INT 21 U - DBLSPACE.BIN - IOCTL - FLUSH OR INVALIDATE INTERNAL CACHES
        AX = 4405h
        BL = drive number (00h = default, 01h = A:, etc)
        CX = 000Ah (size of DSPACKET structure)
        DS:DX -> DSPACKET structure (see below)
Return: CF clear if IOCTL successful -- check DSPACKET for actual status
            AX = number of bytes actually transferred
        CF set on error
            AX = error code (01h,05h,06h,0Dh) (see AH=59h)
Note:   this call is identical to the documented AX=4404h
SeeAlso: AX=4404h"DBLSPACE",INT 2F/AX=4A11h/BX=0000h

Format of DSPACKET structure:
Offset  Size    Description
 00h    WORD    signature 444Dh ("DM")
 02h    BYTE    command code
                46h ('F') flush internal caches
                49h ('I') flush and invalidate internal caches
 03h    WORD    result code
                (return) 4F4Bh ("OK") if successful, else unchanged
 05h  5 BYTEs   padding

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