# SMSW — Store Machine Status Word

Opcode* | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description
--- | --- | --- | --- | --- | ---
0F 01 /4 | SMSW r/m16 | M | Valid | Valid | Store machine status word to r/m16.
0F 01 /4 | SMSW r32/m16 | M | Valid | Valid | Store machine status word in low-order 16 bits of r32/m16; high-order 16 bits of r32 are undefined.
REX.W + 0F 01 /4 | SMSW r64/m16 | M | Valid | Valid | Store machine status word in low-order 16 bits of r64/m16; high-order 16 bits of r32 are undefined.

## Instruction Operand Encoding ¶

Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | ---
M | ModRM:r/m (w) | N/A | N/A | N/A

## Description ¶

Stores the machine status word (bits 0 through 15 of control register CR0) into the destination operand. The destination operand can be a general-purpose register or a memory location.

In non-64-bit modes, when the destination operand is a 32-bit register, the low-order 16 bits of register CR0 are copied into the low-order 16 bits of the register and the high-order 16 bits are undefined. When the destination operand is a memory location, the low-order 16 bits of register CR0 are written to memory as a 16-bit quantity, regardless of the operand size.

In 64-bit mode, the behavior of the SMSW instruction is defined by the following examples:

- SMSW r16 operand size 16, store CR0[15:0] in r16
- SMSW r32 operand size 32, zero-extend CR0[31:0], and store in r32
- SMSW r64 operand size 64, zero-extend CR0[63:0], and store in r64
- SMSW m16 operand size 16, store CR0[15:0] in m16
- SMSW m16 operand size 32, store CR0[15:0] in m16 (not m32)
- SMSW m16 operands size 64, store CR0[15:0] in m16 (not m64)

SMSW is only useful in operating-system software. However, it is not a privileged instruction and can be used in application programs if CR4.UMIP = 0. It is provided for compatibility with the Intel 286 processor. Programs and procedures intended to run on IA-32 and Intel 64 processors beginning with the Intel386 processors should use the MOV CR instruction to load the machine status word.

See “Changes to Instruction Behavior in VMX Non-Root Operation” in Chapter 26 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3C, for more information about the behavior of this instruction in VMX non-root operation.

## Operation ¶

    DEST := CR0[15:0];
    (* Machine status word *)

## Flags Affected ¶

None.

## Protected Mode Exceptions ¶

#GP(0) | If the destination is located in a non-writable segment.
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.
If CR4.UMIP = 1 and CPL > 0.
#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 an unaligned memory reference is made while CPL = 3.
#UD | If the LOCK prefix is used.

## Real-Address Mode Exceptions ¶

#GP | If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
#SS(0) | If a memory operand effective address is outside the SS segment limit.
#UD | If the LOCK prefix is used.

## Virtual-8086 Mode Exceptions ¶

#GP(0) | If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
If CR4.UMIP = 1.
#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 an unaligned memory reference is made.
#UD | If the LOCK prefix is used.

## Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

## 64-Bit Mode Exceptions ¶

#SS(0) | If a memory address referencing the SS segment is in a non-canonical form.
#GP(0) | If the memory address is in a non-canonical form.
If CR4.UMIP = 1 and CPL > 0.
#PF(fault-code) | If a page fault occurs.
#AC(0) | If alignment checking is enabled and an unaligned memory reference is made while CPL = 3.
#UD | If the LOCK prefix is used.