Abstract: A user program directly accesses a storage device for I/O operations, bypassing the operating system. The user registers a virtual memory region and an “extent”, or region, of the storage device with a host bus adapter. A virtual interface is created between the user and the storage device including a queue for transmitting commands from the user to the storage device. To perform I/O, the user creates a descriptor including command information identifying the type of I/O operation, a buffer handle identifying the registered virtual memory region, virtual memory address information identifying the locations of data buffers in the registered virtual memory region to be used for the I/O operation, an extent handle identifying the registered extent, and disk address information identifying an area within the registered extent to or from which data is to be written or read. The user posts this descriptor to the virtual interface by placed it on the associated queue.