ARPL -- Adjust RPL Field of Selector

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