; Convert a string to uppercase 
; s ->
.label upcase
  ; store index
  POPxw
  .label upcase.loop
    LDBxw $00 EQU upcase.end JCNw
    ; check if a lowercase letter
    LDBxw $60 GTH
    LDBxw $7B LTH
    AND upcase.change JCNw
    ; continue
    INCxw upcase.loop JMPw
    .label upcase.change
      ; convert to uppercase  
      LDBxw $20 SUB STBxw
      ; continue 
      INCxw upcase.loop JMPw
    .label upcase.end
      RTS