ibus is a virtual device corresponding to the pmax internal I/O space found on DEC 2100, 3100, 5100 and 5000/200 systems.
Internal I/O space spans the pmax physical address space, and is mapped permanently in the kernel virtual space at the very early time of the kernel startup procedure.
ibus driver manages the internal I/O space of pmax.
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Address range management to avoid confliction of address space of which devices probe by touching hardware port is difficult.
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Interrupt vector management.
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Other utility functions.
ibus is always required to run the
NetBSD kernel.