The ROM cartridge slot principle continues in various mobile devices, thanks to the development of high density low-cost flash memory. For example, a GPS navigation device might allow user updates of maps by inserting a flash memory chip into an expansion slot. An E-book reader can store the text of several thousand books on a flash chip. Personal computers may allow the user to boot and install an operating system off a USB flash drive instead of CD ROM or floppy disks. Digital cameras with flash drive slots allow users to rapidly exchange cards when full, and allow rapid transfer of pictures to a computer or printer.