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When a computer is first turned on (when power is first applied to the computer), or when the computer user completely restarts the computer without turning off the power, the microprocessor must set itself and set the system to a known condition. On each such occasion, this known condition must be the same. This event is called a reset
. A reset does not delete data in permanent storage or save data in system memory. To effect a reset, the microprocessor must have an input that allows the system to initiate it. This input is called the reset input
The figure below shows a power-up indicator, switches S1 and S2, a delay unit, and the reset input of the microprocessor. All but the reset input are part of the computer system external to the microprocessor.
Upon power up, the Power Up Signal transitions from logic low to logic high. This signal is the input to S1 and the output of S1 is the input of S2. The output of S2 is the input of the Delay Unit. The Delay Unit holds its output to the microprocessor low until three conditions are satisfied: It is receiving the power up signal through S1 and S, and a set amount of time has elapsed since the logic high was received from S2. This provides the time required for the microprocessor to become active and the time to recognize the state of its Reset Input as being at logic low. The microprocessor then begins the reset process and does not start the reset process again until the Reset Input goes high and then after that goes low.
In any technology, there are many ways to accomplish this functionality.
Having received a reset signal, functional units within the microprocessor are reset and then a program in system memory is executed to reset system units external to the microprocessor in accordance with the intent of the system programmer. This is called the reset program
. The starting address of the reset program in system memory is often called the rest vector
In order for a reset to be initiated after the computer is already up and running, the Power Up Signal output must be shut off momentarily. This can be done by either S1 or S2. S1 is controlled by the computer user directly (often by means of a push button). S2 is controlled by system software invoked by the user with a keyboard or a pointing device. The momentary interruption of the power-up signal is sustained through a fixed period of time during which the delay unit holds the reset input low. S1 and S2 are both normally closed switches. This means that in the absence of any force or signal control the switch passes its input signal through to its output.
If the reset microinstructions in the microprogram ROM begin at address zero, the system reset sequence can be initiated by using a resettable counter as the microprogram counter. This would give the microprogram counter a reset input, a preset input, and a clock input.