Very interesting. I would consider doing this to my K6XV3+/66, but I suspect that board has severe aging issues, so at this point I don't feel like investing into a board that may die any day...

Limiting factor are the crappy green electrolytics in the VRM circuitry around the CPU socket. So I would recommend a recap before going any further.

I forgot to mention the cacheable areas:

"Write Through"
512K -> 128MB RAM
1024K -> 256MB RAM
2048K -> 512MB RAM

"Write Back"
512K -> 64MB RAM
1024K -> 128MB RAM
2048K -> 256MB RAM

majestyk wrote on 2022-11-16, 08:47:

Limiting factor are the crappy green electrolytics in the VRM circuitry around the CPU socket. So I would recommend a recap before going any further.

This is most likely what I will do when it starts crashing too frequently.

IIRC, if your installed RAM is larger than the cacheable areas, and you don't have a 2+/3+ CPU (with internal cache), then you will experience performance drops, correct? Can stability suffer as well?

Depending on the OS and it´s memory usage there will be performance drops. Stability will not be affected.

I should also add that if you are using a "K6 plus" CPU onboard (then) L3 cache becomes an issue of lower importance that doesn´t play a major role in performance.

Don´t wait too long with a recap. Fading electrolytics cause higher ripple on the supply voltages and can stress the chips.

I found one of my old K6BV3+ /66 the other day that was equipped with just one 1MB cache chip.
I always wanted to upgrade to 2MB but some component was always missing.

The first step is to solder a second 64x128K chip in it´s place.

The second step is to adjust the jumpering. R145 has to be moved to position R143 and R 137 (or R138) to position R203.

The third step is replacing the TAG RAM:
On 512K or 1MB versions there is a 256K chip (32Kx8, 28 pin SOJ 300 MIL) while 2MB boards have a 512K chip (64Kx8, 32 pin SOJ 300MIL). All chips are 8nS.
These 512K chips were made bei Utron and one more manufacturer and are virtually non-existent today. You will find datasheets but no seller offering them.
That´s why I chose a 1MB chip (128Kx8) 10nS hoping a good sample of a 10nS type today might be as good/fast as a bad sample of a 8nS type back in 1999.
These chips are still available, but you have to make sure they´re 300MIL SOJ and that they have the old pinout with Vss and Vcc at pins 32/16. Modern chips have these pins in the middle of the package due to advantages at high frequencies.
Of course there´s one unused address line in the 1MB chip (A16). This line needs to be grounded, the system won´t work with this pin open. There´s a ground pad nearby so this is no effort.

I still have to test this setup to make sure there are no instabilities at 100MHz FSB.

The FIC VA-503+ has 1MB L2 cache and two 512K chips. The TAG RAM is 256K and the layout only provides the 28 landings for this chip.
I have repeatedly read in online boards that there is a "rare 2MB version" of this model. I doubt this ever exixted due to the TAG limitation.

I tested this setup with both 256MB and 512MB RAM:
In the 256MB case all 256MB are being cached in "Write Back" mode, "Write Allocation" gets enabled by BIOS.

If there´s 512MB RAM present, 512Mb are being cached, but BIOS switches to "Write Through" mode to avoid half of the RAM being uncached. Write Allocation is disabled in this case.
Th data from the table of cacheable areas (see above) is correct as we see.

And here´s a picture of the finished product: