Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1686

VERR, VERW—Verify a Segment for Reading or Writing
Opcode
0F 00 /4
0F 00 /5
Description
Verifies whether the code or data segment specified with the source operand is
readable (VERR) or writable (VERW) from the current privilege level (CPL). The source
operand is a 16-bit register or a memory location that contains the segment selector for
the segment to be verified. If the segment is accessible and readable (VERR) or
writable (VERW), the ZF flag is set; otherwise, the ZF flag is cleared. Code segments
are never verified as writable. This check cannot be performed on system segments.
To set the ZF flag, the following conditions must be met:
• The segment selector is not null.
• The selector must denote a descriptor within the bounds of the descriptor table
(GDT or LDT).
• The selector must denote the descriptor of a code or data segment (not that of a
system segment or gate).
• For the VERR instruction, the segment must be readable; the VERW instruction, the
segment must be a writable data segment.
• If the segment is not a conforming code segment, the segment's DPL must be
greater than or equal to (have less or the same privilege as) both the CPL and the
segment selector's RPL.
The validation performed is the same as if the segment were loaded into the DS, ES,
FS, or GS register, and the indicated access (read or write) were performed. The
selector's value cannot result in a protection exception, enabling the software to
anticipate possible segment access problems.
Operation
IF SRC(Offset) > (GDTR(Limit) OR (LDTR(Limit))
THEN
Read segment descriptor;
IF SegmentDescriptor(DescriptorType) = 0 (* system segment *)
OR (SegmentDescriptor(Type)  conforming code segment)
AND (CPL > DPL) OR (RPL > DPL)
THEN
ELSE
FI;
4:384
Instruction Description
VERR r/m16 Set ZF=1 if segment specified with r/m16 can be read
VERW r/m16 Set ZF=1 if segment specified with r/m16 can be written
ZF  0
ZF  0
IF ((Instruction = VERR) AND (segment = readable))
OR ((Instruction = VERW) AND (segment = writable))
THEN
ZF  1;
FI;
Volume 4: Base IA-32 Instruction Reference