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Someone pls explain 486 L2 external cache architecture

T

technoid

Member
Joined
Dec 8, 2011
Messages
36
Location
Cybernet
Hi, new here.

I have a 80486sx-25 system, with Evergreen AMD 5x86 (100-133MHz) Overdrive and 32MB system memory (48MB max onboard). This was bought almost 20 years ago. I put the Overdrive CPU a few years later. It came without external motherboard cache (IC slots empty). And since then I've been trying to get it to work.

Per the technical reference booklet, it requires the following:
- one non-parity instruction chip 8kx8 at 15ns, mandatory
- one parity instruction chip 8kx9 at 15ns, mandatory
- four non-parity 32kx8 or parity 32kx9 data chips, at 25ns

The problem is that once I've put the cache chips in, it freezes right after it senses them. Per the booklet this is supposed to make a cache of 128k, but onscreen it shows as 256k cache, although, as said it freezes right after that.

The chips that I have (28-pin plastic skinny DIPs):
- four 32kx8 25ns
- four 32kx8 20ns
- two 8kx8 15ns
- one 8kx9 35ns

I tried either the 20ns or 25ns data chips. Notice the 8kx9 instruction chip is 35ns, not 15ns, as instructed by the booklet. A 15ns 8kx9 is a bitch to find on the internet. Only those wholesale houses/dealers have them to be bought by auction and minimum costs/quantities. Could the 35ns be the freezing problem (timing issue?), or could there be a motherboard problem?

I want to know why the screen is showing 256kb cache when the booklet says this should be 128kb, unless the booklet is wrong or something. Currently running Windows 98SE and I think it would be a little nice if it had some sort of L2 cache to increase performance a few percent. Anyway, you may request further details. I can also furnish photos or scans, etc. Thanks in advance.

~tech
 
first thing to check is the jumper settings for the cache. If you have the manual, and it would appear that you do, make sure the jumpers are set to 128kb.

If that doesn't work, remove the cache and set it back to where you started. Look at the post screen. if it says anything about cache post it here. If it still says 256kb of cache or "shadow ram" then you have a fake cache board, and the likelihood of making it work with cache is next to nil.
 
luckybob said:
first thing to check is the jumper settings for the cache. If you have the manual, and it would appear that you do, make sure the jumpers are set to 128kb.

If that doesn't work, remove the cache and set it back to where you started. Look at the post screen. if it says anything about cache post it here. If it still says 256kb of cache or "shadow ram" then you have a fake cache board, and the likelihood of making it work with cache is next to nil.
Click to expand...
Yeah I've studied all the jumper settings. There are no jumpers for changing cache quantity, nor does it state that in the tech ref booklet. The only place that states that it is 128kb is that booklet. There are jumper settings to set parity or non-parity cache. In the BIOS, you can set external cache on or off, what type of cache (writeback or writethru) and parity check (I think that last one is only for main memory). Other than that I have no idea how it works. The motherboard silkscreen shows the area of empty chip sockets as "Cache memory" or something (not in front of me right now). Again, there is one socket for 8kx8, one for 8kx9 and 4 sockets for 32kx8. I don't know the architecture, but I assume adding 32k by 4 sockets equals 128k. You must have both 8kx8 and 8kx9 locations populated or the cache won't get recognized. I know that somehow it is working because if I try take one of the data chips off, the system will tell me "bad cache" after boot.

Unfortunately as aforementioned, once it recognizes the cache (at boot), it then freezes after that. It does not make sense that this would be fake cache after all that is written in the tech ref on how to install cache chips. And I've read here and on some other sites/forums that fake cache is usually directly soldered on the board. This PC is from around 1992-93. I am thinking I might try uninstalling all the addon boards (all ISA slots) and see if that causes the issue, although I think that shouldn't make a difference. I also tried this with and without the overdrive cpu. Without the overdrive is the 486sx-25, directly soldered on board. No difference there either, it still hangs.

I wish I could also find 32kx9 cache chips. Those are even rarer. A few other cache configurations I'd like to try are the following, but can't because I don't have these chips:

1. Find 8kx9 15ns chip, tough to hunt down.
2. Find another 8kx8 15ns chip, maybe mine is damaged?
3. Find four parity 32kx9 25ns chip, even tougher to obtain.

Again I don't know how these old cache configurations work. Someone might like to explain to me how one 8kx8 instruction chip runs the four 32kx8 data chips. If the booklet is wrong and these should not be four 32k data cache chips, then what should they be then? The only other reasons I can think of that this is not working is that the external cache system is damaged or I'm using the wrong flavor of chips or there is a timing issue.
 
Based on freezing I would guess either there was a modification to the board that was never updated in the users manual (happens sometimes, especially with different revisions), or a defective cache chip.

The 1st thing I would do is check for a defective chip. It would be far easier to do if you had a 2nd board that used the same chips.
 
Compgeke said:
Based on freezing I would guess either there was a modification to the board that was never updated in the users manual (happens sometimes, especially with different revisions), or a defective cache chip.

The 1st thing I would do is check for a defective chip. It would be far easier to do if you had a 2nd board that used the same chips.
Click to expand...
Yeah, a documentation revision was also in the back of my mind. For example, one thing I discovered recently is that there was a jumper setting missing in the manual. I installed the overdrive cpu in the late 1990's so that I, of course, recently forgot how to disable it and use the onboard cpu. The manual stated that a jumper can be left open to reuse the stock cpu. However, I tried that and it didn't work (hangs on boot with black screen). There wasn't much I could do except to try an undocumented jumper shunt and that worked.

In response to defective chips, the last set of four 32kx8 data chips (the 20ns ones I mentioned) were tested by the seller. I made sure of this and he tested them on his prom burner (he's an electrical engineer). He sold them to me for a dollar each (though the shipping was $10). Unfortunately I don't have another 486 board. If I had to plunk more cash into this mess, then my next try would be to get another 8kx8 instruction chip, if I think my current one is damaged. And I have no way of testing any of these, I don't have any sort of chip/ram tester. The PC works fine on Windows 98SE, just understandably a bit laggy. External cache would help it chug on a little better, heh. Having only two 428mb harddrives doesn't help either, due to the 528mb bios limitation (I have an addon isa drive controller that overcomes this, but I don't want to use it on this pc).
 
if you have 2 sets of cache chips, I find it unlikely that there will be a bad chip in each set. Is there a way we can get a good large picture of the board? Or a model number so we can look at the settings on a site like stason.org (there are others)

http://www.ebay.com/itm/180729687562
I have one of those, they are worth their weight in gold. You may find it could be a different problem than bad cache. I have heard stories where upgrade chips and/or motherboards do NOT like write-back cache.

On another note, if you have 4 cache sockets, are they 32 pin or 28? Also, are you inserting everything in correctly? I know it may seem condescending but its always worth asking. I've fixed probably 3 peoples computers (over the phone) by asking if they were plugged in and they WEREN'T.

Finally, if you want to know more about how cache works, I would like to direct you here: http://www.pcguide.com/ref/mbsys/cache/charCacheability-c.html
 
i would say wrong chip makes system freeze. chip you have most likely for 256kb { it reports to bios} as it is instruction chip. like luckybob says ,more info on system helps.
 
kbit said:
i would say wrong chip makes system freeze. chip you have most likely for 256kb { it reports to bios} as it is instruction chip. like luckybob says ,more info on system helps.
Click to expand...

in a set of 8 he would have 256kb of cache yes, but in a set of 4 its 128kb. *usually* on the motherboards i've seen, when you only have 4 cache slots, (not including tag ram) they are 32 pin sockets that will take 4x 512b chips to make 256kb of cache. These chips are 32 pins but you can put the 28's in them if you had less cache.
 
luckybob said:
in a set of 8 he would have 256kb of cache yes, but in a set of 4 its 128kb. *usually* on the motherboards i've seen, when you only have 4 cache slots, (not including tag ram) they are 32 pin sockets that will take 4x 512b chips to make 256kb of cache. These chips are 32 pins but you can put the 28's in them if you had less cache.
Click to expand...
How odd, I posted earlier this morning (after last midnite) and it seems my posts have to go through a moderator. It hasn't posted yet? :( I listed my specs on that post, including chip pinouts and stuff. My digital mathematics is a little rusty right now. I'm still not following how the non-parity 8kx8 tag/instruction chip adds up the four 32kx8 data chips, i.e. how does 64kb tag run 128kb? I read in one forum that the guy replaced the 8kx8 tag with a 32kx8 and it increased his cache quantity or something.

I have a feeling this post will have to go through moderator too, heh.
 
Sorry, been away here for almost 2 weeks. Below is the post I wrote that didn't get submitted immediately (which I said above). I haven't had a chance to take pics yet. And yeah, I'm still working on this, unfortunately:

----------

luckybob said:
if you have 2 sets of cache chips, I find it unlikely that there will be a bad chip in each set. Is there a way we can get a good large picture of the board? Or a model number so we can look at the settings on a site like stason.org (there are others)

http://www.ebay.com/itm/180729687562
I have one of those, they are worth their weight in gold. You may find it could be a different problem than bad cache. I have heard stories where upgrade chips and/or motherboards do NOT like write-back cache.

On another note, if you have 4 cache sockets, are they 32 pin or 28? Also, are you inserting everything in correctly? I know it may seem condescending but its always worth asking. I've fixed probably 3 peoples computers (over the phone) by asking if they were plugged in and they WEREN'T.

Finally, if you want to know more about how cache works, I would like to direct you here: http://www.pcguide.com/ref/mbsys/cache/charCacheability-c.html
Click to expand...
Ah yeah, post cards, I haven't used those in a long time (I used to work on a line in a motherboard factory doing tests and such, heh). I didn't realize they've become so cheap, I might get one.

Yeah, I've made sure everything is plugged in correctly. However there is a chance I may have initially put the cache chips in wrong those many years ago, but my own memory is hazy... here's the reason why: The cache data sockets are 32 pin, to make room for 32kx9 parity DIPs. To use 32kx8 nonparity dips (28 pin), which again is all I have, you put those chips in aligned to the opposite side of the notch, so there's 4 pins unused on the notch side of socket. This is per the manual as well. I have not fully looked at the chip datasheets to confirm this. The cache instruction chip sockets for 8kx8/8kx9 are 28 pin. I am wondering if I could put a 32kx8 chip in there (as I've read in other places), but since I don't have any 32kx9 for the other instruction socket, it probably won't work anyway.

Hmm yeah there is bios setting as I said above where I can choose between write back or thru. I already tried that, but I think I try again.

This PC is a hard one to find specs/info on the internet. If you Google, you may find half of that info came from me (technoid) on various other forums, lol. It is a VTech Laser 486SX/3 (25MHz). There was something Laser on Stason, can't recall, but not the exact model. I will try to get photos and maybe also a manual scan soon, maybe this weekend.

And no problem asking me "condescending" questions, I need all the info I can get on this thing. :)
 
Update for the curious...

SOLVED: For the first time ever in 20 years, this 486 PC now has a working external cache.

The main suspect was the aforementioned 8k x 9 parity tag/instruction chip. It was required to have the access time of 15ns-20ns. That's all there was to it. I made an earlier error of first buying a slower-timed chip of 35ns, because it was much much cheaper. So after a month or so after that I reluctantly bought a 20ns tag chip that was 6 times more expensive, and that made all the difference. The PC is now happily running a bit faster than it was when bought new. Well, it's still just a wee bit slower than a Sandy Bridge, but I'm not complaining all that much, heh.

Here are the before and after PC specs:

- Before:

Bought new in ~1992,
Non-parity 4MB system memory,
No external cache,
Windows 3.1,
80486SX 25MHz,
No sound card.

- After:

Final upgrades February 2011,
Non-parity 48MB system memory (max onboard),
128k 25ns external cache (in non-parity mode),
Windows 98SE,
AMD 5x86 DX4 100MHz (in Overdrive slot),
Sound card (finally).

Thanks for playing!
 
Sorry to bump this old thread, but i have possible the same pc. I've taken the time to make a few photos:

Laser 486SX pizzabox style
b8c798ff_l.jpeg


486SX and upgrade socket (LIF)

12e05417_l.jpeg


Cache sockets

20c8cf7e_l.jpeg


Tag ram? sockets

0220e57a_l.jpeg


...and a surfeit of jumpers
2e54d524_l.jpeg

70bf6cb5_l.jpeg


On the pcb there are no brands or model numbers silkscreened. With the amount of jumpers on it i'm a bit lost about configuring it. I hope technoid has the same pc with the isa riser (not pictured).
 
As shown in the photo's, there are two sockets to the left of the cpu. I suspect they are tag ram sockets, but the manual makes
no mention of them. Does cache work without tag ram?

2i7tgsz.png
 
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