Thus far, the steady-state operation of a microprocessor has been discussed in which instructions are fetched, decoded, and executed in an order determined by the PC and branch instructions. There are two special cases in which the microprocessor does not follow this regular pattern of operation. The first case is at power-up, when the microprocessor must transition from an idle state to executing instructions. This transition sequence is called reset
and involves the microprocessor fetching its boot code from memory to begin the programmed software sequence. Reset is triggered by asserting a particular logic level onto a microprocessor pin and can occur either at power-up or at any arbitrary time when it is desired to restart, or reboot, the microprocessor from a known initial state. Some microprocessors have special instructions that can actually trigger a soft reset.
The question arises of how the microprocessor determines which instruction to execute first when it has just been reset. To solve this problem, each microprocessor has a reset vector
that points it to a fixed, predetermined memory address where the programmer must locate the first instruction of the boot sequence. The reset vector is specified by the microprocessor’s designer. Some microprocessors locate the reset vector at the beginning of memory and some place it toward the end of the address space. Sometimes the main body of the program will be located in another portion of memory, and the first instruction at the reset vector will contain a branch instruction to jump to the desired location. The second case in which the microprocessor does not follow the normal instruction sequence is during normal operation when an event occurs and the programmer wishes the microprocessor to pause what it is currently doing and handle the event with a special software routine. Such an event is called an interrupt . A common application for an interrupt is the implementation of a periodic, timed operation such as monitoring the temperature of a room. Because the room temperature does not change often, the microprocessor can handle other tasks during normal operation.
A timer can be set to expire every few seconds, causing an interrupt event. When the interrupt triggers, the microprocessor can read the room temperature, take any appropriate action (e.g., turn on a ventilation fan), and then resume its normal operation.
and involves the microprocessor fetching its boot code from memory to begin the programmed software sequence. Reset is triggered by asserting a particular logic level onto a microprocessor pin and can occur either at power-up or at any arbitrary time when it is desired to restart, or reboot, the microprocessor from a known initial state. Some microprocessors have special instructions that can actually trigger a soft reset.
The question arises of how the microprocessor determines which instruction to execute first when it has just been reset. To solve this problem, each microprocessor has a reset vector
that points it to a fixed, predetermined memory address where the programmer must locate the first instruction of the boot sequence. The reset vector is specified by the microprocessor’s designer. Some microprocessors locate the reset vector at the beginning of memory and some place it toward the end of the address space. Sometimes the main body of the program will be located in another portion of memory, and the first instruction at the reset vector will contain a branch instruction to jump to the desired location. The second case in which the microprocessor does not follow the normal instruction sequence is during normal operation when an event occurs and the programmer wishes the microprocessor to pause what it is currently doing and handle the event with a special software routine. Such an event is called an interrupt . A common application for an interrupt is the implementation of a periodic, timed operation such as monitoring the temperature of a room. Because the room temperature does not change often, the microprocessor can handle other tasks during normal operation.
A timer can be set to expire every few seconds, causing an interrupt event. When the interrupt triggers, the microprocessor can read the room temperature, take any appropriate action (e.g., turn on a ventilation fan), and then resume its normal operation.
