The vast majority of 3rd party 16-bit memory expansion boards *do* support mapping memory into the 512-640kB area

Skip94 wrote on 2022-03-31, 17:46:

I can't just add one card and map some of it from 512k-640k and then some of it above that.

As maxtherabbit already mentioned, this feature was quite common on 3rd party memory expansion cards. Usually, it's called "backfill".

Skip94 wrote on 2022-03-31, 17:46:

Also, is the AT (8MHz) actually powerful enough to really make use of anything over 640K? I've got no particular software in mind, it'll probably just play the odd era appropriate game.

You don't need processor power to use memory. The bandwidth of the AT is high enough that software that uses memory above 640k makes sense. But to efficiently use the memory avoce 640K, you should run protected mode software. Borland's C++ IDE "Borland C++ 3.1" is a good example for protected mode software making use of memory above 640k, but I wonder whether in this case, compilation gets slow before the compiler requires more than the conventional RAM.

As you are talking about games, I don't think you will find a lot of DOS games that make good use of extended memory. Games typically either are designed to also work on 3rd party Turbo XTs (8088 based computers with up to 10MHz), which were contemporary with the AT, or they require more graphics performance than an 8MHz AT can offer. A fast graphics card (like the now overpriced ET4000 series) can mitigate issues with graphics performance, though, as the original IBM "PS/2 display adapter" (the ISA board that had the gate array chip called "video graphics array" installed on it) and early 8-bit clones are quite poor on memory thoughput compared to modern cards, especially in 256 color modes.

A very good way to use "second class memory" like extended memory on an AT, or EMS on an XT is a disk cache - at least if you use disks from that era. Every cache beats a typical 30MB RLL hard drive by at least an order of magnitude in performance. On the other hand, when you use a Compact Flash adapter (i.e. install a "SSD"), the performance effect of the cache might be much less notable, if at all.

I'm using an AST Rampage on mine, 16 bit ISA takes SIMMs. works great and fills in the memory hole up to 640k. I've also used an Intel Aboveboard that worked as well.

The ISA RAM expansion cards that take SIMM memory were never that easy to find. Most people who owned early 286s just had the onboard memory (usually 640kb or 1024kb), and if they were professionals may have had a 2MB expansion card that used DIP RAM. By the time the 12MHz 286 rolled around the SIMM/SIPP slots were typically integrated onto the motherboard. There was actually a time when it was probably easier to find those SIMM expansion cards new in box, rather than from a junk bin or a complete system.
It would be kind of nice if somebody did a homebrew SIMM card, because they just weren't produced in large quantities and were never that easy to find.

Thanks all, some thoughts for consideration there.
I'll admit my wording in the initial post was a little poor, I really meant am I actually likely to run software on an 8MHz 286 that will benefit from lots of memory. I think the answer is probably not. I'd really like to upgrade it to 640K, but above that, I've got my IBM 5162 with 8+ megs of RAM and also a 16Mhz 286 clone with 8 Mb, 4 in DIP on the motherboard and 4Mb in SIPPs.
Cheers
Andrew

Hi there!

If you ask me, I would add some UMB card and an EMS card.
Like those cheap 8-Bit cards from Lo-tech or replicas from other manufacturers.

Provided that you're interested in late 80s/early 90s software or professional software.

Some utilities in MS-DOS 6.x can make use of EMS, whereas DOS itself can store parts of it in the UMA (640-1MB) and HMA (64KB minus 16Bytes above 1MB).

Of course, 8-Bit access slows things down.
The others surely will tell you about it no doubt.
They aren't wrong, but also not exactly right, either.

Thing is, back in the day, many 8-Bit ROM chips on older expansions cards
were not interleaved and connected via 8-Bit, anyway.
So it's no ideal by any means, but not murks either. It simply happened.

So if your HDD controller or ethernet card was calling a ROM routine, it ended up being 8-Bit at worst.
That's why "shadow memory" became a thing. It copied slow ROM code into fast RAM.

Same goes for older graphics cards or EMS boards sometimes.
Some AST (?) board models had a 16-Bit connector, but the transfers were 8-Bit really.
Heck, some ancient ISA VGA cards did their whole i/o via 8-Bit and people didn't notice back then. 😉

That's why I think to myself: Little keyboard/mouse drivers don't need much space or bandwidth, either.
But they make room for more demanding programs located in the conventional memory.

A single piece of, say, 1 to 64KB contiguous memory (as UMB) is just a single segment of memory to an old 8086, even.
The transfer should be fast enough to not hog the 8-Bit bus too long.
But I'm speaking under correction, of course.
I do merely have one 4,77MHz machine and my experience with 8-Bit PCs is still lacking.

My point is just: Slow memory is better than no memory, at all.
Plus, an 8MHz 80286 isn't exactly fast, anyway.
So I think it would be okay if it ends up being on a XT or Turbo XT level.

However, just don't spend too much money on 8-Bit expansion cards.
If you can get them at a reasonable price, they might be worth trying out.

That being said, if you're running MS/PC-DOS 3.30 then that's not needed perhaps.
Likewise, if your games/programs fit into 640KB, then everything is okay the way it is.

It's just if you're going to play MOD files, watch retro pictures etc. that memory will be a limiting factor quickly.

Good luck! 😀

Edit: Typos fixed.

IBM made a 16-bit upgrade card for the AT to bump you up to 640k. Here's a photo. You can search for the FRU:
http://www.minuszerodegrees.net/5170/cards/51 … rds.htm#128_meo
Boca Research made a special card for the AT for the sole purpose of upgrading from 512k to 640k. I believe it was called the "Tophat".