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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. [1] Interrupts are signals sent by hardware devices to the CPU alerting it that an event occurred to which it should react.
Common methods are reducing the clock speed along with the CPU voltage and sending parts of the processor into a sleep state. On processors that have a halt instruction that stops the CPU until an interrupt occurs, such as x86 's HLT instruction, it may save significant amounts of power and heat if the idle task consists of a loop which ...
The Intel 8086 and subsequent processors in the x86 series have an HLT (halt) instruction, opcode F4, which stops instruction execution and places the processor in a HALT state. An enabled interrupt, a debug exception, the BINIT signal, the INIT signal, or the RESET signal resumes execution, which means the processor can always be restarted. [ 15 ]
The x86 instruction set refers to the set of instructions that x86-compatible microprocessors support. The instructions are usually part of an executable program, often stored as a computer file and executed on the processor. The x86 instruction set has been extended several times, introducing wider registers and datatypes as well as new ...
Sleep mode (or suspend to RAM) is a low power mode for electronic devices such as computers, televisions, and remote controlled devices. These modes save significantly on electrical consumption compared to leaving a device fully on and, upon resume, allow the user to avoid having to reissue instructions or to wait for a machine to boot .
The x86 processors support five modes of operation for x86 code, Real Mode, Protected Mode, Long Mode, Virtual 86 Mode, and System Management Mode, in which some instructions are available and others are not. A 16-bit subset of instructions is available on the 16-bit x86 processors, which are the 8086, 8088, 80186, 80188, and 80286.
For instance, on x86 processors under Windows 2000, the idle thread will run a loop of halt instructions, which causes the CPU to turn off many internal components until an interrupt request arrives. Later versions of Windows implement more complex CPU power saving methods.
Busy-waiting itself can be made much less wasteful by using a delay function (e.g., sleep()) found in most operating systems. This puts a thread to sleep for a specified time, during which the thread will waste no CPU time. If the loop is checking something simple then it will spend most of its time asleep and will waste very little CPU time.