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aegishjalmur21
January 6th, 2009, 08:32 AM
Hi everybody!,

I am interested in getting an Am386 DX40 CPU for my Compaq Portable 386 but I am not sure if the bus would support it or if I would see any significant speed increase. The motherboard is from 1987-1988 and has an Intel 386DX-20 and an Intel 387 DX16-33 Co-Processor. If anyone knows anything, let me know. Thank you! :)

Terry Yager
January 6th, 2009, 11:41 AM
If you're upgrading, why not go to a DLC? I have upgraded other Compaq models with no problem, but can't swear to the Portable 386. I'd consider it worth a try, tho.

--T

aegishjalmur21
January 6th, 2009, 11:58 AM
are you talking about the Cyrix DLC chips? Those are pretty rare to find (there is one listing on ebay, but the chip looks like its questionable). If you have any extra's or know of someone with an extra Cyrix DLC chip, let me know!

Druid6900
January 6th, 2009, 06:24 PM
I have no idea if it would work at all, depends on the bus speed/multiplier.

However, if you DO find out, I have a working AMD Am386 DX/DXL-40 chip sitting right next to me on the desk here.

patscc
January 7th, 2009, 12:23 AM
The AMD 386 DX40 requires a bus speed of 40 Mhz if you want it to run at 40 Mhz core. It doesn't have any clock doubling circuitry like the 486 DX2.
As far as I know, the bus speed on the Portable 386 is fixed at 20 Mhz, not withstanding the "compatability" slow speed boot.
I don't remember the CPU being socketed, but that's just me. Even if you ramp the bus speed up, the FPU and probably most of the other stuff on the bus wouldn't be able to keep up.

TI made a TX486DLC-25GA which, while specked at 25 Mhz, should still be stable clocked at 20 Mhz
I'm not sure if Cyrix made a 20 Mhz version, but I don't see any reason why the 33 mhz one shouldn't work.

patscc

IBMMuseum
January 7th, 2009, 08:45 AM
The AMD 386 DX40 requires a bus speed of 40 Mhz if you want it to run at 40 Mhz core. It doesn't have any clock doubling circuitry like the 486 DX2.
As far as I know, the bus speed on the Portable 386 is fixed at 20 Mhz, not withstanding the "compatability" slow speed boot.
I don't remember the CPU being socketed, but that's just me. Even if you ramp the bus speed up, the FPU and probably most of the other stuff on the bus wouldn't be able to keep up.

TI made a TX486DLC-25GA which, while specked at 25 Mhz, should still be stable clocked at 20 Mhz
I'm not sure if Cyrix made a 20 Mhz version, but I don't see any reason why the 33 mhz one shouldn't work.

I was going to make the same observation, as the OP seems to have it wrong that dropping a higher speed rated part in will make the system run the CPU at that speed. There are Cyrix/TI/IBM parts that are internally clock-doubled to 40MHz from a 20MHz external clock. Almost all of the 386DX implementations should be PGA (Pin Grid Array).

The 387DX, if installed, will run at the 20MHz external clock. A multi-speed marking ("16 - 33MHz") just means the FPU is the latest stepping, and is able to operate over those ranges of speed. I have often overclocked Intel 387DX FPUs to 25% over rated speed (which is like a 16MHz part at 20MHz, 20MHz part at 25MHz, 25MHz part at 33MHz, 33MHz part at 40MHz) with no troubles.

aegishjalmur21
January 8th, 2009, 11:33 AM
Thanks everybody for your advice! I did find on Ebay a TX486DLC-33GA. I think I will bid on that and install it. I just got an expansion port for it today, so I can put VGA and Soundblaster on it. :-)

patscc
January 8th, 2009, 04:19 PM
aegishjalmur21, if you have the time, do some before/after benchmarks & post the result so that all of us unlucky enough to have missed that 486DLC can see the results of your upgrade.
patscc

Anonymous Coward
January 8th, 2009, 07:40 PM
486DLC is not rare at all. TI made tonnes of them.

However, the chip that you *really* want is the TI486SXL-40. It's almost the same as the 486DLC, except that it has 8kb cache (rather than 1kb), and optional clock doubling so you can run it at the full 40Mhz on a 20MHz bus.

dorkbert
January 30th, 2009, 03:13 PM
Don't recall if it was DLC or DLX, I think one of them will have cache coherency issues without proper motherboard support (an extra pin that invalidate cache on DMA if memory serves.)

Anonymous Coward
January 30th, 2009, 04:11 PM
If you get the last release of the cyrix control software, you can do a pretty good job of taking care of the coherency issues. The software I have in mind is the one that came with the Cyrix 486DRX2 and SRX2 chips.

vwestlife
February 2nd, 2009, 07:57 PM
I am interested in getting an Am386 DX40 CPU for my Compaq Portable 386 but I am not sure if the bus would support it or if I would see any significant speed increase. The motherboard is from 1987-1988 and has an Intel 386DX-20 and an Intel 387 DX16-33 Co-Processor. If anyone knows anything, let me know. Thank you! :)
A 386DX-40 will not give you any benefit. Your computer will still be a 386DX-20 unless you change the CPU clock frequency, which is not possible on most 386 computers. The speed of the CPU chip only indicates its maximum allowable speed, not necessarily the speed at which your computer will run with it installed!

The middle one in the following photo is the chip you want: the Cyrix 486DRx2 is basically a clock-doubled 386DX with internal cache and a 486-compatible instruction set, so while it's really still a 386 at heart, it will appear to be a "486" to your computer and programs. It will also double your Compaq's speed from 20 to 40 MHz, through the clock doubler. It's not as good as a "real" 486, but it's still an effective upgrade and will give a noticeable improvement in speed.

http://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/KL_Upgrade_386.jpg/317px-KL_Upgrade_386.jpg

Note that a 486DRx2-25/50 or -33/66 would work just as well for you. Again, this only indicates the chip's maximum speed rating.

A 486DLC is also a good upgrade, and is much easier to find than the 486DRx2, but it is not clock-doubled, so your computer would still run at 20 MHz, and it has a smaller internal cache compared to the 486DRx2.

Anonymous Coward
February 3rd, 2009, 08:52 PM
There is a DRx2 20/40 on ebay in a vintage CPU vendor's store:

http://cgi.ebay.com/CYRIX-CX486DRX2-20-40GP_W0QQitemZ260282905882QQcmdZViewItemQQptZLH_De faultDomain_0?_trksid=p3286.m20.l1116

However, I highly recommend the TI486SXL-40. It is a 386 pinout chip with the full 8kb internal cache, which makes it quite a bit faster than the Cyrix part.

http://cgi.ebay.com/TI-486-SXL-40-Unsolder_W0QQitemZ230189572240QQcmdZViewItemQQptZL H_DefaultDomain_0?_trksid=p3286.m20.l1116

I don't know why the TI486 is so expensive. I bought a few of them last year for peanuts.

vwestlife
February 4th, 2009, 04:33 AM
However, I highly recommend the TI486SXL-40. It is a 386 pinout chip with the full 8kb internal cache, which makes it quite a bit faster than the Cyrix part.
But the TI is not clock doubled. Installed in a 386DX-20 machine, it will only operate at 20 MHz. The Cyrix will clock-double this to 40 MHz internally.

Anonymous Coward
February 4th, 2009, 01:57 PM
All TI486SXL chips are clock doubled whether they are labeled as such or not. The clock doubling is controlled by one of the registers. By default clock doubling is disabled, and must be turned on by software...even with the 486SXL2 chips.

The Cyrix DRx2 and SRx2 chips are locked in 2X mode, and cannot be changed. All DLC chips (including the ones sold by TI) are 1X chips only.

In my personal experience the DRx2 is more trouble free than the 486SXL, but the 486SXL is much faster once you get both the clock doubling and 8kb cache enabled. I have one installed in an original IBM AT at the moment, using the 2X internal multiplier + 3X external multiplier for a total of 6X.

vwestlife
February 4th, 2009, 05:37 PM
Even better would be an upgrade based on the IBM "Blue Lightning" chip. This was basically a clock-tripled 486DLC with 16 kB internal cache. The Blue Lightning (a.k.a. 486BL3 or 486BLX3) had its 15 minutes of fame for being both the first clock-tripled x86 processor and the first x86 to reach 100 MHz.

IBM also had a 486SLC with 16 kB internal cache, in regular, clock-doubled, and clock-tripled forms, which were widely used in lower-end IBM and Ambra computers at the time. The 486SLC3 was sometimes also known as "Blue Lightning."

To add confusion, IBM later gave the "Blue Lightning" name to their OEM version of the Cyrix 486DX2.

Anonymous Coward
February 4th, 2009, 06:36 PM
Agreed...the Blue Lightning lightning would be your best choice. However, your chances of finding one are extremely thin. I saw one about two weeks ago, but they're very hard to come by.

I have one of these installed on a 386 system, and the thing really flies.

I'm not sure if the 100MHz parts actually exist or not. The only ones i've ever seen are either 60 or 75MHz. I tried to overclock mine to 100MHz, but it didn't work.

http://lh6.ggpht.com/_CzHY0sEvu6A/RpyxCXJUUmI/AAAAAAAAAOc/4NJP4xPa568/s800/Blue%20Lightning.jpg

IBMMuseum
February 4th, 2009, 08:06 PM
Agreed...the Blue Lightning lightning would be your best choice. However, your chances of finding one are extremely thin. I saw one about two weeks ago, but they're very hard to come by.

I have one of these installed on a 386 system, and the thing really flies.

I'm not sure if the 100MHz parts actually exist or not. The only ones i've ever seen are either 60 or 75MHz. I tried to overclock mine to 100MHz, but it didn't work...

On the proper 386DX system (read PS/2) you could CPUID it:

http://www.ibmmuseum.com/interrupts/INT15h/INT15hC9.html

First digit: A (a value IBM used in the level of these CPUs)

Second digit: IBM CPU class (a 3 on the 386SLC, 4 on the 486SLCx CPUs)

Third digit: Clock multiplier value (i.e. 0 on the 386SLC, 2 on the 486SLC2, 3 on the 486SLC3)

Fourth digit: Stepping version: 0 to Fh