Input output port

Input/output scheme

Input output port space

The 8080 supported up to 256 input/output (I/O) ports, accessed via dedicated I/O instructions—taking port addresses as operands. This I/O mapping scheme was regarded as an advantage as it freed up the processor's limited address space. Many CPU architectures instead use a common address space without the need for dedicated I/O instructions, although a drawback in such designs may be that special hardware must be used to insert wait states as peripherals are often slower than memory. However, in some simple 8080 computers, I/O was indeed addressed as if they were memory cells, "memory mapped", leaving the I/O commands unused. I/O addressing could also sometimes employ the fact that the processor would output the same 8-bit port address at to both the lower and the higher address byte (i.e. IN 05h would put the address 0505h on the 16-bit address bus). Similar I/O-port schemes was used in the backward compatible Zilog Z80 and Intel 8085 as well as in the closely related x86 families of microprocessors.