High Quality Content by WIKIPEDIA articles! The Q-bus was one of several bus technologies used with PDP and MicroVAX computer systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts. The Q-bus was a less expensive version of Unibus using multiplexing so that address and data signals shared the same wires. This allowed both a physically smaller and less-expensive implementation of essentially the same functionality. Over time, the physical address range of the Q-bus was expanded from 16 to 18 and then 22 bits. Block transfer modes were also added to the Q-bus. Memory-mapped I/O means that data cycles between any two devices, whether CPU, memory, or I/O devices, used the same protocols. On the Unibus, a range of physical addresses were dedicated for I/O devices. The Q-bus simplified this design by providing a specific signal (originally called BBS7, Bus Bank Select 7 but later generalized to be called BBSIO, Bus Bank Select I/O) that selected the range of addresses used by the I/O devices.

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