ISA Memory Expansion

[southbird]

I'm just looking for clarification. Can an ISA memory board actually provide (slow) extended memory, or is it relegated only to the realm of EMS? I'm assuming the latter since I would imagine the addressing of the ISA bus sits in that first 1MB only. I know I once had a microchannel PS/2 which had a board that DID provide extended memory, but obviously that's apples to oranges here...

 

[krebizfan]

If you are looking at 16-bit ISA cards, it depends on the card. Some only supported EMS, some only supported extended memory and still others supported both EMS and extended memory and the memory could even be split between the two types. I vaguely recall that some cards supported a memory system that was different from both EMS and extended memory; just something to watch out for if buying very early expansion cards.

Check the documentation for the specifics. Some of the jumpered extended memory cards could not correctly handle a system with more than a megabyte of RAM on the motherboard making them useless with the late model 286s or if another memory expansion card was installed. Most of the SIMM based cards were software adjustable and handled larger memory totals easily plus were much simpler to adjust for different program configurations.

 

[southbird]

Do you have examples of any of these?

 

[Raven]

I'd wager that there's no reason an ISA card would be hardware-restricted to EMS, and it's just a matter of a driver. If you really want to, you could probably write a driver for a particular board that would provide XMS instead, or a hybrid of XMS/EMS akin to what Jemm does.

Edit: *angry* I have a 486/Now! I'm not using but unfortunately the pins on the 386 socket all got crushed (and snap off if you try to unbend them) by a cabinet. It probably would work just fine still, but it's ruined now...

Edit 2: Oh and by the way, the 486/Now! isn't socketed, it's an SX33 with the pins poking through the PCB and soldered on the other side.

 

[Chuck(G)]

There are pure extended memory boards that behave as additional memory (up to 16MB). After all, you've got all of the address (24 on ISA) and data (16 on ISA). (I'm not counting 8-bit ISA bus as that pretty much limits you to 1MB because there are only 20 address lines).

An Expanded memory board (EMS) has a bit of extra hardware. It contains circuitry to map the additional memory to one or more "windows" located in the lower 1MB of memory space. Many EMS boards can also be jumpered to serve as extended memory also.

Given that the ISA bus is pretty much a 286 animal (24 addressing lines), that's the story for the 80286. However, the picture is clouded when one gets to the 386 and above. Because of the extra CPU operating modes, (in particular, V86), it's a simple matter to make extended memory behave as expanded (or a combination thereof).

Of course, any ISA memory board is going to be limited to ISA bus speeds, which isn't that great if you're talking about a faster CPU.

 

[krebizfan]

Do you have examples of any of these?

Rather than relying on my memory of 20+ year old hardware, I will link you to a couple of Infoworld pieces on them. The card I used was IIRC AST Advantage Plus, very similar to the AST Rampage 286 Plus in the linked review which had SIMMs, software selectable EMS/XMS layout. I think the early AT expansion cards from IBM and Micron were extended memory only. I can't find an example of a 16-bit EMS only card or one of the strange cards that had an unusual memory standard.

The first is from 1986 and lists lots of memory cards, some that only work on an AT but also do not support EMS. What exactly they do with the memory is unclear.
http://books.google.com/books?id=XC8EAAAAMBAJ&pg=PA25#v=onepage&q&f=false

The second is from 1988 and covers the more modern cards and the whole EMS 3.2/EEMS/EMS 4.0 differences. You probably want one of these cards or a newer version of them.
http://books.google.com/books?id=CjoEAAAAMBAJ&pg=PT69#v=onepage&q&f=false

Next is a 1987 review of an Micron extended memory card which tried to implement EMS in software with poor results.
http://books.google.com/books?id=zzwEAAAAMBAJ&pg=PA67#v=onepage&q&f=false

Finally, a note about a fast AT clone that is problematic with memory cards
http://books.google.com/books?id=BDwEAAAAMBAJ&pg=PA60

There were other reviews in Infoworld. I linked to the ones I could quickly find.

 

[southbird]

Edit: *angry* I have a 486/Now! I'm not using but unfortunately the pins on the 386 socket all got crushed (and snap off if you try to unbend them) by a cabinet. It probably would work just fine still, but it's ruined now...

Sorry to hear that.

Edit 2: Oh and by the way, the 486/Now! isn't socketed, it's an SX33 with the pins poking through the PCB and soldered on the other side.

Well I'm confused, because yes, I just saw a picture online which confirms what you said. But I KNOW I had something that looked exactly like one and I was able to push out the chip it came with (I got it used at a thrift store a very long time ago) and insert a 486DX/2-66 and it worked. There must be varieties of the upgrade part.

In an article I found through Google, someone notes: "Fellow PS/2'er Karsten Harder was kind enough to send me a 486/33PD3-P70LP [Model for 486/Now! as detailed just above in same linked article] fitted with an AMD 486DX-40 CPU from the factory ... I see no reason why clock doubled or tripled CPU's could not be used with this upgrade ... I did test the upgrade with an Intel 486DX4ODPR-100; it came up fine, and benchmarks reported a CPU speed of 120 MHz. Be careful when removing the CPU from the circuit board; the Kingston board is quite thin, and is very susceptible to damage."

So at least what I'm looking for DOES exist, but now I know to be careful to watch out for the soldered-in type. Although I imagine it wouldn't be too bad to modify (just tedious...)

 

[southbird]

Anyway, just saw a "BOCARAM AT PLUS" on eBay pre-populated with 8MB and made an offer 33% under what the guy wanted, and he took it. So soon there will be another adventure... anyone know if it needs driver software or something? I'll start hunting around...