baeuchlein on 27/6/2009 at 17:12

---

I am trying to adapt a few CPUs to games and operating systems of different eras as well. Changing the CPU speed can be important for success, but it's not always enough.

The AMD Socket A Durons can all operate with a FSB of 100 MHz. Some (the ones with the "Applebred" core, running at 1.4, 1.6 and 1.8 GHz) are meant for 133 MHz, but should operate at 100 MHz FSB with lower CPU speed as well. I had one of these 1,8 GHz Durons which was recognized by the mainboard as a 1350 MHz Duron when 100 MHz FSB was applied instead of 133, but I had no time to test whether everything ran OK.

However, I have an Athlon XP 3000+ ("Barton" core, meant for 166 MHz FSB) which works reliable at speeds between 1230 MHz and 1733 MHz. I have a Duron meant for 850 MHz which works well at 765 MHz (90 MHz FSB), 808 MHz (95 MHz FSB) and about 930 MHz (FSB > 100 MHz). An Athlon meant for 900 MHz works with 810 MHz (90 MHz FSB) as well. All operate without any noticeable problems below their maximum speeds. So, getting below maximum speed with the K7 Athlon/Duron CPUs for Socket A depends on what the mainboard does.

One board (Gigabyte GA-7IXE4, an ancient board still using SDRAMs instead of DDR's, and having problems with its AGP port) can set the FSB to 90, 95, 100, 105, 110 and 115 MHz. Anything below the standard 100 MHz FSB works well. 105 and 110 MHz worked well with the hardware I used, except for some memory modules which were PC100 spec, meaning they were designed for 100 MHz FSB. And once they grew old, some of them were even more reliable with FSB < 100 MHz. However, since the PCI bus frequency was derived from the FSB speed as well, I overclocked all PCI cards as well as all on-board PCI devices (IDE and USB controllers and the AGP slot). This was a problem with 115 MHz, were occasionally disks and/or CD-ROM drives were not found by the board.

Another board (Gigabyte GA-7VR; DD-RAM ("DDR")) could set the FSB to 100 or 133 MHz, but allowed me to raise the FSB in steps of 1 MHz to about 165 MHz. Unfortunately, the IDE drives were not detected reliably anymore even with 105 MHz FSB instead of 100.

Since this board did not allow me to lower the FSB speed, the board could only be used with 100 and 133 MHz FSB.

Both of these boards could not change the multiplier of the CPU.

A third board I use is the ASRock K7VT4A Pro, currently housing my Athlon XP 3000+. This board has jumpers to change the multiplicator, but either these do not work, or I have a CPU with its multiplier locked.

However, this board supports 100, 133 and 166 MHz FSB, and then you can lower this FSB in steps of 1 MHz down to 50 MHz or less. That means you can try to let this Athlon XP 3000+ run at its maximum frequency (166 MHz FSB x 13 = about 2158 MHz) as well as 650 MHz (50 MHz FSB). Unfortunately, the computer would not boot with such a drastic change. 140 and 150 MHz did work, though, so anything between 100 MHz FSB (1,3 GHz CPU speed with this CPU) and 166 MHz FSB (2,1 GHz) should be OK. However, I have stability problems with two Athlon XP 3000+ CPUs which vanish below 1,8 GHz, so I usually set my CPU to 1,3 or 1,7 GHz.

Furthermore, Windows 98 and 98 SE have problems with CPUs running at about 2,1 GHz or more. Common symptoms are errors with the "IOS device" (whatever that is), says Microsoft. Although I have not seen error messages with the Athlon at 2,1 GHz, Win98SE definitly became less stable. It usually works well with the CPU at 1,3 and 1,7 GHz.

So, for a Win98 based computer, your maximum speed is at about 2 GHz. The lowest speed you can safely assume to be selectable is the one set with a FSB of 100 MHz and the multiplier of your CPU. Anything else depends on a lot of things including the mainboard, and no one can really guarantee that you can go below the minimum speed attainable with a FSB of 100 MHz.

Some ancient games meant for MS-DOS - games from about 1997 and earlier - have additional troubles with CPUs that are "too fast". Some will run into problems when your CPU has a cache (which is the case since the ancient days of the Intel 486 processor); in this case, configuring the game to run without sound or using the PC speaker instead of a sound card often helps. If you can switch off the on-CPU cache, this problem can be averted sometimes, but I have never seen anything above the ancient Pentium-I CPUs that could do that.

Other DOS games just can't handle "too fast" CPUs. In that case, you can sometimes use one of a few slowdown utilities like Mo'Slo, or try to under-clock your CPU. Sometimes it works, and sometimes it doesn't.

I have little experience with the Socket variants of the P-II and P-III CPUs. These should be based on FSB speeds of 66, 100 and 133 MHz.

I have a Pentium-II meant for 233 MHz (66 MHz FSB x 3,5) whose multiplicator isn't locked, and a MS-6116 mainboard which allows me to change the multiplier. I can set the CPU to 200 MHz (66 MHz FSB x 3,0), and the CPU runs stable. However, anything above 233 MHz means the CPU will overheat and become unstable after some time. Even 250 MHz (100 MHz FSB x 2,5) does not work for prolonged periods of time.

A Pentium-II meant for 350 MHz (100 MHz FSB x 3,5) has its multiplier locked. It runs at 233 MHz (66 MHz FSB) without any problems. No other options are available with this CPU and the MS-6116 board.

As you see, there is not much except for the standard frequencies which is guaranteed to work, so you should check which mainboards and CPUs you can still obtain, and then ask about their limits or figure them out on your own. Another possibility would be to tell us if there are specific games you want to run on that computer; maybe someone has already had the same problem and solved it with that particular game.

Last but not least... don't forget about the peripherals (HDD, CD-/DVD-ROM drive, sound card or sound chip, and so on) you want to use. Most old boards cannot use SATA hard disks because they only have IDE/ATAPI connectors. And several pieces of hardware simply cannot be used in Win98 because there are no drivers for this old OS. A lot of hardware won't work in MS-DOS mode, especially modern soundcards.

Depending on what games you want to play and which hardware is still available, your choices may be very limited.

All right, then. My fingers are falling off, it's time to stop typing...:cheeky:

EDIT: One more thing, though. None of the boards I mentioned are µ-ATX boards.

gerwin on 28/6/2009 at 20:17

---

Some months ago I was also messing around with underclocked P-II and P-III: The results can be read here: (http://vogons.zetafleet.com/viewtopic.php?t=20758) vogons; Slowing a 440BX based PC



Was messing around in the area of 133Mhz minimum to 450Mhz maximum, But there are some caveats. Would like to hear what you choose to use eventually. Myself, I do have a rare unlocked P-III now, but still haven't decided on what system to build around it.

gerwin on 28/6/2009 at 21:54

---

Nice to know you are willing to take care of it, but it is a thing I must do myself. ;)
I was thinking of putting the unlocked P-III in an Intel brand mobo: the SE440BX or SR440BX if I recall correctly, because then one can use the newspeed.exe Dos tool to set the multiplier without rebooting. At least, that is what I understand from reading about it.


Well, of course I did test out that unlocked P-III as soon as I got it, I wrote things down in the same topic I posted earlier:

I don't know why someone would say 5x is the minimum, because 3x obviously works.

The other main point was that certain normal retail P-II CPU's are not multiplier locked, but multiplier limited. Giving you a speed range of 133MHz to 400MHz with a single CPU, although the L2 caches don't work below 200MHz. Still, having 133MHz selectable is quite nice.

None of the CPU's I have tried accepted multiplier 1.5x, which an AOpen Mainboard can set. I wonder if there is any CPU out there that will accept it and run at 100MHz...

gerwin on 28/6/2009 at 22:13

---

It is a Coppermine core yes. Like I said, a P-II Deschutes core seems to go lower then an unlocked Coppermine. So there are some differences, but I suspect all Coppermines are similar in this regard.


I don't know if it is µATX, but an intel SR440BX looks small. (I am a bit of an 440BX chipset fan, because they make such nice Dos machines) If full size mainboards are also an option this incomplete list might be a starter: (http://vogons.zetafleet.com/download.php?id=6271) Slot-1 440BX mainboards