3-375
INSTRUCTION SET REFERENCE
LSLLoad Segment Limit
Description
This instruction loads the unscrambled segment limit from the segment descriptor specified with
the second operand (source operand) into the first operand (destination operand) and sets the ZF
flag in the EFLAGS register. The source operand (which can be a register or a memory location)
contains the segment selector for the segment descriptor being accessed. The destination
operand is a general-purpose register.
The processor performs access checks as part of the loading process. Once loaded in the desti-
nation register, software can compare the segment limit with the offset of a pointer.
The segment limit is a 20-bit value contained in bytes 0 and 1 and in the first four bits of byte 6
of the segment descriptor. If the descriptor has a byte granular segment limit (the granularity flag
is set to 0), the destination operand is loaded with a byte granular value (byte limit). If the
descriptor has a page granular segment limit (the granularity flag is set to 1), the LSL instruction
will translate the page granular limit (page limit) into a byte limit before loading it into the desti-
nation operand. The translation is performed by shifting the 20-bit raw limit left 12 bits and
filling the low-order 12 bits with 1s.
When the operand size is 32 bits, the 32-bit byte limit is stored in the destination operand. When
the operand size is 16 bits, a valid 32-bit limit is computed; however, the upper 16 bits are trun-
cated and only the low-order 16 bits are loaded into the destination operand.
This instruction performs the following checks before it loads the segment limit into the desti-
nation register:
Checks that the segment selector is not null.
Checks that the segment selector points to a descriptor that is within the limits of the GDT
or LDT being accessed
Checks that the descriptor type is valid for this instruction. All code and data segment
descriptors are valid for (can be accessed with) the LSL instruction. The valid special
segment and gate descriptor types are given in the following table.
If the segment is not a conforming code segment, the instruction checks that the specified
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 of the segment selector).
If the segment descriptor cannot be accessed or is an invalid type for the instruction, the ZF flag
is cleared and no value is loaded in the destination operand.
Opcode
Instruction
Description
0F 03 /r
LSL r16,r/m16
Load: r16
<
segment limit, selector r/m16
0F 03 /r
LSL r32,r/m32
Load: r32
<
segment limit, selector r/m32)
