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386 and cache

oblivion

oblivion

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I know no 386 cpu has on board L1 cache but is a AMD 386-40 capable of taking advantage of up to 64kb of motherboard cache? is is higher? lower?
 
I noticed a huge improvement in performance when I upgraded my AM386DX-40 system to 256KB from 128KB, so I think it's quite safe to say that 64KB is no problem. I suspect that it's a motherboard thing anyway and the 386 itself has no idea that cache even exists.
 
Motherboard dependent, usually controlled by Chipset. OPTi seem to be among the fastest.

128KB is the generally recommended amount, some motherboards have problems with 256KB; losing floppy functionality or forcing you to slow memory timings down for example. 64KB should be fine and should give you an advantage over no cache at all.

Trivia: At least some 486 upgrade chips (such as Cx486DLC) have an L1 cache of 1KB, some of TI's (486SXL) had 8KB onboard though it seems to make a rather marginal difference and many motherboards don't support it.
 
The difference is measurable. But the main thing is to do have cache with a 386DX. Going from 0K to 64K is a huge improvement. From 64K to 128K and then to 256K the improvement is still measurable, but it is not that big.
The L1 enabled 386-like CPUs have a big performance increase if the board supports them properly, with a 40MHz Cyrix or Ti with 1K L2 matching a 486SX-33 performance-wise. I've got all these CPUs, but I still lack an 8KB L1 CPU that fits in a 386 socket.
 
I just threw out a 80386 MOBO that had 128KB cache installed on it. 80386 MOBOs with cache were pretty common back in the day.
 
PeterLI said:
I just threw out a 80386 MOBO that had 128KB cache installed on it. 80386 MOBOs with cache were pretty common back in the day.
Click to expand...

Yeah I was just going to post up, I have a 386SX-25 baby-AT motherboard that has cache chips in DIP sockets.
 
Thanks for the info guys. I had no idea cache was a thing on 386 boards. I assumed it was mostly for 486's. The 386 board I just got can be upgraded to a 486 and that's why I assumed it supported so much cache. Luckly it has 128 atm upgradable to 256kb
 
The 386 board I just got can be upgraded to a 486 and that's why I assumed it supported so much cache. Luckly it has 128 atm upgradable to 256kb
Click to expand...

It may also be worth pointing out... that board actually came with 64KB originally. I upgraded it because a couple of the original chips were bad.
 
SpidersWeb said:
Yeah I was just going to post up, I have a 386SX-25 baby-AT motherboard that has cache chips in DIP sockets.
Click to expand...

Cache is common with 386DX boards, but quite rare with 386SX, especially the later ones. I've yet to find an AMD SX-40 board with cache. On the Intel side, what was the fastest 386SX? 25 or 33 MHz?
 
Appears to be a 386SX-33 in a PQFP package. I can find no record of any faster 386SX made by Intel and the EX and SL appear to stop at 25MHz. If you're referring to performance the 33 should still be faster as it is identical and the difference in BUS speed resulting from divisors is very minimal if it is even implemented.
 
What I found is that a lot of it depends on the Chipset and motherboard. I have cache-less 386SX boards that outperform cache-disabled (in BIOS) 386DX systems. As always ANY cache gives you the biggest boost, after that you get diminishing returns. If a motherboard has 9 cache sockets, I always go for 256KB as you just need to buy 9 identical cache. They are also available to purchase new from places such as Elements14. I made a video about this a while ago:


They are 15ns and that's plenty of speed for these 386 boards.
 
Mau1wurf1977 said:
What I found is that a lot of it depends on the Chipset and motherboard. I have cache-less 386SX boards that outperform cache-disabled (in BIOS) 386DX systems. As always ANY cache gives you the biggest boost, after that you get diminishing returns. If a motherboard has 9 cache sockets, I always go for 256KB as you just need to buy 9 identical cache. They are also available to purchase new from places such as Elements14. I made a video about this a while ago:


They are 15ns and that's plenty of speed for these 386 boards.
Click to expand...

heh, should of known you had a video on it. your videos are always great help so thank you. does it matter if you mix speeds? for instance if both my tag chips are 15ns and the standard cache is a mix of 15ns and 20ns will it just run at 20ns or will that cause issues? also whats considered good for cache speeds? i've seen 25,20 and 15ns. is 20ns considered okay since thats what I tend to have around.
 
The cache speed is dependent on the speed of the system. For 40mhz, you only really need 25ns cache... the short explanation being that 40mhz = 40 million cycles per second, and 25ns = one 40 millionth of a second. There's some more complicated stuff about why the speed of the tag chips is more critical, but I don't really understand how that all works.
 
I wonder if 25ns will be quite sufficient for 40MHz, though, considering that 40MHz works out to just exactly a 25ns access time, and there's still any delays imposed by the cache controller to consider...?
 
Yeah. 15ns will work with practically everything and seems to be the most available. 12ns and 10ns are hard to find and are usually not needed.
 
oblivion said:
heh, should of known you had a video on it. your videos are always great help so thank you. does it matter if you mix speeds? for instance if both my tag chips are 15ns and the standard cache is a mix of 15ns and 20ns will it just run at 20ns or will that cause issues? also whats considered good for cache speeds? i've seen 25,20 and 15ns. is 20ns considered okay since thats what I tend to have around.
Click to expand...

Hehe...

The thing with the ratings is that you set the speed in the BIOS, set the FSB and they will either handle it or they won't. Good thing is if Cache has errors, you know straight away. Yes you can mix them if you like.

For me the 15ns was purely because of availability. They also have the smaller one, the one you need for TAG when you go for 128KB Cache, but they are 20ns. I did find a few faster ones on eBay, but 15ns is what I was able to find new and readily available.

They are fast enough to run a 486 at 40 MHz with the tightest timings on my Biostar motherboard.
 
Just found a 386DX-40 with cache. No idea if it works, has battery leakage and previous owner stripped it.
I'll check my SX's and see which ones have cache.
 
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