One of the problems in trying to upgrade 486 based computers is that there are so many different types out there. There were many, many improvements along the way that continuously increased capabilities as time progressed. Naturally, the mobo [MOtherBOard] had to change to accommodate these new features.
Early mobos supported perhaps the 486DX, and maybe the 486DX2 (clock doubled), both of which run from 5 Volts (although there were a few 3.3V DX2 chips around) (SX chips are the same as DX except the "math co-processor" portion is not there). When the faster 486DX4 (clock tripled) chips came out, they were designed to run from 3.3 Volts, so the mobo had to be capable of that to run these chips. A socket 3 mobo (the CPU socket is usually engraved with the type) has “jumpers” that allow the selection of 5 V or 3.3V (and maybe others as well, like 3.45 V, 4 V, etc.). Hopefully there is information stenciled on the mobo to tell you how to configure this.
Towards the end of Socket 3's reign, write back cache was introduced, which is just another one of those techniques designed to help increase system speed. I mention it here because the latest 486 style chips introduced, i.e the Intel P24D (a DX2 with w/b cache), the AMD and Cyrix 5x86, as well as the Intel Pentium ODP-63 and the ODP-83 (overdrive processor for 486 systems) (called the P24T during development) have this capability. If your mobo supports this function, which the later SiS and UMC chipsets did (as well as other chipsets perhaps...I'm out of experience here), you can achieve performance (except in floating point) equivalent to that of the Pentium 75. If your mobo does not support w/b cache and you want to use one of the CPUs that do have this feature, this function must be disabled or the CPU/mobo combo will not play together.
Many people are overclocking their Am5x86 chips to 4x40 Mhz = 160 Mhz. This yields a performance of about a P90 (except in pure floating point). However, some mobos set the PCI/Memory Bus speed at half the System Clock speed when you choose a System Clock of 40 Mhz or above. By extension, this affects the hard drive speed as well. Some mobos effect this by setting a jumper labeled “>40 Mhz” or some such. Just keep that jumper at the “<= 33 Mhz” position when going to 40 or 50 Mhz for faster operation. Other mobos effect this automatically. The way around this is to boot at 33 Mhz, and use the keylock or turbo switch to get you into 40 Mhz after boot. You have to study the jumper settings for both 33 and 40 Mhz to determine which stay open, which stay closed, and which wire to the switch. Get CACHECHK at http://www.shareware.com and run it (from DOS) to see what is really going on with your memory timings.
Cachechk will also tell you how much, if any, L2 cache you have on the mobo. If it shows none, get at least 128 kb.
Not all attempts at overclocking succeed. There are many reasons for failure. Should you want to chase down reasons in order to try and remedy the situation, check out the overclocking section at http://sysdoc.pair.com for advice on things to try.
I should mention at this point that a number of articles came out in the computer mags last Fall (Oh...where has the time gone...it's a year ago allready!) that looked at the upgrading options for both 486 and Pentium systems. Their conclusions concerning the 486 were as follows...(Note: Overclocking was not included as an option in this article).
1. Chip upgrades by themselves did increase performance.
2. You could get as much performance increase by just going to 16 Megs of RAM (if you had 4 or 8 to start with) as you could in upgrading the processor. 32 megs didn't perform much better than 16.
3. Doing both of the above worked the best, i.e. upgrading both memory (to 16 Megs) and the processor.
4. The 5x86 gave slightly better performance than the Intel Pentium ODP (P24T) at a lot lower cost.
Unfortunately, many of the 486 mobos use 30 pin simms (which must be added in sets of 4, i.e. 4 ea 4 Meg simms to give you 16 megs). These are rather expensive today, unlike the 72 pin SIMMS which have dropped in price lately. 1 meg 30 pin SIMMS are cheaper and more available. There are SIMM savers that connect 4 ea 1 meg simms together for a total of 4 meg, and it plugs into a 30 pin socket. Do this 4 times for the 16 meg requirement. I understand it is not as easy as it sounds, however. Maybe there is a memory news group that could help you here if you contemplate this. Some later 486 mobos had 72 pin SIMM sockets, which offers a less expensive upgrade path. With the prices today, you might be able to buy 16 Megs of 72 pin ram and a new 486 mobo for the price of 16 Megs of 30 pin ram.
Anyway, what's a good strategy on CPU upgrades? (But don't forget the memory upgrade part.)
1. Most definitely the safest upgrade (from the standpoint of shelling out cash and having the part work) is the Intel 486DX4-100 ODP, a non writeback cache 5 Volt DX4 Overdrive chip. Spirit or Performance just received a shipment of these benmyers@ma.ultranet.com. Useful for any socket, but probably the only solution for all Sockets before Socket 3.
2. For Socket 3 mobos that support w/b cache, the AMD 5x86-P75 (sometimes advertised as 5x86-133) works the best of what's available today (AMD is no longer producing any, however, so the price of existing stock is rising). Cyrix made some 5x86 CPUs as well, but they’re also no longer in production. (Any of these chips might be available on the used market.) The Intel P-83 ODP (a 5 V chip) works almost as well, but is generally much more expensive.
Setting the mobo jumpers correctly for the chip of choice is very necessary. If you can not chase down the manual on line for the jumper settings, or get them from someone else, it could be hopeless for these chips. However, if the mobo is working with an Intel chip in it, there is perhaps a path to upgrading.
3. For socket 3 mobos that do not support w/b cache, you can, with proper info on jumpers,...
If no jumper info is available, see the discussion in 2. above for mobos w/o documentation.
You might want to check out the "This old 486" info in the reference section at http://www.computercraft.com.
Also, there are additional links mentioned in the upgrading section at http://www.erols.com/chare/hardware.htm
Lastly, you ought to think about the fact that Pentium mobos and chips are coming down in price really fast these days. Why not invest in something much better rather than something just incrementally better?
Alan Tasker
ATasker@comnet.net
Newsgroups: comp.sys.intel
Posted: 10/20/1997
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