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IBM 5x86 in a 486 system with PCI slots?

itsvince725

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I have a 486 system with PCI slots and an Intel i420 chipset that supports 3.3v and 5v CPUs. I also have an IBM 5x86C-100 from a dead ThinkPad 365 laptop. My question is, assuming my motherboard has a 3x multiplier, could I use the 5x86 in that motherboard? The IBM 5x86s run at 3.3v and the little CPU board seems to use the Socket 3 pinout, though I haven't tried inserting it into a Socket 3 socket to find out.
 

Agent Orange

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I have a 486 system with PCI slots and an Intel i420 chipset that supports 3.3v and 5v CPUs. I also have an IBM 5x86C-100 from a dead ThinkPad 365 laptop. My question is, assuming my motherboard has a 3x multiplier, could I use the 5x86 in that motherboard? The IBM 5x86s run at 3.3v and the little CPU board seems to use the Socket 3 pinout, though I haven't tried inserting it into a Socket 3 socket to find out.

Drop it in and give it a shot. I can't see where it would irreversibly hurt any thing.
 

itsvince725

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Thanks to the Wayback Machine I was able to find the information for my motherboard and apparently there's no 3X multiplier, only 2X. I'm not gonna bother then, sure the IBM 5x86C is probably a bit faster at 66MHz than the DX2 but I don't know if it would even run underclocked.
 

GiGaBiTe

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486s were not multiplier or FSB locked, you can generally set the multiplier/FSB to whatever you want.

I've run some 66-133 MHz Am486s at 16/25/33 MHz and they worked fine.

If you wanted to get a 3/4x multiplier, you'd need one of those Evergreen 586 upgrade chips, but they're pretty expensive on ePay.

https://www.ebay.com/itm/372640945884

I have a dead one that burned up in a laptop, I need to get around to replacing the CPU on the interposer one of these days.
 

retro-pc_user

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GiGaBiTe

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It's a shame they didn't try to repair the Evergreen interposer, they're pretty rare these days. You can get Am5x86 PQFP CPUs on Ebay for $20 or so, NoS.

When I eventually get to repairing mine, I'm just going to use an X-Acto knife to cut the legs off and use a blob of solder to remove the remaining legs, not like the burned CPU is worth anything.

I wonder how hard it would be to copy the design of the Evergreen interposer and make replicas today. It looks like a multi-layer PCB, I'd imagine it'd be pretty difficult.
 

itsvince725

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486s were not multiplier or FSB locked, you can generally set the multiplier/FSB to whatever you want.

I've run some 66-133 MHz Am486s at 16/25/33 MHz and they worked fine.

If you wanted to get a 3/4x multiplier, you'd need one of those Evergreen 586 upgrade chips, but they're pretty expensive on ePay.

https://www.ebay.com/itm/372640945884

I have a dead one that burned up in a laptop, I need to get around to replacing the CPU on the interposer one of these days.

Hmm, well, the IBM 5x86 probably does perform better at 66MHz than an Intel DX2 does...
 

GiGaBiTe

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The IBM is just a rebranded Cyrix 5x86, which was a cut down 6x86; Which was basically a Pentium class CPU.

The 5x86 was a very buggy CPU, features it was advertised to have and be shipped with (like branch prediction) were disabled because it made the CPU very unstable. Even with all of the advanced features it was supposed to have being turned off in final silicon, it still suffered incompatibility issues with software and motherboards.
 

itsvince725

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A shame, as benchmarks prove the Cyrix 5x86 is the fastest Socket 3 CPU ever produced (it easily beats the AMD equivalent at the same clock speed). Socket 3 could have lived on into the Pentium age as Socket 7 did into the Pentium III age had the Cyrix chip been more stable.
 

GiGaBiTe

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486 machines did live well into the Pentium 3 age, in fact to nearly the end of it. Upgrades for 486 machines were made all the way until 1999, when AMD ceased to make the Am5x86-133.

They were still popular machines for people that didn't play games and just needed them for basic home office, email and light internet browsing. Windows 98SE ran fine on a fast 486 with a decent amount of RAM, but many people were still running Windows 3.1 and 95.

The death knell of the 486 was when AMD stopped manufacturing the chips and Microsoft dropped support for it in Windows XP, which required the CMPXCHG and CPUID instructions. Though, technically you could run Windows XP on a 486 machine by using a POD63/83, it's just painfully slow.
 

itsvince725

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Hmm, interesting. I personally have yet to find a Socket 3 box that was actually built as an Am5x86 (mine has an Evergreen 5x86 upgrade) but I suppose that doesn't mean they don't exist.
 

Unknown_K

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AMD kept making the am5x86-133 in SMT format for embedded controllers until 1999, which is why the last gasp 486 upgrade chips you see from Evergreen used surface mount chips. There were tons of embedded controls using that chip around until something faster and cheaper came around. Very few people would have bothered with a 486 upgrade in the late 1990's.
 

GiGaBiTe

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Very few people would have bothered with a 486 upgrade in the late 1990's.

I bought mine in the late 90s for a 486 SX/25 system because I was poor. IIRC it ran $99, but it was well worth it. Games that previously ran terrible (like Sim City 2000) then were very playable, and even Quake was somewhat playable at the lowest resolution setting.
 

Krille

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The IBM is just a rebranded Cyrix 5x86, which was a cut down 6x86; Which was basically a Pentium class CPU.
Unless I'm mistaken, the 6x86 came well after the 5x86 so calling the 5x86 a cut down version is like calling the 486 a cut down version of the Pentium.

The 5x86 was a very buggy CPU, features it was advertised to have and be shipped with (like branch prediction) were disabled because it made the CPU very unstable. Even with all of the advanced features it was supposed to have being turned off in final silicon, it still suffered incompatibility issues with software and motherboards.
Can you name examples of software incompatible with the Cyrix 5x86? I'm asking because my first computer* was an AST Advantage 611S with a 100 MHz Cyrix 5x86 and I don't recall any compatibility problems whatsoever.

The death knell of the 486 was when AMD stopped manufacturing the chips and Microsoft dropped support for it in Windows XP, which required the CMPXCHG and CPUID instructions. Though, technically you could run Windows XP on a 486 machine by using a POD63/83, it's just painfully slow.
FWIW, CMPXCHG and CPUID are both available on 486 CPUs (CPUID only on later revisons).

I bought mine in the late 90s for a 486 SX/25 system because I was poor. IIRC it ran $99, but it was well worth it. Games that previously ran terrible (like Sim City 2000) then were very playable, and even Quake was somewhat playable at the lowest resolution setting.

*It was actually my dad's computer and he bought it in august -95 (IIRC). I can still remember the feeling when I saw it boot up for the first time and hearing the Windows 95 start-up sound. Good times! Oh, and I ran Quake on it but it became unplayable as soon as 3 polygons where in view at the same time - at the second level with two Ogres and a torch on the wall it screached to a halt. :)

My dad somehow got the impression that the Cyrix processor was better at floating point operations than the Pentium - I'm guessing the guy at the shop selling the computer told him that (or maybe the guy told him the truth but dad misunderstood/misremembered). Today I know better of course but my dad still believes it and I'm not going to argue with him - we both have fond memories of that computer and I don't think it's necessary to destroy his illusion. Anyway, I still have that computer and I've been meaning to set it up and use it again. The problem is as always lack of space.
 

vwestlife

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Unless I'm mistaken, the 6x86 came well after the 5x86 so calling the 5x86 a cut down version is like calling the 486 a cut down version of the Pentium.
The Cyrix 5x86 was a cut-down version of the M1, which was the prototype of the 6x86:

https://en.wikipedia.org/wiki/Cyrix_Cx5x86

Wikipedia said:
The Cyrix 5x86 processor, codename "M1sc", was based on a scaled-down version of the "M1" core used in the Cyrix 6x86, which provided 80% of the performance for a 50% decrease in transistors over the 6x86 design. It had the 32-bit memory bus of an ordinary 486 processor, but internally had much more in common with fifth-generation processors such as the Cyrix 6x86, the AMD K5, and the Intel Pentium, and even the sixth-generation Intel Pentium Pro.
 

Agent Orange

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IIRC, back in 1991 I purchased a 'bare bones' mini tower from a warehouse distributor located near O'Hare airport, just north of Chicago. The thing consisted of the case, P/S, and motherboard along with a 486SX-25, all for about $700, plus shipping. There wasn't much in the way of used peripherals, so I watched 'Computer Shopper' like a hawk. By the time I outfitted it in with a VGA card, color monitor, sound card, I/O card, RAM, keyboard, serial mouse, and Windows 3.0, I easily had 2K or so invested or maybe more, as I probably didn't want to remember. The modems were another thing back then. I had a 300 baud in my 1000SX and used that for a while and later settled on a 14.4K when the prices came down. It seems that every time you turned around someone had a new modem offer with a new features and speed (my last dial-up was a 56K external US Robotics with Mindspring until DSL showed its ugly head).

The high end 486's were more or less out of the ball park, price-wise, for the casual user. One of my neighbor's taught CAD/CAM at Ford HQ and another was a mechanical engineer, who always had the current AutoCAD. Both always had the latest and greatest hardware and it was all made possible with the use of a tax write-off. So, the 486-25 served me well until I eventually landed an AMD Anthlon something or another. Since I had full use of PC's at the work place, it was hard to justify the expense of an elaborate personal computer thing at home. Then along came gaming.
 

GiGaBiTe

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Can you name examples of software incompatible with the Cyrix 5x86? I'm asking because my first computer* was an AST Advantage 611S with a 100 MHz Cyrix 5x86 and I don't recall any compatibility problems whatsoever.

The Cyrix 5x86 had an identity crisis. It wasn't a 486, and it wasn't quite a Pentium class processor either, putting it in limbo between both generations. The incompatibility issues stem from Cyrix PR touting it as a Pentium class processor, while it wasn't because it lacked several Pentium class instructions. This meant software that expected and was compiled for a Pentium would behave erratically.

It also may have the infamous coma bug (the Cyrix equivalent of the Pentium F00F bug) that plagued the later 6x86 it was based on.

The disabled features of the CPU were also a source of instability if enabled, which was possible because two tools were released to turn these features on and off.


FWIW, CMPXCHG and CPUID are both available on 486 CPUs (CPUID only on later revisons).

CMPXCHG is present on 486 systems, but not CMPXCHG8b, which Windows XP requires.
 

vwestlife

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Even the first-generation 6x86 won't run Windows XP because it is missing the necessary Pentium-compatible CPU ID.
 
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