In the x86

computer architecture In computer engineering, computer architecture is a description of the structure of a computer system made from component parts. It can sometimes be a high-level description that ignores details of the implementation. At a more detailed level, the ...

, HLT (halt) is an

assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence b ...

instruction which halts the

central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...

(CPU) until the next external

interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

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. Most

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s execute a HLT instruction when there is no immediate work to be done, putting the processor into an idle state. In

Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...

, for example, this instruction is run in the "

System Idle Process In Windows NT operating systems, the System Idle Process contains one or more kernel threads which run when no other runnable thread can be scheduled on a CPU. In a multiprocessor system, there is one idle thread associated with each CPU core. ...

". On x86 processors, the

opcode In computing, an opcode (abbreviated from operation code, also known as instruction machine code, instruction code, instruction syllable, instruction parcel or opstring) is the portion of a machine language instruction that specifies the operat ...

of HLT is 0xF4.

History on x86

All x86 processors from the 8086 onward had the HLT instruction, but it was not used by

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few o ...

prior to 6.0 and was not specifically designed to reduce power consumption until the release of the

Intel DX4 IntelDX4 is a clock-tripled i486 microprocessor with 16-kB Level 1 cache.http://www.pld.ttu.ee/~prj/486dev.pdf Intel named it DX4 (rather than ''DX3'') as a consequence of litigation with AMD over trademarks. The product was officially n ...

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 Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...

to implement a "no-hlt" option for use when running on those chips, but this was fixed in later chips. Intel has since introduced additional processor-yielding instructions. These include: * in

SSE2 SSE2 (Streaming SIMD Extensions 2) is one of the Intel SIMD (Single Instruction, Multiple Data) processor supplementary instruction sets first introduced by Intel with the initial version of the Pentium 4 in 2000. It extends the earlier SSE i ...

intended for spin loops. Available to userspace (low-privilege rings). * / in

SSE3 SSE3, Streaming SIMD Extensions 3, also known by its Intel code name Prescott New Instructions (PNI), is the third iteration of the SSE instruction set for the IA-32 (x86) architecture. Intel introduced SSE3 in early 2004 with the Prescott revis ...

for thread synchronization. * (timed pause) and / (userspace monitor/mwait). Available to userspace.

Process

Almost every modern processor

instruction set In computer science, an instruction set architecture (ISA), also called computer architecture, is an abstract model of a computer. A device that executes instructions described by that ISA, such as a central processing unit (CPU), is called an ...

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 In computer science and software engineering, busy-waiting, busy-looping or spinning is a technique in which a process repeatedly checks to see if a condition is true, such as whether keyboard input or a lock is available. Spinning can also be use ...

for sleeping and idling. In most processors, halting (instead of looping) also reduces the latency of the next interrupt.

Use in operating systems

Since issuing the HLT instruction requires ring 0 access, it can only be run by privileged system software such as the

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

. Because of this, it is often best practice in application programming to use the

application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

(API) provided for that purpose by the operating system when no more work can be done, such as Linux's sched_yield(). This is referred to as "yielding" the processor. This allows the operating system's scheduler to decide whether other processes are runnable; if not. If every process is sleeping or waiting, it will normally execute a HLT instruction to cut power usage until the next hardware interrupt.

References

{{Reflist, 30em Machine code X86 instructions

History on x86

Process

Use in operating systems

See also

References