HLT

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In computer science and more specifically the x86 architecture, HLT is an assembly language instruction which halts the CPU until the next external interrupt is fired.^[1] This interrupt usually comes from the clock. The HLT instruction is run when an operating system enters its idle loop and there is no immediate work to be done. In Windows NT for example, this instruction is run in the 'System Idle Process'.

Contents 1 HLT machine instruction 2 Everyday usage 3 See also 4 References

[edit] HLT machine instruction

Almost every reasonably modern processor instruction set includes an instruction 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 fired. 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.

[edit] Everyday usage

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 API provided for that purpose by the operating system when no more work can be done. This allows the kernel to decide if other processes are runnable; if not, it will normally issue the HLT instruction to cut power usage

[edit] See also

• Halt and Catch Fire
• Idle task
• X86
• Computer architecture
• Instruction set

[edit] References