MCU Host Bus Emulation mode makes the chip emulate a CPU with:
a) a multiplexed 8 bit address and data bus
b) an extended 8 bit address bus
c) CS# , ALE, WR# , RD# and OSC signals
d) 2 I/O lines that can be used as extra I/O or to wait for IRQs. These are defined as I/O0 and I/O1.