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Just got my original IBM compatible back! Need some help.....

@Anonymous - I may have. You are correct that you can put a standard SRAM chip in the TAG Cache spot, but that doesn't necessarily mean that it will work. The TAG chip contains the comparator, as you mentioned which is used to build a table for the location of the Cache RAM. On newer systems, like socket 7 boards, you may be correct that the comparator logic is built into the motherboard, but this socket is specifically labeled TAG, which would indicate to me that I would need to install an actual TAG chip for this system. As such the information that I have found shows the accompanying TAG chip for my ATT7C199P-15 is an 8kx8 28-pin ATT7C174P-15 chip. I know the TH99 indicates a 32kx8 chip, but I have been unsuccessful in locating or identifying a 32kx8 TAG chip from what I have come across. I assume they exist, but I have not had luck in finding them. I may have found another handle on getting a TAG chip though. Wish me luck. I am going to try and call the source shortly.

-Edit- I misread your follow up post. I currently have the TAG socket filled with a standard SRAM chip and I can confirm completely that with a standard SRAM in the TAG socket my L1 cache dropped from 64.4MB/s with no L2 to 31.8MB/s with L2. My system ram dropped from 19.8MB/s to 11.8MB/s Very confusing. So yes, it does work, but performance has suffered. I read the different numbers listed in that ultimate 486 benchmark and I am literally half the speed that the 486DX2-66 scored in the exact same benchmarks. It looks like I really do need a TAG chip for this system.
 
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A built in tag comparator is probably a lot more common in 486 motherboards than you suspect. OPTi 493/495/499/895 (very popular chipsets for 386/486 systems) all have the comparator built in. I have the pdf manuals for all of these, and it is stated very explicitly in the cache subsystem section. I am quite certain it is the same for the SiS chisets as well. I am travelling and I don't have all of my documents on hand, but just about every 386/486 chipset pdf manual I have looked at lists the tag comparator as being integrated. Likewise, none of my 386/486 motherboards require tag chips with comparators. If your system requires a special tag chip because the comparator is not built into the chipset, then I think your system is a bit special. If you could tell us which chipset your PB 420 uses perhaps we could locate the datasheet to find out what kind of tag chip is required.


*edit*
I was trying to find a manual for your motherboard. I was not entirely successful, but I did find a support page: http://www.uktsupport.co.uk/pb/mb/410.htm

What's interesting is that your system supports writeback L1 cache. This means that the chipset on your board must be very modern (by 486 stardards). In this case I highly doubt a special tag piece is required. I suspect that you are correct that a 8kx8 SRAM works with 128kb cache, but only in WRITE THRU mode. In order to get your L2 into WRITE BACK mode a 32kx8 tag would be required. It's possible that your L1 cache is being kicked back into WRITE THRU mode when L2 cache is running in WRITE THRU mode. I have seen 486 motherboards that cannot control L1 and L2 caching schemes independently. This might explain the drop in your L1 cache performance when L2 is installed.

*edit 2*
I found a photo of what I believe is your motherboard. It appears to use something called "ACC Micro" 2268 system logic. Sadly no documentation exists.

https://ancientelectronics.files.wordpress.com/2013/09/pbl1154.jpg
 
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Yeah, I saw that data book earlier. It doesn't really tell us much though.
in the AMI ftp archive there is a chipset manual for the ACC2168. I am not sure what software you need to open the file though.
ftp://ftp.ami.com/archive/Other_Manuals/

*edit* the .z11 extension is a compressed file. I was able to open it with winrar. I used wp51 to open the .man file. Sadly there was nothing of interest in this file either.
 
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There really doesn't seem to be much out there to find. I understand from looking at Stason.org at the various Packard Bell motherboards that ACC Micro was used for a couple different systems, including 386 and 486 machines. Aside from that I really can't find any information. I talked with my friend that does all sorts of Electronics and he doesn't think a comparator logic circuit can be built for the SRAM chip I already have to turn one into a TAG chip. I have confirmed with a gentleman online that his cache speeds with 128K with an 8kx8 TAG chip were higher with a 486SX-25 than my 486DX2-66 with the way mine is set up. Just the cache speeds are faster, every other benchmark shows significant improvements versus the SX-25 So the design must need that TAG chip. I have contacted multiple companies that I have been referred to that indicate they have stock, but I have not received a single return call. Very frustrating. I would buy 50 or 100 if I had to and then get them out into the community so that others don't have to deal with this frustration. Want to pull my hair out over a single simple chip. Something that seems so ubiquitous with machines from that era are so difficult to find now.
 
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