Abstract: A point to point connection architecture for a computer I/O subsystem, resulting in a scalable tree structure. A Master I/O Concentrator (MIOC) is connected to the host bus and handles conversion between a bus oriented structure and the tree structure of the I/O subsystem. Ports away from the host bus are downstream ports and conform to a simple byte wide message protocol. Various IOCs and devices can be attached to one of the downstream ports on the MIOC. The MIOC directs transmissions to the appropriate channel based on a geographical addressing scheme. The IOC connections act as further points of branching. Ultimately IOD or I/O devices are reached, having an upstream port for connection to the IOC and a downstream port and internal logic appropriate for the particular peripheral device. Various registers are present in the IOCs and the IODs to allow determination of the topology and particular devices present. Messages and commands are transferred in the I/O subsystem in defined packets.