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THE
80286
INSTRUCTION SET
VERR,VERW-Verifya
Segment
for
Reading
or
Writing
Opcode
OF
00
/4
OF
00
/5
FLAGS MODIFIED
Zero
Instruction
VERR
ew
VERWew
FLAGS UNDEFINED
None
OPERATION
Clocks
14,mem=16
14,mem=16
Description
Set
ZF=1
if seg.
can
be
read, selector
ew
Set
ZF=
1 if
seg.
can
be
written, selector
ew
VERR and VERW expect the 2-byte register or memory operand
to
contain the value
of
a selector.
The instructions determine whether the segment denoted by the selector
is
reachable from the current
privilege level; the instructions also determine whether it
is
readable or writable.
If
the segment
is
determined to be accessible, the zero flag
is
set to
1;
if the segment
is
not accessible, it
is
set to
o.
To
set
ZF, the following conditions must be met:
I.
The selector must denote a descriptor within the bounds of the table (GDT or LDT); that
is,
the
selector must be
"defined."
2.
The selector must denote the descriptor of a code or data segment.
3.
If
the instruction
is
VERR, the segment must
be
readable.
If
the instruction
is
VERW, the segment
must be a writable data segment.
4.
If
the code segment
is
readable and conforming, the descriptor privilege level (DPL) can be any
value for VERR. Otherwise, the DPL must be greater than or equal to (have less
or
the same
privilege as) both the current privilege level and the selector's
RPL.
The validation performed
is
the same
as
if the segment were loaded into DS or ES and the indicated
access (read or write) were performed. The zero flag receives the result of the validation. The selector's
value cannot result
in
a protection exception. This enables the software to anticipate possible segment
access problems.
PROTECTED MODE EXCEPTIONS
The only faults that can occur are those generated by illegally addressing the memory operand which
contains the selector. The selector
is
not loaded into any segment register, and
no
faults attributable to
the selector operand are generated.
#GP(O) for an illegal memory operand effective address
in
the CS, DS, or ES segments;
#SS(O)
for an
illegal address
in
the SS segment.
8-111