Varför tar inte programmet return?

        .org $1000     

RESET:
        SEI            
        CLD            
        LDX #$FF       
        TXS            
        LDA #$81       
        STA $9F26      

READ_LOOP:
        JSR READ_CHAR  
        CMP #12        
        BEQ PRINT      
        STA $100,X     
        DEX            
        BPL READ_LOOP  

PRINT:
        INX                    
PRINT_LOOP:
        LDA $100,X             
        JSR PRINT_CHAR         
        DEX                   
        BPL PRINT_LOOP         

        BRK                    

READ_CHAR:
        LDA $9F21              
        RTS                    

PRINT_CHAR:
        STA $9F20              
        RTS