Книга: Microsoft Windows Embedded CE 6.0 Exam Preparation Kit

Interrupt Service Routines

Interrupt Service Routines

In general, ISRs are small blocks of code that run in response to a hardware interrupt. Because the kernel exception handler masks off all interrupts of equal or lesser priority while this ISR runs, it is important to complete the ISR and return a SYSINTR value as quickly as possible so that the kernel can re-enable (unmask) all IRQs with minimal delay (except the currently processed interrupt). System performance can suffer significantly if too much time is spent in ISRs, leading to missed interrupts or overrun buffers on some devices. Another important aspect is that the ISR runs in kernel mode and does not have access to higher-level operating system APIs. For these reasons, ISRs usually perform no more than the most basic tasks, such as quickly copying data from hardware registers to memory buffers. On Windows Embedded CE, time-consuming interrupt processing is usually performed in an IST.

The primary task of the ISR is to determine the interrupt source, mask off or clear the interrupt at the device, and then return a SYSINTR value for the interrupt to notify the kernel about an IST to run. In the simplest case, the ISR returns SYSINTR_NOP to indicate that no further processing is necessary. Accordingly, the kernel does not signal an event for an IST to handle the interrupt. On the other hand, if the device driver uses an IST to handle the interrupt, the ISR passes the logical interrupt identifier to the kernel, the kernel determines and signals the interrupt event, and the IST typically resumes from a WaitForSingleObject call and executes the interrupt processing instructions in a loop. The latency between the ISR and the IST depends on the priority of the thread and other threads running in the system, as explained in Chapter 3, "Performing System Programming." Typically, ISTs run with a high thread priority.

Оглавление книги