User's Manual

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.
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