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.)
Yes, especially for VGA Graphics cards, which also have a large address space requirement. As motherboards included more memory, it became necessary to have the BIOS and motherboard leave a hole in the address space for the graphics card. On motherboards I used, there was a VGA hole at megabyte 0xE (I think, it may bave been lower).

I designed some non-graphics interface cards that depended on this VGA-hole. Without it, the entire 16 MByte address space could be consumed by RAM.

In some motherboards, the VGA-hole was optional in the BIOS, perhaps also where the hole was located.

When the bus structure evolved from ISA to VL-bus and then PCI, the addressable range increased, the bus structure became configurable, and the VGA-hole was eliminated.