Intel IA-32 - Manual (Page 163)

Vol. 3A 4-33
PROTECTION
3. Checking if the pointer offset exceeds the segment limit.
4. Checking if the supplier of the pointer is allowed to access the segment.
5. Checking the offset alignment.
The processor automatically performs first, second, and third checks during instruction execu-
tion. Software must explicitly request the fourth check by issuing an ARPL instruction. The fifth
check (offset alignment) is performed automatically at privilege level 3 if alignment checking is
turned on. Offset alignment does not affect isolation of privilege levels.
4.10.1 Checking Access Rights (LAR Instruction)
When the processor accesses a segment using a far pointer, it performs an access rights check
on the segment descriptor pointed to by the far pointer. This check is performed to determine if
type and privilege level (DPL) of the segment descriptor are compatible with the operation to be
performed. For example, when making a far call in protected mode, the segment-descriptor type
must be for a conforming or nonconforming code segment, a call gate, a task gate, or a TSS.
Then, if the call is to a nonconforming code segment, the DPL of the code segment must be equal
to the CPL, and the RPL of the code segment’s segment selector must be less than or equal to
the DPL. If type or privilege level are found to be incompatible, the appropriate exception is
generated.
To prevent type incompatibility exceptions from being generated, software can check the access
rights of a segment descriptor using the LAR (load access rights) instruction. The LAR instruc-
tion specifies the segment selector for the segment descriptor whose access rights are to be
checked and a destination register. The instruction then performs the following operations:
1. Check that the segment selector is not null.
2. Checks that the segment selector points to a segment descriptor that is within the descriptor
table limit (GDT or LDT).
3. Checks that the segment descriptor is a code, data, LDT, call gate, task gate, or TSS
segment-descriptor type.
4. If the segment is not a conforming code segment, checks if the segment descriptor is
visible at the CPL (that is, if the CPL and the RPL of the segment selector are less than or
equal to the DPL).
5. If the privilege level and type checks pass, loads the second doubleword of the segment
descriptor into the destination register (masked by the value 00FXFF00H, where X
indicates that the corresponding 4 bits are undefined) and sets the ZF flag in the EFLAGS
register. If the segment selector is not visible at the current privilege level or is an invalid
type for the LAR instruction, the instruction does not modify the destination register and
clears the ZF flag.
Once loaded in the destination register, software can preform additional checks on the access
rights information.