The Osborne 1 has bank switched memory. Unusually for a system based on the Z80, all I/O is memory mapped, and the Z80 I/O instructions are only used to select memory banks. Bank 1 is "normal" mode, where user programs ran; this includes a 4 kB area at the top of the address space which is video memory. Bank 2 is called "shadow". The first 4 kB of this address space is the ROM, and 4 kB is reserved for the on-board I/O ports: The disk controller, the keyboard, the parallel port PIA, the serial port ACIA, and a second PIA chip used for the video system. All memory above the first 16 KB is the same memory as Bank 1. This is the mode of the system on power up, because this is where the boot ROM was mapped. Bank 3 has only 4 kb by 1 bit of memory, used solely to hold the "dim" attribute of the video system.