Compatibility: 3.3+
Expects: AX    6521H
CX    length of text, in bytes
DS:DX address of text to convert
                                                                  
Returns: AX    error code if CF is set to CY
DS:DX the text is converted in place
                                                                  
Info: Converts the specified number of characters starting at the
address at DS:DX into uppercase, using the currently-active
uppercase table for the active country and code page.

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DOS Fn 6521H: Upshift String