Intel 253666-024US - Manual (Page 159)

Vol. 2A 3-113
INSTRUCTION SET REFERENCE, A-M
CLTS—Clear Task-Switched Flag in CR0
CLTS—Clear Task-Switched 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 real-address 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 descrip-
tion 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 infor-
mation about this flag.
CLTS operation is the same in non-64-bit modes and 64-bit mode.
See Chapter 21, “VMX Non-Root Operation,” of the Intel® 64 and IA-32 Architectures
Software Developer’s Manual, Volume 3B, 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.
Protected Mode Exceptions
#GP(0) If the current privilege level is not 0.
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.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
0F 06 CLTS Valid Valid Clears TS flag in CR0.