This is another crippled HX mainboard. You can install 512MB RAM but it´s cacheable area will (probably) be restricted to 64MB.
There´s no onboard TAG-RAM, you would have to use a 512K COAST stick with TWO onboard TAG chips.
As far as I remember the 3 additional TAG-IO lines Tio10:8 are routed from the CELP slot to the memory controller.
So I have tried it with my PC330 mainboard that I upgraded with a VRM socket for dual voltage CPUs. The COAST stick is correctly identified as 512K but the latest IBM BIOS does not support the HX extended cacheable area operation.
You can try to use an alternative BIOS, but be aware that onboard S3 graphics will demand further BIOS modding.
The onboard "National" super-IO chip is another obstacle.
I tried to use the usual Flash-Programs (Uniflash) under DOS but they wouldn´t detect the flash chip. There´s probably some more proprietary stuff at work here.
I doubt the original IBM BIOS can be modded easily for extended cacheable area mode, but maybe someone did it or found another way?

majestyk wrote on 2022-12-04, 17:57:

This is another crippled HX mainboard. You can install 512MB RAM but it´s cacheable area will (probably) be restricted to 64MB. […]
Show full quote

This is another crippled HX mainboard. You can install 512MB RAM but it´s cacheable area will (probably) be restricted to 64MB.
There´s no onboard TAG-RAM, you would have to use a 512K COAST stick with TWO onboard TAG chips.
As far as I remember the 3 additional TAG-IO lines Tio10:8 are routed from the CELP slot to the memory controller.
So I have tried it with my PC330 mainboard that I upgraded with a VRM socket for dual voltage CPUs. The COAST stick is correctly identified as 512K but the latest IBM BIOS does not support the HX extended cacheable area operation.
You can try to use an alternative BIOS, but be aware that onboard S3 graphics will demand further BIOS modding.
The onboard "National" super-IO chip is another obstacle.
I tried to use the usual Flash-Programs (Uniflash) under DOS but they wouldn´t detect the flash chip. There´s probably some more proprietary stuff at work here.
I doubt the original IBM BIOS can be modded easily for extended cacheable area mode, but maybe someone did it or found another way?

no please not tell me another computer with only 64MB of ram… your computer is ecactly the same? i not want to use a modified bios anyway but maximize using cache and ram.

It´s not that bad.

I just did some tests and the board will n o t support 512MB as I falsely recalled, but 128MB in total. The maximum is 32MB in every SIMM slot. (I haven´t tried the "SD-RAM" slot.)

Half of these 128MB =64MB can be cached due to BIOS limitations. That doesn´t mean you cannot use the other half, but what that means is if you have populated 128MB and your OS starts using RAM in the non-cached area, your system will be slower than if you only populate 64MB RAM. If the uncached 64MB are fully used, the OS will use the cached area so any RAM-hungry software you are running can benefit from cached RAM.

So the best memory configuration would be 64MB when this is enough for your needs (OS, software).
If 64MB is not sufficient and running software makes the system swap data to the harddrive all the time it would be best to expand RAM to 128MB - or even more, IF pupulating the "SD-RAM" slot (BANK0) with a 5V stick exceeds 128MB.

AlessandroB wrote on 2022-12-05, 07:33:

only 64MB of ram…

and once again: this is more than people had back then. There is NOTHING you would run on this computer comfortably that would use more anyway
May 1997:
128MB = $300
Ram 16MB lowend, 32MB standard, 64MB would be only available on the absolute top end.
CPU garbage bin (whole system $499) 120MHz Cyrix. lowend ~133MHz Pentium/K5. Standard 200MHz Pentium. Top 200MHz PPro.

July 1998:
128MB = $150
Ram 32MB lowend, 64MB standard/top.
CPU lowend ~200-233MHz Pentium, 266MHz Celeron, 233MHz Cyrix, 233-300MHz AMD. Standard 300MHz P2. Top 400MHz P2.

tldr: computers with 2.5-3x faster CPU were sold with 64MB of ram because 128MB was too expensive and nothing used that much ram.
Even year later in July 1999 128MB was the TOP ram option, with standard still being 64MB. In PC Mag Jul 1999 the only computers with 256MB ram are $5K workstations with silly silicon graphics cards or $5K, $8.2K and $9K servers.

yes I know very well that 64MB is a lot, but I would have liked to install a crazy amount of 512 just because it is possible and in fact I had bought a computer with HX chipset precisely because it could handle 512MB by manual. In this way I have 3 computers: all IBM DX4,P60,P200 all with the same amount of 64MB of ram.

Since this computer manages a modest amount of memory, from your tests, does it at least have the speed of the HX resulting in a very fast Pentium200?

Besides the RAM and cache restrictions caused by the proprietary BIOS this is a typical HX LPX-mainboard and absolutely comparable with it´s competitors. Nothing wrong about it.

just for information, what is a "...typical HX LPX-mainboard..."?

LPX boards have some typical features like onboard video (and sound), riser cards etc. Examples are the Eliteqgroup P5HX-LA or Epson TGS-90301.
Performancewise there´s no significant difference between LPX and (Baby-) AT or ATX boards.

Ok, thanks, 64MB installed, now alla 3 PC have the maximum amount of memory. let's go for the other upgrade...

I checked more thoroughly yesterday and found all 3 additional TAG-I/O lines for the extended cache mode are connected to the respective contacts of the COAST-slot. Even the 10K pull down resistor at TIO10 is present.

As far as hardware is concncerned all conditions for extended cacheable area are met - assumed you insert a 512K cache stick that has two TAG-chips.
The limitations are caused by BIOS. Although the latest BIOS is from 1999(!) it is unable to enable extended cache and it´s probably also preventing you from using more than 128MB RAM.
IBM could have done way better than that here...

majestyk wrote on 2022-12-07, 07:56:

I checked more thoroughly yesterday and found all 3 additional TAG-I/O lines for the extended cache mode are connected to the re […]
Show full quote

I checked more thoroughly yesterday and found all 3 additional TAG-I/O lines for the extended cache mode are connected to the respective contacts of the COAST-slot. Even the 10K pull down resistor at TIO10 is present.
ibm_hx2.JPG

As far as hardware is concncerned all conditions for extended cacheable area are met - assumed you insert a 512K cache stick that has two TAG-chips.
The limitations are caused by BIOS. Although the latest BIOS is from 1999(!) it is unable to enable extended cache and it´s probably also preventing you from using more than 128MB RAM.
IBM could have done way better than that here...

but 128MB i chached or not?

No, with this (stock-) BIOS it´s limited to 64MB.