Uncreative Labs

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

I am wondering if it is possible to not install any memory onto an XT motherboard, and run all memory entirely off of expansion cards. I have seen a few people who claimed to have done this, though I'm not sure if they really knew what they were talking about.

The reason I ask is because I am interested in setting up my XT with Large Page Frame EMS memory. Using LIM 4.0 memory expansion, it is possible to bank switch memory below 640kb. Large Page Frame mode involves disabling some of your conventional memory on the motherboard, and having your EMS board do backfilling. LIM 4.0 boards are able to bank switch the backfilled conventional memory. Normally all but 256kb conventional would be removed. However, in theory LIM 4.0 boards are capable of bank switching the entire 1MB region. As LIM 4.0 was kind of a late development, there wasn't a lot of software written that could take full advantage of it. Though, Windows 3.0 could...but it wasn't very smart about it. Anyway, if such a configuration were possible, it would be interesting to develop some software to take advantage of the full feature set of LIM 4.0.

Joined: 19 Apr 2006 Posts: 261 Location: WisConSin

Or better yet develope an 8088 EMS to Extended memory emulator, such things exist for the 286 and higher, too bad none are for the 8088.

A huge library of software would become possible if the 8088 could address even a little emulated extended memory.

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

Actually, I've been thinking about the EMS to XMS emulator on 8088 for a very long time...which is why I am always asking so many questions about EMS. I didn't want to come right out and say it though, in fear that people might think I'm some kind of moron. Because you have to admit that XMS memory isn't very practical on an XT even if you can make it work.

It will probably be too slow to be usable, and I'm not exactly sure which apps I am missing out on that use extended memory. I am guessing most of the good stuff needs protected mode too. However, I would still like to try it out.

What would be more interesting is to use the EMS memory to emulate unreal mode on an 8088 system. There's no software support for unreal mode, but it would make an interesting platform to write new software for.

Joined: 19 Apr 2006 Posts: 261 Location: WisConSin

Actually it is not that too far fetched, historically there have been discussions and prototypes that were to not only add XMS to an 8088 but use hardware to remove any need for emulation.

The method described was to setup an XT with 512kb of ram, install a custom bios and a special memory card.

Any time the last address bit hits the upper 512kb its masked and an interrupt would be issued, the system would check a secondary memory location (emulation of the 286's extra 4 bits assumably stored in a register) and the cpu would send a standard data value which would be interpretted as the missing address space by the card and the proper memory location would get addressed.

This would cut performance to the upper 512kb and any "extended" memory to about half speed but in reality given the speed of memory at that time it may be more or less than that.

Theoretically the 8088 has a 16 bit register and could address up to 19bit + 16 or 35bit address, more realistically 19bit + 8 or 27bit address using this scheme.

This would only work if a hardware device automatically reinterpretted every incidence of the upper bit.

Software emulation is more complex but not undoable some of the DOS barrons of freedos are considering adding this into their dos versions.
Effectively making EMS emulate XMS would just mean that a lot of page window swapping would occur this wouldn't be any worse that poorly written ems software. And on an XT probaby wouldn't be all that slow (at least compared to the regular speed), also given the fact that most 286 programs that use XMS still are only written in safe 8088 instructions you would probably find a lot of programs would function so long as they didn't ID the CPU.

Cheers
Ryan

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

Hey, that sounds pretty encouraging. Can you point me to some more information on this topic? I'd really like to see what the FreeDOS people are up to.

So I guess the CPUID problem isn't really that serious, as you can probably get around it by writing a TSR, or using a V20 or V30 which is normally seen by software as a 286 anyway.

Joined: 03 Feb 2006 Posts: 488 Location: pennsylvanai

unless you can figure out how to make the 8088 based system look on the expansion card for memory, nothing will ever happen.
everything is constructed so some memory on the mainboard is necessary.
without that it will just sit there dumb.

I don';t know all the machinations of making a 8088 board see more memory, but back in those days there were 2 things people did
1) used some of the unused area between 640 and 1meg for a ramdisk - depending on which video adapter was used.
2) add a board to create a ramdrive and if i recall some LIM memory lotus,intel, microsoft forget if it was expanded or extended memory.
intel said the 8088 would address 1 m and the 80286 16 megs.

various scemes were used to load programs on the ramdisk ( to run faster) and the date on the hard drive or a floppy.
troube is many schemes depended on a batch file and if the system or program crashed ( frequently)
the batch file did not save the data to a permanent place. and it was lost,

when the eqrly pc bios did not support "extensions"
like a xt hard drive controoler or a ega/vga card,,
schemes were developed to boot from a floppy , load a driver and THEN access the hard drive, almost as if you were booting from the hard drive.

aaaagh those were the days... I am glad they are gone.

Joined: 20 Nov 2004 Posts: 589 Location: Shandong, China

Aparently an XMS emulator for EMS DOES exist. It was written by AST, and it's called rex.sys. You can find a copy of it here:

http://www.ari-service.com/support/file/dir.asp?VirtPath=/ftp/epg/

I'm not sure if it will work on non-AST cards. Somebody with an EMS board should try it out and let us know what happens. I am also not sure if the extended memory memory it produces conforms with XMS standards...so it's possible the memory would be useless.

From what I've seen you'll need to load REMM.SYS before loading REX.sys, and it should work on an 8088 system. I have a feeling it's a dead end, but it's still worth examining.

Joined: 19 Apr 2006 Posts: 261 Location: WisConSin

I have refered to that AST Rampage emulator in the past.

In reality it isn't much of an emulator but rather just reinterprets the address space when used on a 286 (or better) processor so that your 286 can use the ems card as extended memory.

I don't think it can operate on an 8088, at least the version I tried would not.

Good Luck