HLT (x86 instruction)

In the x86 computer architecture, HLT (halt) is an assembly language instruction which halts the central processing unit (CPU) until the next external interrupt is fired. Interrupts are signals sent by hardware devices to the CPU alerting it that an event occurred to which it should react. For example, hardware timers send interrupts to the CPU at regular intervals.

The HLT instruction is executed by the operating system when there is no immediate work to be done, and the system enters its idle state. In Windows NT, for example, this instruction is run in the "System Idle Process". On x86 processors, the opcode of HLT is 0xF4.

All x86 processors from the 8086 onwards had the HLT instruction, but it was not used by MS-DOS prior to 6.0 and was not specifically designed to reduce power consumption until the release of the Intel DX4 processor in 1994. MS-DOS 6.0 provided a POWER.EXE that could be installed in CONFIG.SYS and in Microsoft's tests it saved 5%. Some of the first 100 MHz DX chips had a buggy HLT state, prompting the developers of Linux to implement a "no-hlt" option for use when running on those chips, but this was fixed in later chips.

Almost every reasonably modern processor instruction set includes an instruction or sleep mode which halts the processor until more work needs to be done. In interrupt-driven processors, this instruction halts the CPU until an external interrupt is received. On most architectures, executing such an instruction allows the processor to significantly reduce its power usage and heat output, which is why it is commonly used instead of busy waiting for sleeping and idling.

Since issuing the HLT instruction requires ring 0 access, it can only be run by privileged system software such as the kernel. Because of this, it is often best practice in application programming to use the application programming interface (API) provided for that purpose by the operating system when no more work can be done. This is referred to as "yielding" the processor. This allows the operating system's scheduler to decide if other processes are runnable; if not, it will normally issue the HLT instruction to cut power usage.

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