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.

Overview Of Interrupt Handling

An interrupt handler, also known as an interrupt service routine (ISR), is a callback subroutine in an operating system or device driver whose execution is triggered by the reception of an interrupt. Interrupt handlers have a multitude of functions, which vary based on the reason the interrupt was generated and the speed at which the Interrupt Handler completes its task.

An interrupt handler is a low-level counterpart of event handlers. These handlers are initiated by either hardware interrupts or interrupt instructions in software, and are used for servicing hardware devices and transitions between protected modes of operation such as system calls.
When an interrupt is processed, a specific sequence of events takes place. You should write the interrupt service request (ISR) and interrupt service thread (IST) for your device driver with the following sequence of events in mind :

- When an interrupt occurs, the microprocessor jumps to the kernel exception handler.
- The exception handler disables all interrupts of an equal and lower priority at the microprocessor, and then calls the appropriate ISR for the physical interrupt request (IRQ).
- The ISR returns a logical interrupt, in the form of an interrupt identifier, to the interrupt handler and typically masks the board-level device interrupt.
- The interrupt handler re-enables all interrupts at the microprocessor, with the exception of the current interrupt, which is left masked at the board, and then signals the appropriate IST event.
- The IST is scheduled, services the hardware, and then finishes processing the interrupt.
- The IST calls the InterruptDone function, which in turn calls the OEMInterruptDone function in the OAL.

OEMInterruptDone re-enables the current interrupt.

Overview of Interrupts

An interrupt is an unexpected hardware initiated subroutine call or jump that temporarily suspends the running of the current program.
Interrupts occur when a peripheral device asserts an interrupt input pin of the micro-processor. Provided the interrupt is permitted, it will be acknowledged by the processor at the end of the current memory cycle. The processor then services the interrupt by branching to a special service routine written to handle that particular interrupt. Upon servicing the device, the processor is then instructed to continue with what is was doing previously by use of the "return from interrupt" instruction.
Interrupts in general can be divided into two kinds- maskable and non-maskable.
A maskable interrupt is an interrupt whose trigger event is not always important, so the programmer can decide that the event should not cause the program to jump. A non-maskable interrupt (like the reset button) is so important that it should never be ignored. The processor will always jump to this interrupt when it happens.

The function that is called or the particular assembly code that is executed when the interrupt happens is called the Interrupt Service Routine (ISR). Other terms of note are: An interrupt flag (IFG) this is the bit that is set that triggers the interrupt, leaving the interrupt resets this flag to the normal state. An interrupt enable (IE) is the control bit that tells the processor that a particular maskable interrupt should or should not be ignored.

Advantages of Interrupts :
Interrupts are used to ensure adequate service response times by the processing. Sometimes, with software polling routines, service times by the processor cannot be guaranteed, and data may be lost. The use of interrupts guarantees that the processor will service the request within a specified time period, reducing the likelihood of lost data.

Software Interrupt :
The Software Interrupt (SWI) is an instruction that can be placed anywhere within a program. It forces the microprocessor to act as if an interrupt has occurs. The vector for the 6800 is located at addresses $FFFA and $FFFB. The SWI is often used by Monitor Programs to set breakpoints, which stops the program at a particular location so that the contents of the memory and registers can be examined.

Interrupt Latency :
The time interval from when the interrupt is first asserted to the time the CPU recognizes it. This will depend much upon whether interrupts are disabled, prioritized and what the processor is currently executing.

Interrupt Response Time :
The time interval between the CPU recognizing the interrupt to the time when the first instruction of the interrupt service routine is executed. This is determined by the processor architecture and clock speed.