INT 21 - DOS 2+ - "SWITCHAR" - SET SWITCH CHARACTER
        AX = 3701h
        DL = new switch character
Return: AL = status
            00h successful
            FFh unsupported subfunction
Notes:  documented in some OEM versions of some releases of DOS
        supported by OS/2 compatibility box
        ignored by MS-DOS 5+ and DR-DOS 3.41+; DR-DOS 6.0 leaves AX unchanged
SeeAlso: AX=3700h

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