INT 21 - DOS 2.25 only - GET LEAD BYTE TABLE ADDRESS
        AX = 6300h
Return: CF clear if successful
            DS:SI -> lead byte table (see below)
        CF set on error
            AX = error code (01h) (see AH=59h)
Notes:  does not preserve any registers other than SS:SP
        the US version of MS-DOS 3.30 treats this as an unused function,
          setting AL=00h and returning immediately
SeeAlso: AX=6301h,AH=07h,AH=08h,AH=0Bh

Format of lead byte table entry:
Offset  Size    Description
 00h  2 BYTEs   low/high ends of a range of leading byte of double-byte chars
 02h  2 BYTEs   low/high ends of a range of leading byte of double-byte chars
        ...
  N   2 BYTEs   00h,00h end flag

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