A typical ISA driver would need the following include files:

#include <sys/module.h>
#include <sys/bus.h>
#include <machine/bus.h>
#include <machine/resource.h>
#include <sys/rman.h>

#include <isa/isavar.h>
#include <isa/pnpvar.h>

They describe the things specific to the ISA and generic bus subsystem.

The bus subsystem is implemented in an object-oriented fashion, its main structures are accessed by associated method functions.

The list of bus methods implemented by an ISA driver is like one for any other bus. For a hypothetical driver named “xxx” they would be:

xxx_isa_probe() and xxx_isa_attach() are mandatory, the rest of the routines are optional, depending on the device's needs.

The driver is linked to the system with the following set of descriptions.

    /* table of supported bus methods */
    static device_method_t xxx_isa_methods[] = {
        /* list all the bus method functions supported by the driver */
        /* omit the unsupported methods */
        DEVMETHOD(device_identify,  xxx_isa_identify),
        DEVMETHOD(device_probe,     xxx_isa_probe),
        DEVMETHOD(device_attach,    xxx_isa_attach),
        DEVMETHOD(device_detach,    xxx_isa_detach),
        DEVMETHOD(device_shutdown,  xxx_isa_shutdown),
        DEVMETHOD(device_suspend,   xxx_isa_suspend),
        DEVMETHOD(device_resume,    xxx_isa_resume),

	DEVMETHOD_END
    };

    static driver_t xxx_isa_driver = {
        "xxx",
        xxx_isa_methods,
        sizeof(struct xxx_softc),
    };


    static devclass_t xxx_devclass;

    DRIVER_MODULE(xxx, isa, xxx_isa_driver, xxx_devclass,
        load_function, load_argument);

Here struct xxx_softc is a device-specific structure that contains private driver data and descriptors for the driver's resources. The bus code automatically allocates one softc descriptor per device as needed.

If the driver is implemented as a loadable module then load_function() is called to do driver-specific initialization or clean-up when the driver is loaded or unloaded and load_argument is passed as one of its arguments. If the driver does not support dynamic loading (in other words it must always be linked into the kernel) then these values should be set to 0 and the last definition would look like:

 DRIVER_MODULE(xxx, isa, xxx_isa_driver,
       xxx_devclass, 0, 0);

If the driver is for a device which supports PnP then a table of supported PnP IDs must be defined. The table consists of a list of PnP IDs supported by this driver and human-readable descriptions of the hardware types and models having these IDs. It looks like:

    static struct isa_pnp_id xxx_pnp_ids[] = {
        /* a line for each supported PnP ID */
        { 0x12345678,   "Our device model 1234A" },
        { 0x12345679,   "Our device model 1234B" },
        { 0,        NULL }, /* end of table */
    };

If the driver does not support PnP devices it still needs an empty PnP ID table, like:

    static struct isa_pnp_id xxx_pnp_ids[] = {
        { 0,        NULL }, /* end of table */
    };