try something like that
Code:
        DEFINE OSC 40      
        DEFINE HSER_RCSTA 90h 
        DEFINE HSER_TXSTA 20h 
        DEFINE HSER_BAUD 9600
        DEFINE HSER_SPBRG 64
    
Start:        
        Hserout ["Program has started.",13,10] 
        gosub LookForStart
        Goto Start

LookForStart:
        HSERIN 5000,noData,[WAIT ("start")] 
        hserout ["Got it! Backlight ON",13,10]
        Return
 
nodata:
        HSEROUT ["NOPE... Life's so cruel...",13,10]
        Return