Overview of Interrupt Service Routine (ISR)

An interrupt service routine (ISR) is a software routine that hardware invokes in response to an interrupt. ISRs examine an interrupt and determine how to handle it. ISRs handle the interrupt, and then return a logical interrupt value. If no further handling is required because the device is disabled or data is buffered, the ISR notifies the kernel with a SYSINTR_NOP return value. An ISR must perform very fast to avoid slowing down the operation of the device and the operation of all lower priority ISRs. When an ISR notifies the kernel of a specific logical interrupt value, the kernel examines an internal table to map the logical interrupt value to an event handle.

Although an ISR might move data from a CPU register or a hardware port into a memory buffer, in general it relies on a dedicated interrupt thread, called the interrupt service thread (IST), to do most of the required processing.

The system supports two different types of ISRs:
- The driver can register an InterruptService routine to handle line-based or message-based interrupts. (This is the only type available prior to Windows Vista.) The system passes a driver-supplied context value.
- The driver can register an InterruptMessageService routine to handle message-based interrupts. The system passes both a driver-supplied context value and the message ID of the interrupt message.