Driver Load Order

For correct operation the following tasks must be performed before loading the client driver:

Load API driver
Start API driver service
Initialize API device instances

Details about driver load ordering can be found in the Windows DDK documentation.

Windows NT 4 and Earlier

Client drivers under Windows NT 4 and earlier can meet the load order requirements by adding the DependOnService value to the client driver services registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ClientServiceName
Name:"DependOnService", Type:REG_MULTI_SZ, Value:"MGHDLC"

This value causes the service control manager to load the API driver and call the DriverEntry function of the API driver before loading the client driver. The API driver DriverEntry function initializes all device instances (ports).

Windows 2000 and Later

Windows 2000 and later uses a different loading scheme. To ensure the above requirements, the service start type of the API driver must be changed to SERVICE_BOOT_START (0). The plug and play manager starts boot drivers before drivers with other start options. The client driver start option should be something other than SERVICE_BOOT_START, with the specific option depending on the client driver needs.

The driver start option is determined by the Start registry value in the driver service registry key:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MGHDLC
Name:"Start", Type:REG_DWORD

Possible values:

SERVICE_BOOT_START (0x0)
    SERVICE_SYSTEM_START (0x01)
    SERVICE_AUTO_START (0x02)
    SERVICE_DEMAND_START (0x03)
    SERVICE_DISABLED (0x04)

Contents