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3rd Register=End Message Marker. The register must contain the ASCII code of a delimiter character that will be taken as end of message. If this is set to a value of -1, then no end marker will be used, and non-printable characters will be stored in original byte form. If this is set to a value of 0, no end marker will be used, and all non-printable characters (ASCII 1 to 31 and 128 to 255) will be stored in the result string as "\001", "\031", "\128", etc.
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