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Requirements for Routing PC Card Interrupts

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In order to use PCI interrupts instead of ISA interrupts, 16-bit PC Cards must meet the following two basic requirements.

  • Support for PCI level-triggered interrupt generation. Most 16-bit PC Cards comply with this requirement, because the PCMCIA standard specifies that PC Cards and CardBus cards must support level-triggered interrupts.

  • Support for interrupt sharing. Many 16-bit PC Cards do not comply with this requirement. PCI interrupts are sharable, so the system cannot assign a PCI interrupt number to a 16-bit PC Card if the card does not support sharable interrupts. Common reasons why a 16-bit PC Card cannot share interrupts include:

    • Failure of the interrupt service routine (ISR) to return the number of the interrupt. On a PCI bus, an ISR must indicate which interrupt it is servicing by returning the number of the interrupt after it completes. ISRs for some 16-bit PC Cards do not do this.
    • Failure of the PC Card to indicate the interrupts it asserts in its hardware registers. A PC Card must indicate the number of the interrupt it is asserting in its hardware registers to avoid contention for the same interrupt number with other devices. Some 16-bit PC Cards do not do this.
    • Generation of spurious interrupts when powering up. PC Cards must not generate any interrupts other than those that are assigned to them by the system. Some 16-bit PC Cards generate spurious interrupts when powering up, before the system has assigned them interrupts.

A PC Card that does not fulfill these requirements can cause an interrupt storm that provokes a system crash.