This asks the driver to identify whether it supports a particular generic
IOCTL function.  Generic IOCTL fns have a variety of uses, ranging from
setting up code page fonts to formatting a hard disk.

Offset Size Contents
+0      1  bLen         ??H (length of this request packet)
+1      1  bUnitNo      unit number (block devices only)
+2      1  bCmd         19H (command code)
+3      2  rStatus      OUTPUT: Device Status Word must fill before exit
+5H     8  res          (reserved)
+0dH     1  bCategory    INPUT: device category
+0eH     1  bMinorCode   INPUT: minor code
+0fH     4  lRes         (reserved)
+13H     4  pfIoctlData  (not used; do not modify)
23               size of this structure

rStatus  On exit, the driver must place a DevStatusWord here.  Always
set the Done bit (bit 8).

If you don't support the specified category/MinorCode, set the
Error bit (bit 15) and put Device Error Code 03H (unknown
function) in the low byte.

bCategory  is the device category as passed in CH from DOS 440cH or 440dH
(Generic IOCTL fns):
01H=COMn (DOS 3.3+)
03H=console (screen + keyboard) (DOS 3.3+)
05H=parallel printer (DOS 3.2+)

bMinorCode  is a subfn code, as passed in CL from DOS 440cH (Generic IOCTL
for Character Devices):
45H (set iteration count)             3.3+
4aH (select a prepared code page)     3.3+
4cH (begin preparing a code page)     3.3+
4dH (done preparing a code page)      3.3+
5fH (set display information)         4.0+
65H (query retry iteration count)     3.3+
6aH (query active code page)          3.3+
6bH (get list of prepared code pages) 3.3+
7fH (query display information)       4.0+

or a subfn code, as passed in CL from DOS 440dH (Generic IOCTL
for Block Devices):
40H (set device parameters)           3.2+
41H (write track)                     3.2+
42H (format and verify track)         3.2+
46H (set media ID)                    4.0+
47H (set disk access flag)            4.0 only
60H (get device parameters)           3.2+
61H (read track)                      3.2+
62H (verify track)                    3.2+
66H (query media ID)                  4.0+
67H (query disk access flag)          4.0 only
68H (sense media type)                5.0+

Notes:   Only device drivers capable of handling generic IOCTL requests
need support this request (see Device Attribute bit 6).

  The driver must interpret the bCategory and bMinorCode fields
and if the specified combination is not supported, set rStatus
to indicate an "unknown fn" Device Driver Error.

See Also: Device Requests
- -

Device Request 19H: IOCTL Support Query