TODO: Additional undocumented commands

INT 21h AH=0x23 Get File Size for FCB

INT 21h AH=0x26 Create New Program Segment Prefix

Example using INT 21h AH=0x26 to exec to COMMAND.COM in DOS 1.x

INT 21h AX=0x3307 DL=0/1 Windows 95 Set/Clear DOS_FLAG

INT 21h AX=0x5D0B Get DOS swappable data areas (MS-DOS 4.0)

INT 21h AX=0x3702 Specify \DEV\ prefix use (MS-DOS 2.0 and 3.3+)

INT 21h AX=0x3800 Get country-specific information (MS-DOS 2.11+ code, MS-DOS 2.0-2.10 uses a different struct)

INT 21h AX=0x3800 Get country-specific information (alternate version for MS-DOS 2.0-2.10)

INT 21h AH=0x38 DX=0xFFFF Set country code (MS-DOS 3.0+)

INT 21h AH=0x5C FLOCK Lock/unlock region of file (MS-DOS 3.0+)

