That´s an important point: Be very careful when probing at the BGA-balls under the chip! The thin green pcb is flexible and the balls can get loose (and lose contact) very easily.

I´m happy this solution does work, because due to a dead HX MCH is was unable to verify this.

The "semi-fake cache" some vendors sold was not even noticed by most consumers and system integrators so they got away with it.
It´s great that we can fix many such flaws today.

Hello Again!

Admittedly the circuit is by majestyk but I'll do my best to explain.

This Post:
This post is about the circuit we came up with to add the second tag ram chip.

Overview:
Thanks to majestyk for this info.

On boards with 2 active tag chips most of the data lines are wired up in parallel, except that TIO 0 - 7 are wired to the first tag with TIO 8 - 10 wired to the three of the eight I/O lines of the second tag chip. The three extra lines from the HX Northbridge could be wired to a socket on board or the COAST module or even BOTH.
In the example I have and will be using to demonstrate the mod, only TIO 10 is routed as stated already. If you're lucky it could be that the TIO lines are routed to an unpopulated CELP, see the attached image for CELP pin locations but just know that I haven't (and can't currently) test that it works.
In our circuit we'll also be grounding the 5 unused I/O lines of the second tag chip via 1K resistors.

TL;DR:
Take a second TAG chip, wire (nearly) everything in parallel to the first TAG chip, pick any 3 I/O lines and connect them to the top of the Northbridge or 3 pins of the (unpopulated) CELP slot.

As you can guess, the circuit is very simple and looks a little something like this.

PCB:
So taking the circuit that majestyk had described, I made a little circuit board that would fit in the desired location. Designed to be more or less a drop in replacement of the original DIP package, just double check for yourself if there is clearance before ordering anything. The Kicad project files are attached. Please note: I am fairly sure I labelled the CELP ports on the PCB correctly but even if I'm wrong it doesn't matter.

Tag#1 handles the original 8 TIO lines with Tag#2 taking the last 3 TIO lines so if you were to install only one tag ram, use the TAG#1 slot.

A word about SMD parts:
I haven't made a board that would take SMD tag ram chips but I would if there is interest and someone can point to an SMD sram chip that is still available and will work. I know of a few on mouser that I believe will work but I don't know enough about sram to know for sure.

So as you can see the circuit is straightforward and so is the PCB. Installing the board and modding the BIOS, that will be fun!
-Lime

Welcome back!

This Post:
This post will be about physically installing the tag ram PCB. This should be fairly straight forward but also very specific to the example motherboard I have to test with.
I'll be soldering the wires to the chipset not from the CELP slot.

BOM:

• Compatible sram chip x 2
• DIP28 narrow sockets x 2
• 1K through hole resistor x 5
• Some narrow Flexible wire
• 2.54mm pitch pin headers that will go into the socket of the first tag ram. The standard square ones are fine.

PCB:
I didn't take any pictures of the PCB going together. It is very straight forward though.
Solder the pin headers facing down.
Solder the sockets.
Solder the resistors.

After that I would advice installing the module so you can get your runs of wire right.

Northbridge solder mask:
I thought this part would have been more tricky than it turned out to be. It it just required patience and a dull hobby blade. I very slowly scraped away at the solder mask until I got a useable amount of copper exposed.
The traces on the chipset routed to the edge of the PCB but rather than aim for the edge of the PCB I aimed for the little tented via that I assume went down to the solder ball underneath. it gave me more work surface but I did have to be mindful while soldering not to upset the solder ball too much.
Note: while it looks like the traces from the chipset just flow onto the main board, they don't. I did have to double check and those traces go to different balls.

Northbridge wires:
This part is a bit tricky. Just have patience and a steady hand. Solder your wires to the chipset using some flexible wire. I used the very inexpensive wire wrapping wire you can find nearly everywhere at this point.
Dab your flux on and if you get any under the chip be sure to flush it out with plenty of IPA. I had stability issues that could have been from not flushing out the flux well enough in the first place.
After you get the wires on there, support them with something. I temporally used some Kapton tape while testing before going back and securing them better once I confirmed that the mod worked.
In my case I soldered my third wire to the edge of the pull resistor, just solder the wire to the TIO side of the resistor, not the 5 or so volts side. You will likely be able to just follow the trace from the chipset.

All together now:
Now lastly we solder the wires to the PCB we added in. You don't have to solder the wires to the pads how they are labelled, I just added labelling because it makes me feel better.
You'll want to add support to the wires coming from the chipset, so in the event of a snag your mod is less likely to rip a trace from the chipset. Ultimately I coated the wires on the chipset with some curable solder mask and added a small bit of super glue to the wires over the motherboard. I don't like super gluing wires to PCB's like this but I thought it made sense here for the sake of pictures if nothing else.
You could use Kapton forever but I find it tends to come undone, or a drop of epoxy, your choice really.

A word about the sram
I know I used 15ns tag for the second tag ram it was just cheaper to get for the sake of the experiment. It works for the moment anyway.

Physically the mod is now installed. However we're more than likely not done yet.
Since these boards came with no option to install a second tag ram, the option in BIOS to set the L2 cacheable size is hard set to 64MB and you will need to modify your bios to enable 512MB

BIOS Modding
It's worth checking if you actually have to do this, just go to chipset features look for L2 Cache Cacheable Size and see if it lets you select that option for editing. If you can't select the option, or your can't see it, then you'll need to mod your BIOS.
It is a fairly straight forward bios mod as far as bios mods go.

Step by Step:

1. Check out this video by Atheatos, he does a great job covering BIOS modding essentials.
2. Look for L2 Cache Cacheable Size under chipset features.
3. Set the "item-status" of that option to normal. It could be set to DISABLED or SHOW ONLY from factory.
4. Save your modded BIOS as Atheatos shows you.
5. Load up your BIOS on a programmer or if the board lets your flash.
6. Set the option to 512MB.
7. Save and quit.

Testing
To test if the mod has worked, you need to have greater than 64MB system ran installed, set L2 Cache Cacheable Size to 512MB in BIOS and run ctcm7 (attached to this post) from dos.
Look for "Cacheable Area L1" and "Cacheable Area L2", if those numbers are higher than 64MB then the mod has worked.

Personally I has 96MB of ram installed for testing this and you can see in my results screen that the mod worked.

-Lime

I think that's everything.

Hopefully the mod helps you out and that the guide is clear. In the end I wound up soldering the PCB directly to the board due to issues with the original crusty socket and my cheap replacement socket.
The bios doesn't have a handy "SRAM error" beep code it seems, so if you turn on this mod and your machine is extremely unstable or doesn't post at all, look at the socket you installed the PCB into. Give it a good clean and try again.

I'm not sure I'd be of much help with any questions but I'll try.

Thanks for reading and a special thanks again to majestyk for proposing this mod in the first place.

-Lime

Wow! Very impressive.

You did a very good job one this. Very good guide also!

Had a thought for the OCD among us:
For those with bga skills it may be also possible to make a north bridge interposer. Maybe even a purpose shaped pcb that routes to both the socket and the north bridge for a super clean look. Kinda like this:

512MB cacheable area on Intel 430HX mainboard with AMI BIOS?