Floppy disk storage is provided by a built-in, 3.5 inch floppy disk drive. Disks are 80 track, double sided, and formatted as 11 sectors per track, 512 bytes per sector (over 900,000 bytes per disk). The disk controller can read and write 320/360K IBM PC(TM) (MS-DOS(TM)) formatted 3.5 or 5.25 inch disks, and 640/720K IBM PC (MS-DOS) formatted 3.5 inch disks. Up to three extra 3.5 inch or 5.25 inch disk drives can be added to the Amiga. The A2000 and A3000 also provide room to mount floppy or hard disks internally. The A3000 has a built-in hard disk drive and an on-board SCSI controller which can handle two internal drives and up to seven external SCSI devices. The Amiga has a full complement of dedicated I/O connectors. The circuitry for some of these peripherals resides on the Paula custom chip while the Amiga's two 8520 CIA chips handle other I/O chores not specifically assigned to any of the custom chips. These include modem control, disk status sensing, disk motor and stepping control, ROM enable, parallel input/output interface, and keyboard interface. The Amiga includes a standard RS-232-C serial port for external serial input/output devices such as a modem, MIDI interface, or printer. A programmable, Centronics-compatible parallel port supports parallel printers, audio digitizers, and other peripherals. The Amiga also includes a two-button, opto-mechanical mouse plus a keyboard with numeric keypad, cursor controls, and 10 function keys in the base system. A variety of international keyboards are supported. Many other input options are available. Other types of controllers can be attached through the two controller ports on the base unit including joysticks, keypads, trackballs, light pens, and graphics tablets.