clts

Clear Task-Switched Flag

CLTS

Clears the TS flag in CR0 (Privileged).

Details

Clears the Task-Switched flag (TS, bit 3) in CR0. This privileged instruction (CPL=0 only) allows the processor to use the FPU without raising a #NM exception; it is typically executed after an FPU context save. No flags are affected.

Pseudocode Operation

CR0[3] ← 0

Example

CLTS

Encoding

Binary Layout
0F
+0
06
+1
 
Format System
Opcode 0F 06
Extension System

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
0F 06 CLTS ZO Valid Valid Clears TS flag in CR0.

Description

Clears the task-switched (TS) flag in the CR0 register. This instruction is intended for use in operating-system procedures. It is a privileged instruction that can only be executed at a CPL of 0. It is allowed to be executed in realaddress mode to allow initialization for protected mode. The processor sets the TS flag every time a task switch occurs. The flag is used to synchronize the saving of FPU context in multitasking applications. See the description of the TS flag in the section titled “Control Registers” in Chapter 2 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A, for more information about this flag. CLTS operation is the same in non-64-bit modes and 64-bit mode. See Chapter 27, “Virtual Machine Control Structures,” 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

CR0.TS[bit 3] := 0;

Flags Affected

The TS flag in CR0 register is cleared.

Exceptions

Protected Mode Exceptions

#GP(0) If the current privilege level is not 0. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0) CLTS is not recognized in virtual-8086 mode. #UD If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#GP(0) If the CPL is greater than 0. #UD If the LOCK prefix is used. CLTS—Clear Task-Switched Flag in CR0 Vol. 2A 3-152