti-
nation operands, the PUNPCKLWD instruction interleaves the low-order words of the
source and destination operands, the PUNPCKLDQ instruction interleaves the low-
order doubleword (or doublewords) of the source and destination operands, and the
PUNPCKLQDQ instruction interleaves the low-order quadwords of the source and
destination operands.
These instructions can be used to convert bytes to words, words to doublewords,
doublewords to quadwords, and quadwords to double quadwords, respectively, by
placing all 0s in the source operand. Here, if the source operand contains all 0s, the
result (stored in the destination operand) contains zero extensions of the high-order
data elements from the original value in the destination operand. For example, with
the PUNPCKLBW instruction the high-order bytes are zero extended (that is,
unpacked into unsigned word integers), and with the PUNPCKLWD instruction, the
high-order words are zero extended (unpacked into unsigned doubleword integers).
In 64-bit mode, using a REX prefix in the form of REX.R permits this instruction to
access additional registers (XMM8-XMM15).
Operation
PUNPCKLBW instruction with 64-bit operands:
DEST[63:56] ← SRC[31:24];
DEST[55:48] ← DEST[31:24];
DEST[47:40] ← SRC[23:16];
DEST[39:32] ← DEST[23:16];
DEST[31:24] ← SRC[15:8];
DEST[23:16] ← DEST[15:8];
DEST[15:8] ← SRC[7:0];
DEST[7:0] ← DEST[7:0];
PUNPCKLWD instruction with 64-bit operands:
DEST[63:48] ← SRC[31:16];
DEST[47:32] ← DEST[31:16];
DEST[31:16] ← SRC[15:0];
DEST[15:0] ← DEST[15:0];
PUNPCKLDQ instruction with 64-bit operands:
DEST[63:32] ← SRC[31:0];
DEST[31:0] ← DEST[31:0];