Compatibility: 4.0 only  Undoc 
Expects: AX    440dH
BX    drive (0=default, 1=A, 2=B, etc.)
CL    47H
CH    08H (device category: block device)
DS:DX addr of a two-byte structure:
DS:[DX+0]  0
DS:[DX+1]  00H=lock the drive
01H=unlock the drive; deny future access
Returns: AX    error code  if CF is set to CY
Info: Locks or unlocks all read/write access to the specified drive.
The driver may disable all disk access, for instance, when it
detects unformatted media or an invalid boot record.

Note: This function is not documented in official MS-DOS references.

See Also: Fn 44H (IOCTL)
Fn 440dH (generic IOCTL for block devices)
Fn 440dH 67H (query access flag)
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DOS Fn 440dH CL=47H: IOCTL Set Access Flag