Yeah they will work fine at 100MHz.

appiah4 wrote on 2022-08-04, 12:59:

Yeah they will work fine at 100MHz.

Thank you for a quick reply! Now to ebay!

A Cyrix 5x86 with a 4x multiplier would likely be the best option if it works on your board but my experience is AMDs are much more compatible

Cyrix 5x86 with 4x multiplier are for the wealthy these days. I've seem Cx5x86-133 sell for over $500 🙁

debs3759 wrote on 2022-08-04, 13:44:

Cyrix 5x86 with 4x multiplier are for the wealthy these days. I've seem Cx5x86-133 sell for over $500 🙁

Dafuq?

appiah4 wrote on 2022-08-04, 14:15:

debs3759 wrote on 2022-08-04, 13:44:

Cyrix 5x86 with 4x multiplier are for the wealthy these days. I've seem Cx5x86-133 sell for over $500 🙁

Dafuq?

I thought some of the lower speed grades sometimes had X4 enabled instead of just x2-x3

Don’t remember anymore how you identified one from the other

https://forum.vcfed.org/index.php?threads/cyr … 6-133mhz.20956/

rmay635703 wrote on 2022-08-04, 16:04:

I thought some of the lower speed grades sometimes had X4 enabled instead of just x2-x3 […]
Show full quote

appiah4 wrote on 2022-08-04, 14:15:

debs3759 wrote on 2022-08-04, 13:44:

Cyrix 5x86 with 4x multiplier are for the wealthy these days. I've seem Cx5x86-133 sell for over $500 🙁

Dafuq?

I thought some of the lower speed grades sometimes had X4 enabled instead of just x2-x3

Don’t remember anymore how you identified one from the other

https://forum.vcfed.org/index.php?threads/cyr … 6-133mhz.20956/

Some that aren't officially 4x are capable, but it's a lottery. I must get round to testing the 3 x 5x86 I have 😀 I think the are some Cx5x86-100 that are 4x 25 as well, they may be cheaper. I've stopped buying so much lately, so not sure.

Monteto wrote on 2022-08-04, 12:51:

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a proce […]
Show full quote

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a processor with integrated voltage regulation to get the necessary 3.45V or 3.3V instead of the original 5V to the processor.
...
My question is: can I expect 4x33 processors to work on my system underclocked at 100 MHz? Or will they just simply not work?
...

An Am486/"5x86" /133 will work with 25 MHz as well as it would work with 33 MHz FSB.
Despite the requirement for the voltage reduction:
1) On some motherboards the CPU initialisation may fail because the BIOS does not know what to do with that CPU.
2) With a higher multiplicator the L1 cache's operating mode write-back will become more important. If your board does not support write-back CPUs, you won't get a performant system. Don't expect too much if you have to run your CPU in write-through mode.
3) The new CPU will have a double sized L1 cache. This may lead in a performance benefit tough.

You should be aware that there is a service manual online:
http://ps-2.kev009.com/pccbbs/aptiva/63g2028.pdf

Perhaps you have more benefit from installing a Pentium Overdrive...

Disruptor wrote on 2022-08-04, 17:40:

An Am486/"5x86" /133 will work with 25 MHz as well as it would work with 33 MHz FSB. Despite the requirement for the voltage red […]
Show full quote

Monteto wrote on 2022-08-04, 12:51:

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a proce […]
Show full quote

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a processor with integrated voltage regulation to get the necessary 3.45V or 3.3V instead of the original 5V to the processor.
...
My question is: can I expect 4x33 processors to work on my system underclocked at 100 MHz? Or will they just simply not work?
...

An Am486/"5x86" /133 will work with 25 MHz as well as it would work with 33 MHz FSB.
Despite the requirement for the voltage reduction:
1) On some motherboards the CPU initialisation may fail because the BIOS does not know what to do with that CPU.
2) With a higher multiplicator the L1 cache's operating mode write-back will become more important. If your board does not support write-back CPUs, you won't get a performant system. Don't expect too much if you have to run your CPU in write-through mode.
3) The new CPU will have a double sized L1 cache. This may lead in a performance benefit tough.

You should be aware that there is a service manual online:
http://ps-2.kev009.com/pccbbs/aptiva/63g2028.pdf

Perhaps you have more benefit from installing a Pentium Overdrive...

Thank you for really interesting info to take into consideration!

I do not really know if the board supports write-back or not, I will do some research. Thank you for the link to the service manual, however it does not list my exact configuration (it seems like there was a lot of different PS/1 configurations haphazardly tossed out there without much documentation , but one of them gets close. This is an European model.).

It also seems like it is suddenly more 100MHz than 133MHz Cyrixes available for a reasonable price (not the case last time i searched ebay), so I might try to buy a couple and see if I get lucky and it has a 4x multiplier 😀

Very hard to find voltage regulator adapters though....

Monteto wrote on 2022-08-04, 21:59:

Thank you for really interesting info to take into consideration! […]
Show full quote

Disruptor wrote on 2022-08-04, 17:40:

An Am486/"5x86" /133 will work with 25 MHz as well as it would work with 33 MHz FSB. Despite the requirement for the voltage red […]
Show full quote

Monteto wrote on 2022-08-04, 12:51:

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a proce […]
Show full quote

I am aware that all the alternatives listed below would require a voltage regulator adapter, further hardware hacking or a processor with integrated voltage regulation to get the necessary 3.45V or 3.3V instead of the original 5V to the processor.
...
My question is: can I expect 4x33 processors to work on my system underclocked at 100 MHz? Or will they just simply not work?
...

An Am486/"5x86" /133 will work with 25 MHz as well as it would work with 33 MHz FSB.
Despite the requirement for the voltage reduction:
1) On some motherboards the CPU initialisation may fail because the BIOS does not know what to do with that CPU.
2) With a higher multiplicator the L1 cache's operating mode write-back will become more important. If your board does not support write-back CPUs, you won't get a performant system. Don't expect too much if you have to run your CPU in write-through mode.
3) The new CPU will have a double sized L1 cache. This may lead in a performance benefit tough.

You should be aware that there is a service manual online:
http://ps-2.kev009.com/pccbbs/aptiva/63g2028.pdf

Perhaps you have more benefit from installing a Pentium Overdrive...

Thank you for really interesting info to take into consideration!

I do not really know if the board supports write-back or not, I will do some research. Thank you for the link to the service manual, however it does not list my exact configuration (it seems like there was a lot of different PS/1 configurations haphazardly tossed out there without much documentation , but one of them gets close. This is an European model.).

It also seems like it is suddenly more 100MHz than 133MHz Cyrixes available for a reasonable price (not the case last time i searched ebay), so I might try to buy a couple and see if I get lucky and it has a 4x multiplier 😀

Very hard to find voltage regulator adapters though....

Try to find IBM branded Cyrix 5x86s, they reliably work at 120MHz (40x3) and with registers enabled easily match Am5x86-133

Try to find IBM branded Cyrix 5x86s, they reliably work at 120MHz (40x3) and with registers enabled easily match Am5x86-133

But that will leave me with 75MHz with my 25MHz FSB(25x3)? What I need is certainly 25x4, or rather 33x4 underclocked to 25x4?

Just a remark - if you want more speed, try to get AMD's 5x86 instead of Am486. The 5x86 has double the L1 cache, which is really nice to have when the FSB speed is low. 😉

Monteto wrote on 2022-08-05, 09:11:

Try to find IBM branded Cyrix 5x86s, they reliably work at 120MHz (40x3) and with registers enabled easily match Am5x86-133

But that will leave me with 75MHz with my 25MHz FSB(25x3)? What I need is certainly 25x4, or rather 33x4 underclocked to 25x4?

My remark was advice in general, in case you own a board that does 40Mhz at some point. Otherwise what you want is an AMD Am5x86-PR75 (or -133; same part).

Ah, I see. Good advice!

Had any luck? I got my hands on a PNY Powerstacker (an interposed AM5x86-P75) and I'm trying to get it to work with a PS/1 2133-F43 (originally with a 20Mhz CPU) but I can't get it to POST 🙁

werfu wrote on 2022-10-21, 19:18:

Had any luck? I got my hands on a PNY Powerstacker (an interposed AM5x86-P75) and I'm trying to get it to work with a PS/1 2133-F43 (originally with a 20Mhz CPU) but I can't get it to POST 🙁

In my IBM 486 Intel CPU work all models, AMD 5x86 no way to make it boot, Cyrix 5x86 work correctly (no tuning, just the cpu) Pentium Overdrive Work but only with the Hard disk in the second ide channel. IBM are not the compatibility-Champion

4x25 for 100 is no problem.

Btw, if you do come around to the idea of a voltage interposer check out the socket 1/2/3 voltage interposer tweaker

Re: Socket 1/2/3 Voltage Interposer Tweaker (Alpha, seeking testers)

Currently seeking alpha testers.

werfu wrote on 2022-10-21, 19:18:

Had any luck? I got my hands on a PNY Powerstacker (an interposed AM5x86-P75) and I'm trying to get it to work with a PS/1 2133-F43 (originally with a 20Mhz CPU) but I can't get it to POST 🙁

IBM went to great lengths to cripple their boards from being upgradable.
My guess is any chip you install needs to support the overdrive pinout to disable your soldered chip.
Best bet are Intel DX2-4 and p24t overdrives, a 5x86 overdrive would need to support the overdrive pinout and not all did.

Unfortunate that it’s so hard to find BIOs upgrades for all the IBMs, IBM was even stingy in distributing bios updates unless you bought official upgrades from them that required an upgrade.

My school library had original generation IBM model 25 286’s upgraded to SLC chips and they got updated bios’s in the package so the systems actually posted with the correct info.

Have to imagine it was a pretty penny getting IBM certified upgrades let alone having an IBM maintenance agreement and network

rmay635703 wrote on 2022-10-22, 02:04:

IBM went to great lengths to cripple their boards from being upgradable. My guess is any chip you install needs to support the o […]
Show full quote

werfu wrote on 2022-10-21, 19:18:

Had any luck? I got my hands on a PNY Powerstacker (an interposed AM5x86-P75) and I'm trying to get it to work with a PS/1 2133-F43 (originally with a 20Mhz CPU) but I can't get it to POST 🙁

IBM went to great lengths to cripple their boards from being upgradable.
My guess is any chip you install needs to support the overdrive pinout to disable your soldered chip.
Best bet are Intel DX2-4 and p24t overdrives, a 5x86 overdrive would need to support the overdrive pinout and not all did.

Unfortunate that it’s so hard to find BIOs upgrades for all the IBMs, IBM was even stingy in distributing bios updates unless you bought official upgrades from them that required an upgrade.

My school library had original generation IBM model 25 286’s upgraded to SLC chips and they got updated bios’s in the package so the systems actually posted with the correct info.

Have to imagine it was a pretty penny getting IBM certified upgrades let alone having an IBM maintenance agreement and network

Can anyone tell me what exactly about the overdrive pinout it is that disables the chip? Is it the same as the 487 main processor disable signal? Because the tweaker can do that. But if there is another signal I can add it also.