lsl

Load Segment Limit

LSL r, r/m16

Reads segment limit from descriptor.

Details

Loads the segment limit from the descriptor table entry referenced by the source selector into the destination register. The instruction reads the descriptor from the GDT or LDT, extracts the limit field, and writes it to the destination. The ZF flag is cleared if the selector is valid and the descriptor is accessible; ZF is set if the selector is invalid or the descriptor type is unsuitable. This instruction is privileged and operates only in protected mode or 64-bit mode.

Pseudocode Operation

selector ← src[15:0];
descriptor ← load_descriptor(selector);
if (descriptor is valid AND descriptor.type in {code, data, tss, gate}) {
  dest ← descriptor.limit;
  ZF ← 0;
} else {
  ZF ← 1;
}

Example

LSL rax, bx

Encoding

Binary Layout

Format System

Opcode 0F 03

Extension System

Operands

dest
General-purpose register
src
16-bit register or memory

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	Description
0F 03 /r	LSL r16, r16/m16	RM	Valid Valid	Load segment limit from specified descriptor.
0F 03 /r	LSL r32, r32/m16	RM	Valid Valid	Load segment limit from specified descriptor.
REX.W + 0F 03 /r	LSL r64, r32/m16	RM	Valid Valid	Load segment limit from specified descriptor.

Description

Loads the segment limit from the segment descriptor (see below) 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. If the source operand is a memory address, only 16 bits of data are accessed. The destination operand is a general-purpose register. The processor performs access checks as part of the loading process. Once loaded in the destination 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 4 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) as read from the descriptor. 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 destination 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 16 bits, a valid 32-bit byte limit is computed; however, the upper 16 bits are truncated and only the low-order 16 bits are loaded into the destination operand; the upper bits of the destination are unmodified. When the operand size is 32 bits, the 32-bit byte limit is loaded into the destination operand; the upper bits of the destination are cleared. When the operand is 64 bits, the 32-bit byte limit is zero-extended to 64 bits and loaded into the destination operand. (The behavior with 32-bit and 64-bit operand sizes is identical.) This instruction performs the following checks before it loads the segment limit into the destination 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 Table 3-59. • 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. LSL—Load Segment Limit Vol. 2A 3-573 Table 3-59. Segment and Gate Descriptor Types Type Protected Mode IA-32e Mode Name Valid Name Valid 0 Reserved No Reserved No 1 Available 16-bit TSS Yes Reserved No 2 LDT Yes LDT1 Yes 3 Busy 16-bit TSS Yes Reserved No 4 16-bit call gate No Reserved No 5 16-bit/32-bit task gate No Reserved No 6 16-bit interrupt gate No Reserved No 7 16-bit trap gate No Reserved No 8 Reserved No Reserved No 9 Available 32-bit TSS Yes 64-bit TSS1 Yes A Reserved No Reserved No B Busy 32-bit TSS Yes Busy 64-bit TSS1 Yes C 32-bit call gate No 64-bit call gate No D Reserved No Reserved No E 32-bit interrupt gate No 64-bit interrupt gate No F 32-bit trap gate No 64-bit trap gate No

Operation

IF SRC(Offset) > descriptor table limit
THEN ZF := 0; FI;
Read segment descriptor;
IF SegmentDescriptor(Type) ≠ conforming code segment
and (CPL > DPL) OR (RPL > DPL)
or Segment type is not valid for instruction
THEN
ZF := 0;
ELSE
temp := SegmentLimit([SRC]);
IF (SegmentDescriptor(G) = 1)
THEN temp := (temp << 12) OR 00000FFFH;
ELSE IF OperandSize = 32
THEN DEST := temp; FI;
ELSE IF OperandSize = 64 (* REX.W used *)
THEN DEST := temp(* Zero-extended *); FI;
ELSE (* OperandSize = 16 *)
DEST := temp AND FFFFH;
FI;
FI;





LSL—Load Segment Limit                                                                                                                         Vol. 2A 3-574

Flags Affected

The ZF flag is set to 1 if the segment limit is loaded successfully; otherwise, it is set to 0. The CF, OF, SF, AF, and PF flags are not modified.

Exceptions

Protected Mode Exceptions

#GP(0) If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register is used to access memory and it contains a NULL segment selector. #SS(0) If a memory operand effective address is outside the SS segment limit. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and the memory operand effective address is unaligned while the current privilege level is 3. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD The LSL instruction cannot be executed in real-address mode.

Virtual-8086 Mode Exceptions

#UD The LSL instruction cannot be executed in virtual-8086 mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0) If the memory operand effective address referencing the SS segment is in a non-canonical form. #GP(0) If the memory operand effective address is in a non-canonical form. #PF(fault-code) If a page fault occurs. #AC(0) If alignment checking is enabled and the memory operand effective address is unaligned while the current privilege level is 3. #UD If the LOCK prefix is used. LSL—Load Segment Limit Vol. 2A 3-575