Fig. 18.7.1 shows the memory address space of a fictitious computer and where I/O peripherals have been assigned. There are 8 address bits, so there are only 256 addresses, from 00000000 up to 11111111. 8 of these addresses are allocated to device A while 8 others are allocated to device B. Memory gets all the rest. All the allocated words for a single device are contiguous, although main memory's addresses are not necessarily contiguous.