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ARPL -- Adjust RPL Field of Selector




Opcode    Instruction          Clocks    Description

63 /r     ARPL r/m16,r16       pm=20/21  Adjust RPL of r/m16 to not
                                         less than RPL of r16

Operation




IF RPL bits(0,1) of DEST < RPL bits(0,1) of SRC
THEN
   ZF := 1;
   RPL bits(0,1) of DEST := RPL bits(0,1) of SRC;
ELSE
   ZF := 0;
FI;

Description

The ARPL instruction has two operands. The first operand is a 16-bit memory variable or word register that contains the value of a selector. The second operand is a word register. If the RPL field ("requested privilege level"--bottom two bits) of the first operand is less than the RPL field of the second operand, the zero flag is set to 1 and the RPL field of the first operand is increased to match the second operand. Otherwise, the zero flag is set to 0 and no change is made to the first operand.

ARPL appears in operating system software, not in application programs. It is used to guarantee that a selector parameter to a subroutine does not request more privilege than the caller is allowed. The second operand of ARPL is normally a register that contains the CS selector value of the caller.

Flags Affected

ZF as described above

Protected Mode Exceptions

#GP(0) if the result is in a nonwritable segment; #GP(0) for an illegal memory operand effective address in the CS, DS, ES, FS, or GS segments; #SS(0) for an illegal address in the SS segment; #PF(fault-code) for a page fault

Real Address Mode Exceptions

Interrupt 6; ARPL is not recognized in Real Address Mode

Virtual 8086 Mode Exceptions

Same exceptions as in Real Address Mode; #PF(fault-code) for a page fault

up: Chapter 17 -- 80386 Instruction Set
prev: AND Logical AND
next: BOUND Check Array Index Against Bounds