Similarly, the "bus" of a micro-computer started out as a pretty simple thing: parallel, 8 bits of data, 16 bits of address, with memory-mapped or I/O-mapped fi

Similarly, the "bus" of a micro-computer started out as a pretty simple thing: parallel, 8 bits of data, 16 bits of address, with memory-mapped or I/O-mapped fixed-function peripherals managed by the (single) CPU. Memory and I/O devices on the same bus. But the modern PCIe "bus" is really more of a packet-switched network. Memory is now separate (although, see CXL). Devices on that network can talk directly to each other, or to the CPU. In some ways, this is closer to a mainframe architecture.