Function:

VERR verifies that the segment selector in memory, pointed to by op, is
readable and accessible with the current privilege level (CPL).
If so, the Zero Flag is set to 1, if not, the Zero Flag is cleared.

VERW verifies that the segment selector in memory, pointed to by op, is
writable and accessible with the current privilege level (CPL).
If so, the Zero Flag is set to 1, if not, the Zero Flag is cleared.

On some 386 both instructions allow a NULL selector to be specified,
accessing selector zero in the GDT, instead of failing unconditionally with
ZF=0, which would be the normal procedure. Workaround is to fill descriptor
zero in the GDT with all zeroes. Accessing it will then always fail and
produce the desired effect.

On some 386 both VERR and VERW can hang the CPU until an INTR, NMI or RESET
occurs. This bug occurs when there is no memory operand, JMP or CALL
instruction in the <prefetch queue> along with the VERR or VERW.
Workaround is to code a JMP or Jcondition instruction right after the VERR
or VERW, with the added condition that _the last byte_ of the VERR / VERW
and the _complete_ JMP instruction must fit in the same aligned doubleword.

A data breakpoint set to the mem16 operand of either VERR or VERR can be
missed on some 386es if the segment with the selector at mem16 is not
accessible. (see also <debugging>)




WBINVD Write back & invalidate both internal & external caches
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