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AST Bravo LC2 4/25s cache chips

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First post, by raaboof

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Hi Vogons,

I have an AST Bravo LC2 4/25s, with a 80486DX33 CPU upgrade. The system came to be without any cache installed, so one "project" related to the system has been to see if I can enable the cache. The motherboard is this, with one subtle difference I'll come to later: https://stason.org/TULARC/pc/motherboards/A/A … 202560-XXX.html

From stason:

The attachment stason.png is no longer available

So, I went ahead an ordered a bunch of 32K x 8 DIP-28 chips. When I was about to install those, I realized the TAG bank is, in fact, DIP-32:

The attachment IMG_1840.jpg is no longer available

This is the only part where the above stason spec seems to differ from my motherboard. Not sure if that's an error in stason spec, or if I have a slighly different revision of the mobo or something. Anyway, my instinct was then to acquire a DIP-32 version of the 32K x 8 SRAM chip. But this proved to be difficult -- even by googling it, it looks like either there is no such thing as DIP-32 32K x 8, or it's rare.

What I have been able to find is 512K DIP-32 SRAMs, which I guess translates to 64K x 8. This would be double the capacity indicated by stason mobo spec. I ordered one of these, just to give it a try, but did not receive it yet, so let's see...

I don't know if I even have an exact question about any of this, but just wanted to leave this here -- at least I can then report the outcome once I get the chip intended to be used for the TAG slot. Meanwhile, it anyone more knowledgeable about this stuff wants to share some wisdom, this is what I have been pondering about:

- TAG memory is apparently some sort of "index" or "mapping table" about which parts of the main memory have been cached, and pointers/mappings between cache entries and main memory locations. Is this roughly correct assumption?
- How likely it is that the system will work if I have 32K x 8 cache chips (as in the mobo spec) but then 64K x 8 TAG chip (as opposed to the mobo spec)? Does it harm if TAG capacity is bigger than specified?
- Is there such a thing as DIP-32 32K x 8 SRAM? Has there ever been?
- And, of course, if anyone has references of a working cache chip setup on that mobo, that would be highly appreciated -- it would then make it real easy for me to simply replicate the same setup (provided the DIP chips are easily available).

Reply 1 of 1, by raaboof

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Some updates, mostly to just record my findings in case someone else runs into similar situation:

- TAG memory is indeed a "mapping table" or "index" of which parts of main memory have been cached and what not.
- I tried with a 32-pin DIP SRAM ( 64K x 8 ), but the system did not boot at all. Looks likely that the size of the TAG memory must match with the spec. I also found a discussion elsewhere here in the forums where it was mentioned it should usually be ok to plug in a 28-pin DIP memory module into a 32-pin socket. I tried that, too, by leaving the socket pins 1,2,31, and 32 empty. However, I'm not sure how that could work even in theory, because pin 32 is the Vcc input pin. Having a 28-pin memory module plugged in that way leaves it without power in my understanding. So, it is still unclear to me what kind of TAG module I should use, because it would need to be 32K x 8, but there is no 32-pin version of that.
- One thing I may try at some point is to see if anything changes if I use 10ns or 12ns memory instead of my current 20ns modules. The AST BIOS I have does not support changing memory timings in any way, and maybe it's simply that I need to get faster 10ns memory to make it work. Some Vogons discussion sort of have led me think that the access timing may be a critical factor.

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