is it NetBSD problem or hardware impossibility.
i tried it configure same irq on buildin coms and extra 2-port ISA com
adapter. it is detected at boot fine but then it simply doesn't work (no
traffic).

is ISA hardware incapable of interrupt sharing or just NetBSD can't do it?

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Instead of asking why a piece of software is using "1970s technology,"
start asking why software is ignoring 30 years of accumulated wisdom.