Multiple Hardware Instances

The device driver samples shipped with Windows 95 support multiple instances of the hardware in a single machine. For example, it is possible to install three sound cards in the same machine. If there are enough free resources, all three devices will start and use the same driver set but remain independently installed devices.

MMSYSTEM considers each device node a separate device installation. The device driver should respond with the appropriate information for that specific instance of the hardware. For example, given two sound cards and each supports one wave output device, the driver should respond with "1" to the WODM_GETNUMDEVS message for each device node.

Additionally, to properly support multiple device instances the device drivers must perform the following tasks:

• Maintain hardware instance specific data structures using the device node as the key.
• "Thunk" I/O trapping procedures to easily determine device node association.
• Create copies of the ring-3 ISR code for each hardware instance.
• Modify private interfaces to understand multiple hardware instances.
• Avoid the use of global variables.

The DDK code samples demonstrate all of these techniques. Most developers will use one of the sample device drivers and associated VxDs as their device driver base.