Intel 指令

Intel AVX instructions are encoded using an encoding scheme that combines prefix
bytes, opcode extension field, operand encoding fields, and vector length encoding
capability into a new prefix, referred to as VEX.

In the VEX encoding scheme, the VEX
prefix may be two or three bytes long, depending on the instruction semantics.
Despite the two-byte or three-byte length of the VEX prefix, the VEX encoding format
provides a more compact representation/packing of the components of encoding an
instruction in Intel 64 architecture. The VEX encoding scheme also allows more headroom for future growth of Intel 64 architecture.

Advanced Encryption Standard

ARPL—Adjust RPL Field of Segment Selector
Compares the RPL fields of two segment selectors. The first operand (the destination
operand) contains one segment selector and the second operand (source operand)
contains the other. (The RPL field is located in bits 0 and 1 of each operand.) If the
RPL field of the destination operand is less than the RPL field of the source operand,
the ZF flag is set and the RPL field of the destination operand is increased to match
that of the source operand. Otherwise, the ZF flag is cleared and no change is made
to the destination operand. (The destination operand can be a word register or a
memory location; the source operand must be a word register.)
The ARPL instruction is provided for use by operating-system procedures (however, it
can also be used by applications). It is generally used to adjust the RPL of a segment
selector that has been passed to the operating system by an application program to
match the privilege level of the application program. Here the segment selector
passed to the operating system is placed in the destination operand and segment
selector for the application program’s code segment is placed in the source operand.
(The RPL field in the source operand represents the privilege level of the application
program.) Execution of the ARPL instruction then ensures that the RPL of the
segment selector received by the operating system is no lower (does not have a
higher privilege) than the privilege level of the application program (the segment
selector for the application program’s code segment can be read from the stack
following a procedure call).
This instruction executes as described in compatibility mode and legacy mode. It is
not encodable in 64-bit mode.

compatibility mode and legacy mode