Hi all,
I want to know exactly what happens in the hardware when HLT command (Halt) is executed in a micro-processor or micro-controller. I have read that it functions in power-saving mode. Could any please help me explaining the exact working of HLT command and the hardware functioning on execution of this.
Regards,
Raghu
The microprocessor has different operating states that is imposed by instructing it. For example Fetch, execute or halt. IN the fetch and execute the processor is fully operating while in the HALT state it is in the sleep or idle mode. So, from the principle point of view, the state of powering of the processor is either off or on or in a sleep or idle mode. The sleep mode of the processor is accomplished by power down of all logic circuits which are not required to keep the processor ready to run once an instruction is input. The sleep mode is an intermediate active state. The hard ware designers see for a power management circuit which controls the activation of the every logic unit in the processor.
As an example, all synchronous logic circuits has a clock input whether a a logic input is present or not. So, the clock can it self not be made off but can be prevented to enter the logic circuits by switching off its line.
Therefore whenever there is no instruction to execute by the processor, it is put in the HALT state by a HALT instruction issued by the operating system.
Not only the processors have this property but also most of the electronic systems.
AS an example, the chip sets of any mobile phone has a power management circuit to control the power down state and save precious electrical energy and reduce the heating of the circuits.
The Halt instruction freezes the value of Program Counter. Since Program Counter is not getting updated and keeps its value as its is, fetching of any instruction is not possible. This will keep microprocessor in steady position. Clock is internally active but not connected to the Program Counter.
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