This asks the driver to determine whether the media in the drive has been
changed.

DevReq01Rec
Offset Size Contents
                                                                          
+0      1  bLen         13H (length of this request packet)
+1      1  bUnitNo      unit number to check
+2      1  bCmd         01H (command code)
+3      2  rStatus      OUTPUT: Device Status Word must fill before exit
+5H     8  res          (reserved)
+0dH     1  bMediaID     INPUT: Media Descriptor DOS thinks is there
+0eH     1  bRetFlag     OUTPUT: ffH=no change
00H=don't know
01H=changed
+0fH     4  pfVolID      OUTPUT: Addr of ASCIIZ  volume ID (3.0+)
19               size of this structure

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
the Error bit (bit 15) and put a Device Error Code in the low
byte.

bMediaID  On entry, this contains the Media Descriptor byte of the disk
that DOS thinks is in the drive.

bRetFlag  On exit, you must set this to indicate whether a different
disk is in the drive.  If you support Removable Media, and
return ffH, you must set pfVolID.

pfVolID  With DOS 3.0+ drivers that support Removable Media (see
Device Attribute), you should set this to the address of an
ASCIIZ  string of the volume label of the drive that was
previously in the drive.  Then, when DOS issues critical error
0fH (see INT 24H), it can prompt the user to insert the right
disk.

If you don't know the volume label, put the address of the
string "NO NAME" here.

Notes:   Diskette performance is much faster when the device driver can
quickly indicate a "no change", since DOS need not re-read the
FAT on the disk before each disk access.

See Also: Device Requests
- -

Device Request 01H: Media Check