> I thought the maximum was 20. After all, there are _some_ ISA cards that
> can address up to 16MB of memory, aren't there? Perhaps you're talking of
> the old 8bit ISA bus?
Let us clear some confusion here: the term ISA is most often used to refer
to the AT expansion bus, 24 address and 16 data lines, using 2 connectors.
This bus is a superset of the original PC/XT bus, which has 20 address and
8 data lines, using a single connector. At some time in the past, MoBo
were differentiated by the number of 16 bit slots (AT) and 8 bit (XT)
> > Microchannel, EISA, and PCI support all 16 I/O address lines.
But often the Super-I/O chip (Serial/parallel...) only decodes 10 or 11
bits, 11 because some additional register for new parallel port modes
"unaliases" one bit to allow additional registers. The subtractive
decoding (last chance decoding or "if nobody volunteers to claim this PCI
cycle, I'll give it a try but I don't promise anything") of the ISA bridge
does the trick.
> Hmm, yeah. At least, Intel did manage to come up with a really nice bus in
> the guise of PCI, and what's more, it can be expanded to 64bit in the
Don't get me started on PCI. It has its own problems and is not "really
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