VOGONS


First post, by Monteto

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I have an IBM PS1 2133 (surface mounted 486SX 25 MHz) I have been upgrading lately, and I want to get more out of the system by upgrading the CPU. Currently I am running a 486DX2 66MHz underclocked at 50 MHz in the upgrade socket (I did the necessary hardware hack to get non-overdrive processors working in that socket, thanks to epictronics for the know-how).

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.

I know there exists some AMD Am5x86 and Cyrix Cx5x86 models that actually support 25x4 MHz for a 100 MHz system, but I haven't been able to come by any of these.

My question is: can I expect 4x33 processors to work on my system underclocked at 100 MHz? Or will they just simply not work? Some sources mentions the Cyrix processors require 33 FSB, but I suspect this is only for maximum performance. There seems to be a lot more of these processors around than the 25x4 ones.

I do not want to switch the crystal for a 33 MHz FSB on this motherboard as I think it will stray to much from the original and I also don't know if I feel competent enough (seems harder on this board than others I have seen modified).

IBM PS/1 2133 540 10 MB RAM 486SX 25Mhz

Reply 4 of 14, by debs3759

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

See my graphics card database at www.gpuzoo.com
Constantly being worked on. Feel free to message me with any corrections or details of cards you would like me to research and add.

Reply 5 of 14, by appiah4

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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?

Retronautics: A digital gallery of my retro computers, hardware and projects.

Reply 6 of 14, by rmay635703

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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/

Reply 7 of 14, by debs3759

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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 […]
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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.

See my graphics card database at www.gpuzoo.com
Constantly being worked on. Feel free to message me with any corrections or details of cards you would like me to research and add.

Reply 8 of 14, by Disruptor

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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...

Reply 9 of 14, by Monteto

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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....

IBM PS/1 2133 540 10 MB RAM 486SX 25Mhz

Reply 10 of 14, by appiah4

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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

Retronautics: A digital gallery of my retro computers, hardware and projects.

Reply 11 of 14, by Monteto

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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?

IBM PS/1 2133 540 10 MB RAM 486SX 25Mhz

Reply 12 of 14, by GigAHerZ

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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. 😉

"640K ought to be enough for anybody." - And i intend to get every last bit out of it even after loading every damn driver!

Reply 13 of 14, by appiah4

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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).

Retronautics: A digital gallery of my retro computers, hardware and projects.