Intel® Architecture Instruction Set Extensions Programming Reference

INSTRUCTION SET REFERENCE
VEX.256 version: The instruction will gather four qword values. For dword indices, only the lower four indices in
the vector index register are used.
Note that:
• If any pair of the index, mask, or destination registers are the same, this instruction results a UD fault.
• The values may be read from memory in any order. Memory ordering with other instructions follows the Intel-
64 memory-ordering model.
• Faults are delivered in a right-to-left manner. That is, if a fault is triggered by an element and delivered, all
elements closer to the LSB of the destination will be completed (and non-faulting). Individual elements closer
to the MSB may or may not be completed. If a given element triggers multiple faults, they are delivered in the
conventional order.
• Elements may be gathered in any order, but faults must be delivered in a right-to-left order; thus, elements to
the left of a faulting one may be gathered before the fault is delivered. A given implementation of this
instruction is repeatable - given the same input values and architectural state, the same set of elements to the
left of the faulting one will be gathered.
• This instruction does not perform AC checks, and so will never deliver an AC fault.
• This instruction will cause a #UD if the address size attribute is 16-bit.
• This instruction should not be used to access memory mapped I/O as the ordering of the individual loads it does
is implementation specific, and some implementations may use loads larger than the data element size or load
elements an indeterminate number of times.
• The scaled index may require more bits to represent than the address bits used by the processor (e.g., in 32bit
mode, if the scale is greater than one). In this case, the most significant bits beyond the number of address
bits are ignored.
Operation
DEST SRC1;
BASE_ADDR: base register encoded in VSIB addressing;
VINDEX: the vector index register encoded by VSIB addressing;
SCALE: scale factor encoded by SIB:[7:6];
DISP: optional 1, 4 byte displacement;
MASK SRC3;
VPGATHERDQ (VEX.128 version)
FOR j 0 to 1
i j * 64;
IF MASK[63+i] THEN
MASK[i +63:i] 0xFFFFFFFF_FFFFFFFF; // extend from most significant bit
ELSE
MASK[i +63:i] 0;
FI;
ENDFOR
FOR j 0 to 1
k j * 32;
i j * 64;
DATA_ADDR BASE_ADDR + (SignExtend(VINDEX[k+31:k])*SCALE + DISP;
IF MASK[63+i] THEN
DEST[i +63:i] FETCH_64BITS(DATA_ADDR); // a fault exits the instruction
FI;
MASK[i +63:i] 0;
ENDFOR
MASK[VLMAX:128] 0;
DEST[VLMAX:128] 0;
(non-masked elements of the mask register have the content of respective element cleared)
Ref. # 319433-014 5-217