## Physical Device Driver Reference

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### POWER_OEMFUNCTION (45h) - ParmLength

ParmLength Contains the size of the Parameter Packet in bytes (including ParmLength). On input, this length must be either 28, to specify use of the general purpose and index registers, or 36 to also include the DS, ES, FS and GS segment registers in the interface.

Note: The segment registers can only be used by ring 0 code through the IDC interface. Ring 3 code using the IOCTL interface cannot specify segment registers, and will receive a PowerBadLength error return code if this is attempted.