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LSL - Load Segment Limit

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Code Mnemonic 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

Description
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 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). 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:

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.

Type Name Valid
0 Reserved No
1 Available 16-bit TSS Yes
2 LDT Yes
3 Busy 16-bit TSS Yes
4 16-bit call gate No
5 16-bit/32-bit task gate No
6 16-bit interrupt gate No
7 16-bit trap gate No
8 Reserved No
9 Available 32-bit TSS Yes
A Reserved No
B Busy 32-bit TSS Yes
C 32-bit call gate No
D Reserved No
E 32-bit interrupt gate No
F 32-bit trap gate No
Operands Bytes Clocks
r16, r16 3 8 NP
r32, r32 3 8 NP
r16, m16 3 + d(0, 2) 8 NP
r32, m32 3 + d(0, 2) 8 NP

Flags
ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF unaffected
VM unaffected ZF sets to 1 if the segment limit is loaded successfully; otherwise, it is cleared to 0
RF unaffected AF unaffected
NT unaffected PF unaffected
IOPL unaffected CF unaffected
OF unaffected