This tells the driver that a block-device file or character device is
+0 1 bLen 0dH (length of this request packet)
+2 1 bCmd 00H (command code)
rStatus On exit, the driver must place a DevStatusWord here. Always
set the Done bit (bit 8). On an error, set the Done bit and
Notes: Only drivers capable of device Open/Close-processing need
DOS uses this request whenever an application opens or creates
a file or opens a device (fn 3dH) and DOS issues DvRq 0eH
(Device Close) when the device or file gets closed (fn 3eH).
Drivers supporting this request can uses these fns to help with
maintenance of internal buffers, etc.
Character devices (such as PRN drivers) can watch for Open
requests and send control data to the device (such as font
downloads) when it is opened after being closed. In that
(IOCTL fns) in order to provide a means to receive download