Question: The system bus of the IBM PC had 8 data lines and 20 address lines, in a logical correspondence to the 8088 CPU.

The AT added a second inline edge connector to expand this to 16 data lines and 24 address lines, again in a logical correspondence to the new 286 CPU. And certainly the wider data bus made the system run faster.

But I'm wondering were the extra address lines really used? Of course there was the infamous problem with the 1-megabyte address space limit being baked into the API by the time the AT came on the scene, but it seems to me there is another issue.

Most of the address space in an 8088 machine would naturally enough tend to be used for RAM. An IBM PC could take up to 64K on the motherboard, but you could stuff it with memory cards to bring it up to 256K, run Lotus 1-2-3 and turn your spreadsheets into graphs that made you look good in presentations, and that was the combination that made the IBM PC decisively superior to the Apple II in terms of what you could do with it, never mind the IBM nameplate.

But the SIMM was invented in 1982, two years before the AT came out. So in the 286 era, you didn't need to put memory cards on the system bus anymore. It was still used for lots of other things, of course. But none of them consumed address space wholesale the way RAM did.

So: did anything ever actually use the extra address space of the 16-bit ISA bus?

Answer: 
Yes, there were 16-bit ISA cards which used the extra address lines; for example IBM’s various [memory expansion options](http://www.minuszerodegrees.net/5170/cards/5170_cards.htm), most of which could be configured to provide extended memory beyond 1MiB. You can see the address lines referenced in the corresponding schematics, *e.g.* for [the 512KiB/2MiB expansion board](http://www.minuszerodegrees.net/oa/OA%20-%20IBM%20PC%20AT%20512KB_2MB%20Memory%20Expansion%20Option.pdf).

(The PC AT didn’t have SIMM sockets on the motherboard; only the larger memory expansion options used SIMMs.)
Back with the 8088 I used a MDA which mapped into 4KB of video memory 0B0000h. The address decode did not even resolve it properly so it was copied over 32KB of address space.

I used one of them with my old IBM PC.

The PC AT with the 80286 had new slots. In DOS however the same world as before.

You needed a different operating system to use protected mode to recognize 16MB of address space and use the 16-bit slots more effectively.

The 80386 was more DOS address space. Again a different operating system was needed to use the features.

I had BSD on a 386 and that actually could use all the memory natively. Had to use different commands but it worked OK. DOS was still limited.