VOGONS


First post, by megatron-uk

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I have been offered a lovely little Olivetti PCS 11 desktop... it looks to a very small LPX implementation, barely taller than a 3.5" floppy and single slot ISA card.

It appears to be in really nice condition except a broken power supply. However, it's the absolute bottom of the barrel 386SX-16 model.

I'm interested in desoldering the QFP processor and replacing it with something a bit better; I could do a 386SX-25, which the board was equipped to handle, but I'm thinking about possibly one of the available QFP-100 format upgrades - I think it comes down to the following options:

- Cx486SLC / Tx486SLC: 1KB cache, 5v tolerant, 20 to 40MHz parts.
- Tx486SXLC: 8KB cache, up to 40MHz.
- Tx486SXLC2: 8KB cache, clock doubling (20, 25, 33MHz input) for 40 to 66MHz parts.
- IBM 486SLC: 16KB cache, 33MHz.
- IBM 486SLC2: 16KB cache, clock doubling (25, 33MHz inputs) for 50 and 66MHz parts.

My understanding is that the Cyrix and Ti cores are identical, just like the DLC versions, but that the IBM SLC core is entirely different and derived from Intel. However, that's almost entirely irrelevant as the availability of IBM SLC processors appears to be virtually non existent, whereas the SXLC and SXLC2 processors are much more common - though finding 5v rated parts is a little harder.

Are all the above options pin compatible with the 386SX? Anyone have a success story to share relating to swapping out a surface mount 386SX with one of them?

I did buy one of those old stock Kingston 486 Now! IBM PS/2 model 50 upgrade cards many years ago and never found a use for it - it has a 50G6950 marked 486SLC2 on it, but I've had a hard job finding specs on it. That's a possible option....

My collection database and technical wiki:
https://www.target-earth.net

Reply 1 of 7, by BitWrangler

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I beleive it's the Cyrix SLC you want for direct replacement https://en.wikipedia.org/wiki/Cyrix_Cx486SLC in the 90s I had an a 386sx25 and 486SLC25 version of the same machine open on the bench and I couldn't see a single trace or capacitor difference .

2017: Basement full of ancient PC stuff, starting to go through it. 2021: Still starting, heh, many setbacks. So what's this BitWrangler guy's deal ??? >>> Taming the pile, specs to target?

Reply 3 of 7, by rmay635703

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maxtherabbit wrote on 2021-06-10, 22:28:

maybe try overclocking the sx16 to 20 or 25 before you go to all the effort to yank it?

I had a small 40mhz board with an sx16 onboard, just had to peel a sticker to see the real chip

Reply 4 of 7, by Deunan

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IBM chips have the pinout 100% identical to 386SX but the cache control signals are different than Cyrix/Ti. Good luck finding any info on that, or any code to set up the control registers. Possibly IBM needs a chipset that actually works with cache and doesn't have the built-in workaround that Cyrix has. Also, IBMs are all 3V3 with 5V tolerant I/O, at least as far as I know.

Cyrix/Ti also comes with 3V3 models, those usually have letter V in the chip designation somewhere. I am running two 3V3 SXLC2-G50 chips on 5V but I can't guarantee how long these will last. These particular ones seem to able to take it, but they do run hot (well that's true in general for all of these).

Also, SXL(C) = SXL(C)2, they all have clock doublers. So you can pick a 33 or 40MHz variant and run it on 16MHz bus with clock doubling, that way you do not need to overclock the whole chipset (and by quite a lot, too). Without BIOS support you will need to run a configuration program in AUTOEXEC but that's still acceptable solution.

EDIT: Forgot to add, good luck finding a 5V SXLC, those are rare and pricey. Cyrix SLC is easier to find but lacks clock doubler. 8 vs 1 KiB cache is not that much of a difference until you enable the doubler.

Reply 5 of 7, by megatron-uk

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Deunan wrote on 2021-06-10, 22:54:
IBM chips have the pinout 100% identical to 386SX but the cache control signals are different than Cyrix/Ti. Good luck finding a […]
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IBM chips have the pinout 100% identical to 386SX but the cache control signals are different than Cyrix/Ti. Good luck finding any info on that, or any code to set up the control registers. Possibly IBM needs a chipset that actually works with cache and doesn't have the built-in workaround that Cyrix has. Also, IBMs are all 3V3 with 5V tolerant I/O, at least as far as I know.

Cyrix/Ti also comes with 3V3 models, those usually have letter V in the chip designation somewhere. I am running two 3V3 SXLC2-G50 chips on 5V but I can't guarantee how long these will last. These particular ones seem to able to take it, but they do run hot (well that's true in general for all of these).

Also, SXL(C) = SXL(C)2, they all have clock doublers. So you can pick a 33 or 40MHz variant and run it on 16MHz bus with clock doubling, that way you do not need to overclock the whole chipset (and by quite a lot, too). Without BIOS support you will need to run a configuration program in AUTOEXEC but that's still acceptable solution.

EDIT: Forgot to add, good luck finding a 5V SXLC, those are rare and pricey. Cyrix SLC is easier to find but lacks clock doubler. 8 vs 1 KiB cache is not that much of a difference until you enable the doubler.

Yes, I've noticed that there is quite a lot of availability of SXLC2 chips, but almost all are those Tx486SXLC2-G rated parts, which are those 3.3/5v designs you mention, I'm not sure I like the idea of putting 5v through one of those continuously. Pure 5v rated parts like the Tx486SXLC2-0 are much less common - and pricier, as you say!

Sounds like the IBM 486SLC isn't really an option. Pity, they seem to have a really good reputation (and I wish I had a way to use that Kingston module - none of my generic 286 boards work with it).

My collection database and technical wiki:
https://www.target-earth.net

Reply 6 of 7, by keropi

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Having tried to upgrade some 386sx systems in the past in my opinion do not bother with exotic upgrades in the realm of the cyrix 486slc/slc2. (the IBM etc parts that are truly beefier are not just a drop-in replacement).
Just replace the stock 16mhz part with the 25mhz one and do what mobo changes need to be done so 25mhz is supported.
The classic 486slc and slc2 upgrades offer so little performance gain it is not worth it. I have done this upgrade on the 386sx amstrad megapc motherboard and used a bios that offers direct support for the 486slc , if you look at the end of this post Re: Amstrad Mega PC you will see this conclusion - this is 25mhz 386sx Vs. 486slc

So... how does it compare to a 386sx system?
386sx -> 486slc
Sysinfo: 18.1 -> 33.8
Landmark: 34.15 -> 92.76
Wolf3d: 14.1 -> 14.4 fps
3Dbench: 7.8 -> 8.4

there is significant benchmark difference but real-life difference? not even 0.5 frame in Wolfenstein3d. 🤣

🎵 PCMIDI mpu site buy+info
🎧 WIP Orpheus soundcard site
💻 WTB Amstrad PC7486SLC-33 system

Reply 7 of 7, by Deunan

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Not all systems can take even a small overclock of the CPU bus, and while 25MHz seems still low, it's a huge 56% step up from 16MHz. In general I agree that the upgrade will not make a world of difference but it might just make some games or programs more responsive. I did it mostly for the fun of it, and because of curiosity about the clock doubler.

Also, not all code with benefit equally. First, the small L1 is mostly meant to help with memory-spilled registers - something that 16-bit x86 code is really doing a lot of - and tight loops. But a typical optimization for 286 and 386 is to unroll loops so it goes against the small cache. Cyrix CPUs excel at multiplication, their 486 cores (DLC/SLC including) are way faster at it then even Intel 486. This makes the poorly optimized code that uses loops and often multiplication for address calculation way faster, while a hand-crafted assembly for 286/386 that uses shitfs, LEAs, etc instead might not see a lot of improvement.

If the mobo can handle 25MHz then I would look hard for 5V 50MHz SXLC(2) part (or try yet another 3V3 one, but that's me). A 25MHz SLC should more or less match 33MHz 386SX (if cache is enabled, without it there's maybe 5-10% improvement from faster CPU pipeline but it'll be 100% code fetch bound). A clock-doubled 50MHz should get between 33 and 40 MHz 486, that would be significant - if still bus limited.