Donald,

You know, many years I've been used the similar code for debugging with small exchanges ro reduce its size

TXTOUT: POP DPH ;
POP DPL ;
SMBNXT: CLR A ;Get
MOVC A,@A+DPTR ;byte
INC DPTR ;New address
JNZ SMBOUT ;
JMP @A+DPTR ;End of text
SMBOUT: CALL RSOUT ;Send byte
JMP SMBNXT ;

Null is the end of a string.

Regards,
George