486dx2 System Question

[Invisix]

Hey all,

Since so many of us here are classic gamers, etc and even though this is not Amiga related, I still didn't know where else to go to ask this, so here goes.

I recently acquired a complete 486dx2-50Mhz system (Monitor, System, Printer, Keyboard, Mouse, etc) for around $15 USD (cheap compared to eBay prices), and I found an 80Mhz DX2 produced by Cyrix I had laying around, and I am wondering how "fast" the CPU is running at now that I replaced the default 50Mhz DX2 with the 80Mhz DX2.

If I remember correctly 486 dx2 are a "speed doubler" technology from the system bus which as far as I know is 25Mhz bus (the computer is a Tandy 4850 EP)... so my question is this... what speed is the CPU really running at? My assumption is 50Mhz, even though the CPU is 80Mhz? If that's the case i'll just put the default Intel 486 DX2-50Mhz cpu back in and leave the Cyrix 486 DX2-80Mhz in case I ever get a 40Mhz bus 486 board.

[koaftder]

Run msd.exe if you have dos, select the computer button, it might tell you there.

[koaftder]

Select the letter corresponding to what you want to know more about. I don't have a machine running DOS but if i remember correctly, it will tell you processor brand and speed.

If you have win95 or 98, i think it still has msd, also your system might have msinfo, which can help out too.

[Ronmor]

If I remember correctly the dx2 ran at 66 Mhz and the dm4 at 144 Mhz. It's been years, I got the 486 dx4 to play Battle Isle 2200 I got hooked on Battle Isle on the Amiga.

[BlackMonk]

"To attain full operating speed the Am486DX2-80, the Am486DX4-120, the Cyrix 80 MHZ Cx486DX2, and the Cyrix 5x86-120 must have an external bus clock running at 40 MHZ. VLB (Vesa Local Bus) motherboards were not speced to run over 33 MHz. Running VLB motherboards at 40 MHz is over-clocking. This is a risky gamble."

Pay no attention to that rambling about DM4 at 144MHz or whatever.

DX2 = 2x clock
DX3/DX4 = 3x clock

486DX4 75MHz = 25 MHz x 3
486DX4 100MHz = 33 MHz x 3
486DX4 120MHz = 40 MHz x 3

I couldn't find your exact model offhand, but the pictures may be of use:

http://www.jumpers.computed.net/m/m486_8.htm

You WILL have to play around a bit.  Use the 80 MHz CPU to do this.

I would guess your motherboard is similar to:

http://www.jumpers.computed.net/m/S-T/33762.htm

I *think* your options will be to run at a 25 MHz system bus or a 33 MHz system bus.  You currently are running at 25 MHz.  You can at least get that 80 MHz to run at 66 MHz.  Just plop it in there and tell your system to run at 33 MHz and that you have a clock-doubled CPU.  If you try this with the 50 MHz rated CPU, you might just burn it up or break something else.

The most common 486 bus speeds were 16 MHz, 25 MHz, and 33 MHz.  There was a (rare) true 50 MHz but it was expensive and most CPUs of the time wouldn't be able to hit 100 MHz and work in those motherboards.  Some later motherboards could also do 40 MHz but again, that's going to be a bit more rare and you'll not likely run into that.

So your best bet is trying to get that 33/66 deal going with that 80 MHz CPU you have.

MSD and MSInfo will be useless to you, don't bother with them.

You might have better luck with some of the stuff here:

http://www.opus.co.tt/dave/utils.htm

Specifically, the section starting with PC-CONFIG.

Old DOS and 486 systems are obscure and require an obsolete set of knowledge to hack properly.  Be careful of any information you receive if it's from the Amiga community.  No offense to the blokes around here, but this is the AMIGA community, not the 486forever community, if ya know what I mean.  I'm mostly a PC guy and know very little about the Amiga 500 I play around with from time to time.

[DrDekker]

I should image it's running at 50MHz.  My first IBM compatible was a 486 DX2 @50MHz and I upgraded it to 80MHz too.  Although it was along time ago, I'm pretty sure I re-jumpered the motherboard to clock at 40MHz.

Perhaps a 100MHz DX4 'overdrive' CPU may work in your Tandy?

[BlackMonk]

It depends on his motherboard.  He might not have jumpers to change the bus speed.  If he got a DX4-100, it would run at 75 MHz in his system the way it is currently set up.  He currently has 25 MHz with a double-clocked CPU, making the CPU run at 50 MHz.

About 80% of all DOS-based programs will run rather well on that CPU.  Just make sure you have 8 MB or more of RAM and know how to use LOADHIGH and DOS=HIGH, UMB and all that junk, and try to use MS-DOS 6.22 if possible.

His other limiting factor would be the video card, it's probably a built-in deal or he only has an ISA expansion card.  You CAN get some ISA-based video cards, but most of them will only be "windows accelerators", meaning they don't do jack for DOS or VESA-mode gaming, but they accelerate the drawing of Windows primatives like dialog boxes and... lines.  Wooo.

The best DOS-based gaming cards were PCI or VLB based, #9 Imagine128, Matrox Millennium, and perhaps some old Diamond Weitek-based VLB card.

IF he can get that sucker upgraded to a 66 MHz CPU via messing with the bus speed, and maybe throw in some more RAM, that's probably the extent of what he can do without better gear or more specialized knowledge.

If you have a better system, you might want to just use DOSBox.  But if you don't want to go that route, I'm there with ya... I have several old 486-class systems myself.

[da9000]

Follow what BlackMonk says, and to reiterate:

If it says "DX2" on the chip, it's _DEFINITELY_ not a chip that runs at a 50Mhz *BUS* clock (I'm not talking about the internal/core speed). Those chips were very rare and they were marked as DX-50, not DX2-50. They ran at 50Mhz BUS frequency (very fast) and 50Mhz CORE frequency.

Now, the DX2 chips used either a 25Mhz or 33Mhz or 40Mhz BUS frequency and they had a fixed internal multiplier like BlackMonk listed. The bus frequency was determined by the motherboard (and this infact means that you could even use a 16Mhz or 20Mhz clock, which would severely underclock the chip - I've done that in the past while trying to figure out what the jumpers do).

So if you want you system to be the fastest, no matter if you use the DX2-50 or the DX2-80, set the frequency to 33Mhz (or higher if it allows) and see what happens. Of course use a fan, to make sure you don't have any melt-down, but in general the 486s where tough - never seen one melt (internally or externally - usually the melt-down is internally, which makes the CPU just not work - like a brick). Next best thing to do to find out the actual CORE frequency is to use some software for benchmarking (either dhrystones or somesuch, but it's been a while since I've run some to remember names - also, some cache/memory testing programs will give you Mhz rating via using either CPUid instruction or doing instruction timing -- come to think of it, the easiest way is to use a bootable Linux distribution, like Knoppix and seeing the Bogomips of the booting kernel -- or faster yet, a boot disk from Slackware will do the same thing as soon as it boots the kernel)

[Invisix]

Thank you all for your comments, and help. I checked an old 1994(?) post from a DOS newsgroup and it stated the fastest CPU that the Tandy 4850 EP can handle is a DX4 75Mhz (DX4's are triple-clocked the system bus speed). My system BUS is apparently locked-in at 25Mhz, so my CPU is definately running at 50Mhz so I went ahead and switched out Cyrix I put in, and put back-in the default Intel 486 DX2 50Mhz chip.

The mainboard has a riser slot, so I have 3 ISA slots via a riser card. As for graphics, it is currently 512k on-board SVGA graphics, I can upgrade it to 1mb via on-board ram chips if I chose too for 1024x768x256-colour. The system doesn't have any VLB slots.

For now i'm happy with the system, I am just curious, as for now I am currently looking for a total of 32mb 30-pin SIMM's so I can max-out it's ram. I'll probably just get 8 4mb 30-pin SIMM's so I can fill-up all the SIMM slots, thus have 32mb RAM.

As for Dosbox, yes I have a modern PC... 64-bit AMD Dual Core, and all the bells & whistles... however just emulating the Amiga, Commodore, etc... nothing can beat having the real thing. :-D

[DamageX]

Old DOS and 486 systems are obscure and require an obsolete set of knowledge

Ah yes, one of my various areas of obsolete expertise

If you were to install a DX4 in place of a DX2, you may run into a problem with voltage or with write-back cache (which needs to be supported by the BIOS). That's what those adaptor boards that fit between the CPU and socket were for, to drop the voltage to 3.3 or 3.45V and disable the write-back cache. Then the DX4 or AMD 5x86 (and sometimes the Cyrix 5x86) can be used in an old board.

There is also the Pentium Overdrive chip (IIRC you need socket 3 for that, as socket 1 didn't have all the pins)

You'd have to try hard to cook a 486 chip. I ran a 3.3V Cyrix 486-100 (which has write-through cache) in a 5V board for quite some time before I learned that it wasn't supposed to work, yet it didn't have a problem.

40 and 50MHz VLB boards were not uncommon, though sometimes the higher speed settings were undocumented, and in practice there weren't as many CPUs around that officially supported those speeds anyway. But the 5x86 and Intel DX4 CPUs supported different multipliers, 2x, 2.5x, 3x, and 4x, depending on the paricular model CPU. I have a couple of VLB socket 3 boards that support 60 and 66MHz bus. I had a Cyrix 5x86-120 in there running at 60x2 with a Trident 9440 and it could do 50MB/sec. That combo wasn't completely stable though. Later I got a PCI socket 3 board and ran the Cyrix chip in there also at 60MHz x2 (had to divide the PCI clock down to 30MHz, else the onboard IDE would crap out). That one was stable, and the L2 cache was faster on it but the video and IDE was not as fast.

Your onboard video may be faster than what the ISA bus can manage. ISA cards max out around 5MB/sec. You can check with the program SVGASPD http://www.hyakushiki.net/junk/memspd.zip

I have at least one set of 4 4MB 30-pin SIMMs in my pile, as well as various 486 motherboards, VLB and ISA cards, and CPUs

[Invisix]

@DamageX

If you are willing to part with anything (4 4MB 30-pin SIMMs in my pile, as well as various 486 motherboards, VLB and ISA cards, and CPUs) how much would you like for each? Also what type of ISA cards? I'm hoping to hook myself up with a Sound Blaster 16.

Thanks.

[koaftder]

@invisix

I have an SBPro 2 ISA and SB Vibra 16 ISA card you can have for post if you want 'em.

I'll look and see if I have any spare sims from that era.

[Belial6]

To make things more difficult, you need to keep in mind that Cyrix was using a PR-rating.  It was an unrealistic hope as to how fast the chip was in comparison to an Intel chip.  So, if the chip is labeled as an PR-80, it is likely not an 80Mhz chip.

[Invisix]

koaftder wrote:
@invisix

I have an SBPro 2 ISA and SB Vibra 16 ISA card you can have for post if you want 'em.

I'll look and see if I have any spare sims from that era.

:-o :-o :-o Dude, that would completely rock! PM me with details on how much shipping (post) would cost to Tampa, FL USA for them, i'm definately interrested in SIMM's too if you have any.

[koaftder]

I found the box of SIMMs. I don't remember much about them. Some are 32 pin, others 72. I only remember seeing the 32 pin ones on 386 machines. Do you have 72 pin SIMM sockets? I'm guessing the EDO variety doesn't work on most 486's? I snapped a picture of them. Is there an easy way to identify them?

Picture of old ram modules