SIMMS, Parity, and Nanoseconds - oh my!

[Raven]

I grew up in the era where SIMMs were rapidly disappearing and DIMMs were becoming cheaper by the day. Considering I didn't do computer repair at the age of 2 or 3 (though I did learn how to configure my soundblaster! :D) I don't have much experience with SIMMs.

My method thus far has been to haphazardly put SIMMs into a machine, and if it shows an error or refuses to POST, swap the latest added SIMM (yes I know this should be dangerous, but so far nothing has ever fried or died..). This has worked splendidly for all pre-EDO/Parity/etc. boxes like 386s, and for all post-EDO/Parity/etc. boxes like early Pentiums. However, the 486 72-pin machines are a pain. I had 24MB of RAM in my ValuePoint. During my ZIP drive adventures I ended up reducing it to 16MB and am not even sure it was properly seeing the 24MB to begin with. Anywho, I have become aware that it should support up to 64MB, and in the spirit of maxing out this machine, looked through my gigantic bin of RAM (yes, I have a bin of RAM... :D) and tried many sticks. After a long period of "fun" trial and error, I decided to look at the 16MB that has been working so well. It's 70ns Parity RAM, 8MB stick * 2. I looked through my collection, noting that most were unlabeled. I tried many more sticks to no avail. I found a 70ns 4MB stick, and so tried to pair it with various other 4MB sticks in the hope that one of them would be a 70ns parity stick - no such luck. I found an 80ns 4MB parity stick, but it will not work properly in the machine (not sure why, the chip configuration or the speed is unhappy with the other stick I guess). I learned online how to identify parity sticks roughly by counting the chips, and tried any logical sticks.. no luck..

While this is not my model, this motherboard looks most similar to mine (still different) out of the ValuePoints on stason: http://stason.org/TULARC/pc/motherboards/I/IBM-CORPORATION-486-PS-VALUEPOINT-6387-T-TYPE-1.html

It has a chart for RAM which I ripped from the page to post here:

Size Bank 0 Bank 1 Bank 2 Bank 3
16MB (1) 2M x 36 (1) 2M x 36 NONE NONE
32MB (1) 2M x 36 (1) 2M x 36 (1) 2M x 36 (1) 2M x 36
32MB (1) 4M x 36 (1) 4M x 36 NONE NONE
64MB (1) 4M x 36 (1) 4M x 36 (1) 4M x 36 (1) 4M x 36
64MB (1) 8M x 36 (1) 8M x 36 NONE NONE

According to this I have two choices to attain 64MB: 4x4Mx36 sticks, or 2x8Mx36 sticks. Now, I was under the impression that a stick marked 4Mx36 was a 4MB stick and that a stick marked 8Mx36 was an 8MB stick, and that typically a SIMM was marked something along the lines of "4MB 2x36", not like in the table above. I ask for help in understanding this table and selecting a variety/source of some RAM for my quest here.

Thanks!

Edit: Also, could someone explain why it restricts you to certain combinations that include blank banks? That doesn't make much sense to me, I understand you need to pair and match varieties of SIMM, but why leave blank banks? Why can I not put 4x8Mx36 and get 128MB [maybe the BIOS doesn't support that much RAM, is there another reason?]?

 

[Druid6900]

Ok, the short answer is to take the number AFTER the x (YMxnn), divide it by 9 (8 bits plus parity bit) and multiply the number BEFORE the x by the result.

For example: 4Mx36 - we divide 36/9 which gives us 4. We multiply the 4M by 4 for 16M.

On non-parity strips, you'd do the same thing, by divide by 8 IE 4Mx32

 

[Raven]

There isn't a chance in hell that I could get 256MB in there with this set (http://cgi.ebay.com/4-x-64MB-256MB-...emQQptZLH_DefaultDomain_0?hash=item563b6ec7ca) is there?

Edit:

What's the deal with EDO vs NON-EDO and whether what will work in what box? I've seen boxes run with a mix of EDO and non-EDO, some reject EDO, what's up?

I stuck two 8M kingston sticks in the other two slots and it only threw a "memory size error". Usually when that happens I go into the bios and it informs me that there's 28MB when I put in 24MB or some such. This time I put in 32MB and it says 32MB, so I chose to ignore it and try memtest.. it's running now...

Edit 2:

Some bastard just HAD to scratch the sticker off of this stick.. I have a PNY 4x36 "16 MB Upgrade" parity stick. I also have one that looks quite similar (same number and configuration of chips, 8 per side 16 total), and the chips are marked P which I believe means parity on both. I think I've tried these in a bank and they did not work.. but who knows because the sticker is gone! Gah.. I'll try it again I suppose after this memtest is done.

Edit 3:

If I have 2x8MB sticks in there, and then I put 2x16MB sticks in the second two slots, and it throws errors, is it possible that it would work if I put 4x16MB sticks in?

Edit 4 (The Next Day):

Today I have been using my ValuePoint and find that all kinds of weird software errors are cropping up, most notably "DOS Memory-Arena Error".. I think I better remove that added 16MB from last night.

Edit 5:

Seems my IBM DX4 CPU is borked, and the 16MB isn't to blame....

 

[Tetrium]

.....Edit: Also, could someone explain why it restricts you to certain combinations that include blank banks? That doesn't make much sense to me, I understand you need to pair and match varieties of SIMM, but why leave blank banks? Why can I not put 4x8Mx36 and get 128MB [maybe the BIOS doesn't support that much RAM, is there another reason?]?

Quite possibly it's the chipset that doesn't support more then 64MB. I haven't taken a look at the link you posted above though.

About the EDO and NON-EDO:
The very short story is that there used to be a standard memory made for all motherboards, back in the 386/486 era. This was FPM. Then someone found out how to make FPM memory faster without increasing manufacturing costs. This later became EDO memory. The one downside, EDO was not backward compatible with chipsets that didn't specifically support it!
On some motherboards it might be possible to mix FPM and EDO, but all the memory will work at (the slower) FPM speed.
Mind you, my story about FPM and EDO is a grossly simplified version of how it realy happened!

 

[FishFinger]

Edit: Also, could someone explain why it restricts you to certain combinations that include blank banks? That doesn't make much sense to me, I understand you need to pair and match varieties of SIMM, but why leave blank banks? Why can I not put 4x8Mx36 and get 128MB [maybe the BIOS doesn't support that much RAM, is there another reason?]?

Most likely it's a chipset or motherboard limitation. 64MB was a LOT in the 386/486 era, so most boards limit you to 32 or 64MB to reduce costs/complexity (simpler chipset + fewer PCB traces), just as many 286 boards limit you to 4MB even though in theory the 286 can handle 16MB. A typical 486 probably only had 4/8/12/16MB, so manufacturers didn't see the point in providing the hardware to support more than 32 or 64 at a time when practically nobody would have that much.

 

[Minerva10210]

I had a 386DX40 with a whopping 20MB RAM a few years ago. It was plainly too much to be of any value. So I ended up crating a RAMDrive with DOS and copied my .mod music files onto the RAMDrive, was nice and fast after that :D

I also recall having cheaper 486 MB's that just would not support mix & match RAM simms. Used to work on my nerves, as at that time I was still at school and scrounged where and when I could :D