3-704
INSTRUCTION SET REFERENCE
VERR/VERWVerify a Segment for Reading or Writing
Description
These instructions verify 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.
For the VERW instruction, the segment must be a writable data segment.
If the segment is not a conforming code segment, the segments 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 is performed when a segment selector is loaded into the
DS, ES, FS, or GS register, and the indicated access (read or write) is performed. The segment
selector's value cannot result in a protection exception, enabling the software to anticipate
possible segment access problems.
Opcode
Instruction
Description
0F 00 /4
VERR r/m16
Set ZF=1 if segment specified with r/m16 can be read
0F 00 /5
VERW r/m16
Set ZF=1 if segment specified with r/m16 can be written
