Compatibility: 5.0+
Expects: AX    ad81H
Returns: CF    Carry Flag is set if there is an error
AX    0001H (error code) if the code page is not valid
Info: This is similar to using the DOS command, Mode CON CP SEL=nnn; it
activates the specified code page (character set) for the

As with that Mode command, a code page cannot be selected unless
the KEYBOARD.SYS device driver has been installed and unless it
has been previously prepared (Mode CON CP PREP=).

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INT 2fH ad81H: Set KEYB.COM Active Code Page