and how in the world would it work "disabling" the 386 CPU that is left underneath

It's attached directly to the CPU pins.

486 class CPU with 16-bit data bus? Horrible idea and I wonder if 386DX can outperform it. Cyrix 486DLC upgrade certainly will.

Hm. That CPU is an 486SLC (TI486SLC/E-33MAB ?)..
At least it has got on-chip cache, which the 386SX hasn't got. 😀

But yeah, the 386SX wasn't a genius invention.
At least nor from a techical point of view.
(Edit: If it was, it would have had a 286 compatible pinout at least.)

AFAIK, the real 80386 also supported 16-/24-Bit I/O,
as it predecessor did, the 80286.

The only positive things of the 386SX that come to mind right now:
It was cheap and gave smart, quality chipsets of the 286 era a new life.

(But then, again, someting like the later released 486SLC would have been much
better right from the start, since that on-chip cache could compensate the small datapath.)

Edit: I forgot to say : Thanks for posting! 😀
(I believe there also were similar 286 upgrades from the MakeIt 486 series..)

The Serpent Rider wrote:

486 class CPU with 16-bit data bus? Horrible idea and I wonder if 386DX can outperform it. Cyrix 486DLC upgrade certainly will.

In my experience a 486SLC-33 runs roughly similar to a 386DX-40. But only in a cached motherboard. Without cache it's like a miserable 386SX, the more advanced core is completely held back by that 16-bit bus - even though its Landmark result is through the roof, like a real 486.

Fun thing is that it needs both the internal and external cache to perform reasonably. And it freezes at the end of any Turrican II level if its internal cache is enabled. But without it palpably lags behind the vanilla 386DX.

(Of course this experience is only based on the two 486SLC boards I have/had.)

- How it works:
The 386SX has a FLT# input (active-low FLOAT signal) that will make the CPU tristate all the bus signals, including HLDA (hold acknowledge). It still draws some power (especially the Intel ones that need to be executing NOPs even when halted) but is otherwise "not present". The pin has internal pull-up and is not connected on mobos that don't have a jumper for it so the socket with the upgrade simply has it connected to ground and will disable the mobo CPU when the device is installed.
This particular upgrade is 5V one but there are 3V3 chips that will come with a voltage regulator on the PCB (Intel did that too with some Overdrive chips except they've put the regulator on top of the ceramic package).

- How fast is it:
Depends on mobo/BIOS since to properly use the internal cache in the upgrade there should be some support present. Best if the mobo has FLUSH# (and also KEN#) signals - these are not present on 386SX but many later '90 boards were released after Cyrix made the 486SLC and so the chipset support it. These are connected to unused pins on 386 and not so much for the upgrade sake but to make it possible to install the 486SLC in the factory. All you need to do is flip some BIOS settings.
If there is no mobo support you can use something called BARB mode by enabling it via DOS program, but this only works properly if the mobo supports hidden RAM refresh (as in, does not requst HOLD to perform the refresh cycle). This mode however will suffer serious perfomance degradation if there is heavy DMA traffic.
Without the cache the CPU itself will be only some 50% faster, with it up to 100% - so if you had 386SX at 33MHz you now have "66MHz" chip so to speak.

- Why bother?
Because it's way cheaper than a DX mobo, you only have to route 16 data lines and up to 24 address ones, also no new chipset is required since this 486 can't do burst transfers (because it has to be compatible with 386 bus). I have ALI M1217 based 386SX motherboard that I've converted to 486SXLC. This CPU has clock-doubler (needs to be enabled) so with clock generator replaced to low 25MHz I can run it at 25x2 = 50MHz. At this speed it pretty much matches a 486SX-40. And that is a cacheless mobo the size of a notepad. I think it's cool.

Wow, you all make interesting points, guys, thanks!

Jo22 wrote:

I forgot to say : Thanks for posting! 😀
(I believe there also were similar 286 upgrades from the MakeIt 486 series..)

My pleasure, Jo22, thank you for replying!!!
Wow, from 286 to 486!!! That would have sent me nuts as a kid when I had the 286.

Deunan wrote:

- How it works: The 386SX has a FLT# input (active-low FLOAT signal) that will make the CPU tristate all the bus signals, includ […]
Show full quote

- How it works:
The 386SX has a FLT# input (active-low FLOAT signal) that will make the CPU tristate all the bus signals, including HLDA (hold acknowledge). It still draws some power (especially the Intel ones that need to be executing NOPs even when halted) but is otherwise "not present". The pin has internal pull-up and is not connected on mobos that don't have a jumper for it so the socket with the upgrade simply has it connected to ground and will disable the mobo CPU when the device is installed.
This particular upgrade is 5V one but there are 3V3 chips that will come with a voltage regulator on the PCB (Intel did that too with some Overdrive chips except they've put the regulator on top of the ceramic package).

- How fast is it:
Depends on mobo/BIOS since to properly use the internal cache in the upgrade there should be some support present. Best if the mobo has FLUSH# (and also KEN#) signals - these are not present on 386SX but many later '90 boards were released after Cyrix made the 486SLC and so the chipset support it. These are connected to unused pins on 386 and not so much for the upgrade sake but to make it possible to install the 486SLC in the factory. All you need to do is flip some BIOS settings.
If there is no mobo support you can use something called BARB mode by enabling it via DOS program, but this only works properly if the mobo supports hidden RAM refresh (as in, does not requst HOLD to perform the refresh cycle). This mode however will suffer serious perfomance degradation if there is heavy DMA traffic.
Without the cache the CPU itself will be only some 50% faster, with it up to 100% - so if you had 386SX at 33MHz you now have "66MHz" chip so to speak.

- Why bother?
Because it's way cheaper than a DX mobo, you only have to route 16 data lines and up to 24 address ones, also no new chipset is required since this 486 can't do burst transfers (because it has to be compatible with 386 bus). I have ALI M1217 based 386SX motherboard that I've converted to 486SXLC. This CPU has clock-doubler (needs to be enabled) so with clock generator replaced to low 25MHz I can run it at 25x2 = 50MHz. At this speed it pretty much matches a 486SX-40. And that is a cacheless mobo the size of a notepad. I think it's cool.

Wow, all these technicalities... Can I ask you something, Deunan? Do you know all these things by studying by yourself or because of academic studies of some sort? What's your background?

Thanks everybody!

I have this MakeIt 286 to 486slc with fpu underneath. Haven't tested it yet...

kixs wrote:

I have this MakeIt 286 to 486slc with fpu underneath. Haven't tested it yet... […]
Show full quote

I have this MakeIt 286 to 486slc with fpu underneath. Haven't tested it yet...

Woooooow!!!! ↑↑↑ 😲

For 386SX systems, I don't know that the regular SLC upgrades are quite worth the trouble, but the SXLC2 upgrades bring clock doubling and 8kb cache to the table, which gets you closer to a low end 486. The 16-bit memory access is not as big of a deal as lack of L2 cache in my opinion.

Lack of L2 or L1 on the SLC Cyrix vs SXLC Ti? I'd argue L2 effect is very overrated on these mobos. Look at this IBM SLC2 66MHz with 128kB of L2: Help ID this 386SX/486SLC mobo

The read timings are 24-40-72, and writes are 32-66-169 - which is terrible but then again it's another OPTi chipset.
My TI486SXLC2 at 50MHz (25MHz bus) transplanted to a most basic ALI cacheless 386SX mobo gets 27-74 for reads and constant 42 for writes since it's write-through chip.

So I loose a bit on the reads - this is significant because reads are the most common memory operations, but my writes (directly to memory!) are better than that OPTi L2 by a wide margin. Depending somewhat on the application my system is still within spitting distance. The only interesting thing about IBM mobo is VLB slots since 16-bit or not, the slower ISA clock drags games performance down a lot.

aries-mu wrote:

Wow, you all make interesting points, guys, thanks! […]
Show full quote

Wow, you all make interesting points, guys, thanks!

Jo22 wrote:

I forgot to say : Thanks for posting! 😀
(I believe there also were similar 286 upgrades from the MakeIt 486 series..)

My pleasure, Jo22, thank you for replying!!!
Wow, from 286 to 486!!! That would have sent me nuts as a kid when I had the 286.

You're welcome. ^^
I also had a 286, a 286-12 with CD-ROM and a PAS16.
It could even play some QuickTime/AVI videos and large *.wav songs on Win3.1.

Speaking of the 286 and the Cyrix 486es (SLC, DLC, SXL)..
The Commodore Amiga platform had so-called "Bridgeboards", the Atari ST and Acorn PCs, too.
You may find that interesting, too. Back in the day, they were what VMs are today.
Applications could be run in a window, side-by-side with the host operating system.
In the mid-80s, the Amiga originally had a socalled sidecar (see model A1060) which was an 8088 PC in a box.
Later on, Atari STs also had similar emulator boards that could be soldered inside of the computer (like PCSpeed/ATSpeed).
This was much faster than PC emulators of the day (believe it or not, they already existed in the 80s, just have a look for PC-Ditto)
Anyway, back to the Amiga. These bridgeboards often had 286 chips or 486 chips on them,
which made these platforms PC compatible. So installing these 486 upgrade chips on them wasn't too uncommon.
Some bridgeboards designs were even based on the 286 bridgeboard models, but included a 486 CPU core instead.
Hot-rod versions also came with high-clocked 486 chips, probably SLC/SXL types form Cyrix.

By the way, here's a quick explanation of the different types
of these Cyrix/Texas Instrument processors.:

486SLC has 386SX pin out (Single Word - 16-Bit, Low Cache)
486DLC has normal 386 pinout (Double Word - 32Bit, Low Cache)
486SXL has normal 386 pinout with bigger cache

Jo22 wrote:

The Commodore Amiga platform had so-called "Bridgeboards", the Atari ST and Acorn PCs, too.

Yup... Knock your self out with interresting reading HERE.
Lot's of nice stuff, like C64, AtariST, PC and Apple solutions for the various Amiga models.

It might be a dumb question but any idea where to get such tasty snap-in for my 386sx? eBay is empty here 🙁 asking as a last hope 😀

Marco wrote on 2024-04-01, 18:59:

It might be a dumb question but any idea where to get such tasty snap-in for my 386sx? eBay is empty here 🙁 asking as a last hope 😀

they are rare like a dwarf af two meter… and if you find one you must pay double of the maximum of you can think, forget it.

You can probably still build a decent 486 system for half the price of one. They pop up every few months, there's probably a dozen members on here trying to outbid you too.

I was afraid of these answers but also not suprised. So thanks