From Academic Kids
8-bit CPUs normally use an 8-bit data bus and a 16-bit address bus which means that their address space is limited to 64 kilobytes; this is not a "natural law", however, and thus there are exceptions.
The first widely adopted 8-bit microprocessor was the Intel 8080, being used in many hobbyist computers of the late 1970s and early 1980s, often running the CP/M operating system. The Zilog Z80 (compatible with the 8080) and the Motorola 6800 were also used in similar computers. The Z80 and the MOS Technology 6502 8-bit CPUs were widely used in home computers and game consoles of the 70s and 80s. Many 8-bit CPUs or microcontrollers are the basis of today's ubiquitous embedded systems.
There are 28 (256) possible permutations for 8 bits.