Verify a Segment for Reading or Writing (verr, verw)

verr    	r/m16
verw    	r/m16

Operation

1 -> ZF (if segment can be read or written)

Description

VERR and VERW contains the value of a selector in the two-byte register or memory operand. VERR and VERW determine if the indicated segment can be reached in the current privilege level and whether it is readable (VERR) or writable (VERW). If the segment can be accessed, the zero flag (ZF) is set to 1, otherwise the zero flag is set to 0. For the zero flag to be set these conditions must be met:

• The selector denotes a descriptor; the selector is "defined".

• The selector is a code or data segment; not a task statement, LDT or a gate.

• For VERR, the segment must be readable, for VERW, writable.

• The descriptor privilege level (DPL) can be any value for VERR. otherwise the DPL must have the same or less privilege as the current level and the DPL of the selector.

Validation is performed as if the segment were loaded into DS, ES, FS, or GS and the indicated write or read performed. The validation results are indicated by the zero flag. The value of the selector cannot result in an exception.

Example

Determine if the segment indicated by the effective address (addressed by the EBX register plus an offset of 5) can be reached in the current privilege level and whether it is readable (VERR):

verr 5(%ebx)