Y_pos = $FB
        X_pos = $FC        ; 16-bit value (uses FD)
        byte_to_paint = $FE
        btp_mem_pos = $F9  ; 16-bit value (uses FA), byte to paint memory position
        C = $F7            ; 16-bit value (uses F8)
        B = $F5            ; 16-bit value (uses F6)