VOGONS


First post, by uscleo

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Hi Guys! I’ve been reading through the forums and new to tinkering with the Cyrix 5x86. I just wanted to ask a few questions regarding using my Cyrix 5x86 CPU in a very “early” 486 board that does not have clock multiplier jumpers. The computer I’m doing this on is an IBM P75 - a 486 based portable. I’ve modded it so I can swap out the CPU clock crystal and I have had the original 486 DX swapped in with a 486 socket so I can swap out the CPU and change the clock speed (IBM purists, please don’t hate me!).

Right now I have a blue heatsink Cyrix / Gainberry / IBM 5x86c (says IBM26 5x86-3v3100GF under the heatsink) plugged in and running normally with the default 66.6 Mhz crystal . I can boot to DOS and run Windows 95 no problems, very stable. The clock speed however is low (I think CHKCPU mentioned it was measuring 5x.x Mhz or so). Incidentally, does anyone know if the stamp on my CPU means it can take overclocking well? 😀

How do I go about enabling the clock multiplier ? I have read several methods in several threads so I want to make sure I do this right:

1) Download CLKMUL utility and set it through the software (seems like there are limitations on what multipliers I can set through this method)

2) Route the CLKMUL pin from the CPU / socket to a ground pin? (according to this thread - to enable / disable multiplier of 2x or 3x on boot?) https://www.vogons.org/viewtopic.php?f= ... &start=160 post by gerwin

3) Download the utilities mentioned here: https://www.vogons.org/viewtopic.php?f= ... 86#p250032 - do these activate the clock multiplier too, or do I have to use CLKMUL? If they have to be used separately, which utility do I use first? Which utility is better? The thread mentions the IBM utility as the one to use out of the 5 included, but there is no documentation for it in the download.

Does anyone have the typical settings that should be tried first with the IBM utility? Any tips on how to configure it to enable the most stable options?

Does anyone know what clock crystals would work well over 66.6? I tried 80Mhz but that threw memory errors on POST, so I have ordered 70, 72, 75 Mhz, which are in the mail)

This forum has been an amazing resource, thanks in advance for all the help guys!! 😀

Reply 2 of 5, by uscleo

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derSammler wrote on 2020-01-16, 12:53:

Are you aware that you are about to kill your 5x86 CPU? You are running a 3.45V CPU with 5V.

Sorry I forgot to mention I’m using an interposer - it’s running at 3.45 V - I actually found another interposer that had a switch for clock multiplying and got it up to 100 MHz (66mhz Crystal divided / 2 x3) so I’m now waiting on the 7x.x MHz crystals in the mail.

Any ideas on settings to use in the IBM program for an old 486 like this?

Cheers!

Reply 3 of 5, by uscleo

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Oh and I tried out SETMUL.exe - seems to be able to only half the clock down to 33 MHz if I do SETMUL 1, but if I change the multiplier to 2 nothing changes, SETMUL 3 takes it back up to 100 and SETMUL 4 stays at 100 MHz

Reply 4 of 5, by uscleo

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Ok, so I have some results to share in case anyone else is as crazy as me and gets a kick on using 1995 CPUs in motherboards from 1990.

So, as the motherboard / BIOS does not support write back, the fine tuning of settings on the CX 5x86 did not increase performance drastically.

I found that the MS-DOS version of the utility (Peter Moss's) was "better" over the IBM GUI application, and with Peter's utility, you can insert a line into autoexec.bat so your computer starts up pre-configured

Here is what worked on my stepping 1 revision 3 Cyrix 5x86 at 100 Mhz

/BTB_EN=ON /LSSER=ON /BWRT=ON /MEM_BYP=ON DTE_EN=ON /FP_FAST=ON

/LOOP_EN=ON resulted in windows throwing an protection fault and not starting.

Again this is all with write through, as I cannot use write back. Benchmarks in dos and windows improved by maybe 5%. I'm not sure which will perform faster with overclocking, the AMD 5x86 133Mhz or the Cyrix 5x86, will do some tests and see.

Hope this helps someone!

Reply 5 of 5, by Anonymous Coward

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I use 5x86s on motherboards from 1991 and 1992. They're more or less the same as the 1990 models. Your findings are consistent with what I can remember.

Cx5x86-100 can more often than not overclock to 120MHz in my experience (especially if you have active cooling).
Am5x86-133 can usually overclock to 160MHz without issue.
Pushing either of these chips faster usually isn't successful (unless you disable L1 cache).
The AMD has a stronger ALU, but Cyrix has a faster FPU, so which one you should choose depends on what software you run.

I also have three Cyrix 5x86 chips with 4x multiplier (i think). The 100/4X is the only one I have tested and I have with me at the moment, and I was unable to get it to work on my older 486 boards. Actually, it's kind of funny, because normally on older 486 boards I can only get 2x/3x cx5x86s to post if set to "2.5X" on the interposer. Unfortunately that trick doesn't seem to work with my 3x/4x chip. Not sure if there's something wrong with this one, or if these chips are just incompatible with older 486 boards. That would be be unfortunate, because I was hoping that the rare 133MHz Cyrix 5x86 would give me a slight edge in integer scores over a 160MHz AMD.

I'd really like to know if anyone with a Cyrix 5x86-133 has ever gotten it to work in an older 486 board, and if there are any benefits to doing so.

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium