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Big News! P24T works in Socket 1!

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Raven

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A few days ago I contacted joekster about making an adapter for the P24T in Socket 1 systems. I had already analyzed the Socket differences and found that it basically just needs voltage applied to those extra pins (vs a 486), so it shouldn't be too bad.

He asked me if I'd ever just tried putting one in the board to see what happens, and I said no - I only own one P24T so I'd not risk it.. joekster owns many, so he tried it and it WORKED.

Apparently Socket 2 was just a gimmick - without those extra pins connected to anything I managed to play Quake for a long while, no signs of instability, and the CPU is functioning as though it were a Socket 2 board.

It would seem that those of you who run Socket 1 systems, either by choice or because of a proprietary form factor (as is the case with my Presario 425 systems and Prolinea Net/1) are not barred from running the P24T - in fact, since it has a built-in VRM it works without any modification or adaptation. Just line up the pins in the notched corner, insert, and go!

Edit: For those of you with a Presario 425 or Prolinea Net/1, change jumper 5 to "2/3" for it to not complain about this chip on boot.
 
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Yeah, it's the Pentium Overdrive "for 486 systems". Came in 63 and 83Mhz varieties (25 and 33Mhz bus), and some 83Mhz chips can be overclocked to 100Mhz by going to 40Mhz FSB (but this can also kill chips, if the chip can't manage). The fan is powered by the chip's power inputs, which is interesting. They have a built-in VRM, so can be used in 5v slots. If you take the fan off, it changes it's multiplier from 2.5 to 1.0. The minimum socket that they were intended to be used in is Socket 2, also compatible with Socket 3, and unofficially it seems compatible with Socket 1.

I often call them PODs, it's more popular to call them "PODP83" or "PODP63" (or P24T), which iirc are the model names.

Grats, you now know everything I do about them.. xD
 
There is a difference between running the PODP in a socket1 (probably socket2 as well) vs socket3. The L1 cache will only work in write-thru mode. I estimate that this would degrade performance by 15-25%.

In theory, if the chipset supported write-back mode, you could wire the write-back pin of the PODP to the write-back pin of the chipset...
 
joekster said:
There is a difference between running the PODP in a socket1 (probably socket2 as well) vs socket3. The L1 cache will only work in write-thru mode. I estimate that this would degrade performance by 15-25%.

In theory, if the chipset supported write-back mode, you could wire the write-back pin of the PODP to the write-back pin of the chipset...
Click to expand...

A possible check of the chipset support for WB/WT could be to try an Intel S-spec SX955 486DX2-66 or SX158/SX209/SK896/SK096 486DX4-100 (all of the DX4s are 3 volt CPUs). The DX2 model has a CPUID of 0436h in Write-Through mode, and 0470h in Write-Back mode. Models of the DX4 listed, have CPUID 0483h in WT mode, and 0490h in WB mode (the L1 cache on these 486DX4-100s is also increased to 16Kb).

Did you know that Raven?... ;)

All of the experimentation I did on Intel 486 CPUs (including the 486 PODs) is here...

And here is all of the other coding for the Intel 486s:

Intel 486SX: "P23"
Intel 486SX SL-enhanced: "P23S"
Intel 487SX Math Coprocessor: "P23N"
Intel 486 OverDrive: "P23T"
Intel 486 OverDrive Replacement: "P4T"
Intel 486DX: "P4"
Intel 486DX SL-enhanced: "P4S"
Intel 486DX2: "P24"
Intel 486DX2 SL-enhanced: "P24S"
Intel 486DX4 SL-enhanced low voltage: "P24C"
Intel 486DX2 SL-enhanced write-back enhanced: "P24D"

The last entry is for the CPUID 0436h/0470h model above (and an S-spec SX954 486DX2-50 I didn't talk about), and could also be for the unlisted 0483h/0490h units too...
 
No I did not know that, but the CPU tool I use (chkcpu) shows you what mode the L1 cache is in anyway, as long as the CPU is in real mode. However, for times when it is not, that is a useful bit of info to have!

I can confirm that it is running in WT mode, and I don't think this machine explicitly supports WB mode. After a bit of searching I found some indirect references for the chipset.. They turned up almost no information, though, except that it does seem to support VLB, interesting that they'd leave the slots out on this machine. I just can't find any information at all, really. Contacted NXP (bought VLSI in '99) to see if they can help. Anybody got VLSI datasheets laying around? :P

Edit: ZOMG I found one. A datasheet for one of the support chips, ironically the cache controller.
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-40/DSA-790462.html

Hell yeah, this might have what I need!

Edit 2: Well damn, this even supports up to 1MB L2 cache (that the board doesn't have). Maybe one of you could help me to come up with a way to add that...

Edit 3: It states that it supports Write-Back but it doesn't seem to have a pin for it..?
 
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Looking through my notes, I found something from over eight years ago. I don't have it sourced where I found the information, but had the POD63/POD83 documentation during that time:

"Intel recommends that the differing location WB/WT pins on the DX2/DX4 & POD63/POD83 be tied together in a Socket 3 design to handle both types of CPU"

For the DX2/DX4 that should be pin B13, and for the POD63/POD83, pin T1...

There are two different planars for the PS/2 Model 76 and 77, the "Bermuda" (which is a Socket 1) and the "Lacuna" (Socket 3). I know the Bermuda planar can run the WB/WT 486DX2. You just overhang the extra pins to put a 486POD into a Socket 1?

It could be interesting to put a POD83 in a Bermuda, then tie its pin T1 to the socket B13...

I also know that the PS/2 Model 9595 with a 'N' CPU complex (Socket 1) doesn't support the WB/WT 486DX2. That complex is verified to check CPUIDs (it will let 486DX CPUIDs of 041xh run, but not 040xh CPUID 486DXs). Check your motherboard to see if anything from the chipset is wired to B13, because other than supporting a WB/WT 486DX2, it should be a No Connect.
 
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Yeah - there are some pins that hang off in the middle too. Line up the corner with the notch, it is missing the extra row so it's easy to line up.

Thanks for that bit of info, that's half of the puzzle - now I need to figure out what to do with said pin. xD
 
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