486SX and memory problems

[villerk]

Yes, I know, this is propably a bit out-dated thread

Recently I got a MikroMikko 486SX/25 for free. It didn't have any memory, so I installed two 16 MB 60 ns SIMMs. (I have all kinds of old stuff left, if you're wondering where the heck I could get a hold on a few SIMMs). It didn't even boot. Next I tried two 8 MB SIMMs. Nothing. Only 4 MB SIMMs worked properly. Then I got an idea and installed 4 MB SIMM to slot 0 and a 16 MB SIMM to slot 1. Tada! It booted and reported that it had 20 MB RAM! But, when I tried to install an OS, it crashed every single time. I tried DOS 6.22, Windows 95, Mandrake and Slackware Linux (with about fifteen different kernels; I compiled a few of them myself). Only Linux told something usefulll: FPU error. I gather that as the SX hasn't an FPU (i.e. it isn't DX model), it can't handle so big memory space.

Is there any way I could put over 8 MB RAM to this good ol' pal of mine?

[Technoid]

never heard that SX 486's would be limited in memory in any way...

Could be that the bigger SIM's are "EDO" and that the smaller ones are "FPM"

EDO usualy only works in p133+ machines....

[villerk]

How can I tell what they are? Does it read somewhere on the chip?

And if they are EDO's, how come the BIOS recognizes them correctly?

[rugger]

hmmm, it sounds like a cheap board

Given that it is a cheap 486 board, and there probably wasn't anything above 4meg simms common back then, the actual address wires to handle > 4meg simms are probably not correctly wired.

The FPU error is probably a symptom of memory errors causing problems in the linux kernel, maybe in the FPU emulator.

[Och]

Dude, you need to install memory in pairs on that board, like 4mb in simm1 and 4 in simm2. sim 3 & 4 also have to be in pairs, but they dont have to be same as 1 & 2. for example u can have 4 & 4 in simm 1 & 2, and 8 & 8 in simm 3 & 4. Also, all memory either has to have parity or not have it, otherwise you'll be getting tons of errors.

[Wombat]

A 486 can handle a much larger address space than a few megs. And it's been said before, but SIMMs must be installed in matching pairs.

[Tempest]

I had a 486DX machine that required that all 4 SIMMs are of the same type and size. Two SIMMs did not work, and neither did four SIMMs of 2+2 and 4+4 megs.
With some luck you might actually be able to find the manual for your MikroMikko somewhere

ps. from http://www.memorysales.net/faq-new.htm

-386DX, 486SX and 486DX computers that use 30 pin SIMMs require that upgrades to be done in increments of 4 SIMMs of the same size.
-386DX, 486SX and 486DX computers that use 72 pin SIMMs can be upgraded 1 SIMM at a time.
-30 Pin SIMMs with 3 or 9 chips on them are Parity. 30 Pin SIMMs with 2 or 8 chips on them are Non-Parity.
-Install memory starting with the lowest numbered socket or bank (see your owners manual or UpgradePro for more information).
-All 586 computers require SIMMs to be installed in pairs of the same size.
-EDO memory (Extended Data Output) will only work in computers that specify EDO memory.

[CHHAS]

EDIT:

Bleh, that's what you get for forgetting to post when you write, no reason to iterate

[Marshmallowman]

if you are using 36 pin simms you must use pairs for an sx machine 4 for adx machine.

sx's are 16bit and 36 pin simms are 8bit so 2 for sz
dx's are 32bit so they need 4
they should be matched,

if your using 72pin simms you only need one and they don't have to be matched, but best if they are.

EDO are not designed to work with a 486's only normal dram and fast page will work.

486 have problems addressing some simms
(especially double sided stuff)
the simm slots must be populated in the correct order simm 0 to 3..etc
if you use double sided you may have to populate every second simm slot
I found with 486's the best bet is to use 1's 2's 4's and the odd 16m stick
if you want 32pin simms I have a whole tray load.

[Wombat]

huh? I really don't think that the sx and dx have different bus widths. They're still the same processor after all. One just has the connection to the math co-processor severed. They're even the same die.

Now, to cut down on the confusion, is this a 30- or 72-pin setup?

Also, I think EDO RAM is always backwards compatible. EDO is an extra functionality, and you have to engage EDO mode to use it. I know I've put EDO RAM into system that weren't made for it, and it worked perfectly.

[xortam]

Originally posted by Wombat
huh? I really don't think that the sx and dx have different bus widths...

Internally that's true but externally the SX is 16-bit data vs. 32-bit data for the DX. I've still got my 486-66/DX2 PC (w/ P83 overdrive chip) sitting next to me, though I haven't fired it up in several months.

[dZeus]

Wasn't the difference for 386 SX vs. DX the 16 vs. 32, and for the 486 is only was disabled vs. enabled math coprocessor?

[KeiFront]

Yep 486 "SX" just means that it doesn't come with a math coprocessor.

A 386 SX has a 16-bit data bus and no math coprocessor, while a 386DX is a true 32-bit processor. I'm not sure if a 386 (DX) has a coprocessor I believe neither has one.

[Sasq]

Yup neither of the 386 varieties had a coprocessor

Dan

[KeiFront]

I Just remember that you could add a coprocessor to a 486SX I believe it was called a 487SX. Would be funny if I still had my 486SX-25 I feel a bit sorry that I selled it.