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Intel Pentium CPU Instruction Set Reference

LMSW instruction - Load Machine Status Word

Code Mnemonic Description
0F 01 /6 LMSW r/m16 Loads r/m16 in machine status word of CR0


Loads the source operand into the machine status word, bits 0 through 15 of register CR0. The source operand can be a 16-bit general-purpose register or a memory location. Only the low-order 4 bits of the source operand (which contains the PE, MP, EM, and TS flags) are loaded into CR0. The PG, CD, NW, AM, WP, NE, and ET flags of CR0 are not affected. The operand-size attribute has no effect on this instruction.

If the PE flag of the source operand (bit 0) is set to 1, the instruction causes the processor to switch to protected mode. While in protected mode, the LMSW instruction cannot be used clear the PE flag and force a switch back to real-address mode.

The LMSW instruction is provided for use in operating-system software; it should not be used in application programs. In protected or virtual-8086 mode, it can only be executed at CPL 0.

This instruction is provided for compatibility with the Intel 286 processor; programs and procedures intended to run on the Pentium Pro, Pentium, Intel486, and Intel386 processors should use the MOV (control registers) instruction to load the whole CR0 register. The MOV CR0 instruction can be used to set and clear the PE flag in CR0, allowing a procedure or program to switch between protected and real-address modes.

Operands Bytes Clocks
r16 3 8 NP
mem16 3 + d(0 - 2) 8 NP


ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF unaffected
VM unaffected ZF unaffected
RF unaffected AF unaffected
NT unaffected PF unaffected
IOPL unaffected CF unaffected
OF unaffected