Yesterday I upgraded my HP Compaq nc4000 laptop from its default 512MB RAM to 1024MB RAM by adding another module.

When the system booted again, the memory was recognized by BIOS and top shows total memory as 1024MB. However, the system seemed unstable - fonts were rendered slightly different, and the CPU load went up and down like a yo-yo.
At 02:30 last night the system hung completely and a battery-removal was needed to reset, after which the PC would not boot.

Removing the RAM solved it, but it took another 2 reboots before the wireless LAN would work again.

I used Crucial's memory configurator engine to select the RAM based on my exact computer model, but now I'm worried about trying the RAM again. Could this be that the RAM chip is faulty, or that the web-configuration-engine recommended the wrong chip for my PC? Is there any way to find out without burning my PC ?
Is this a common problem?

And - when more RAM is inserted, does that perhaps move the video-RAM used by X so that I need to reconfigure X11 to avoid memory crashes - could that be it? (the BIOS reserves 32M or 64M of total system memory for graphics)

-Y1

Mixing memory (even supposedly the "same") is a real lottery.
Linux seems very sensitive to mismatches - say in timings.
Others who better understand these things will no doubt respond, but from the experience of others like yourself, I'd say "don't do it".

Here's someone else that had issues.

I agree with syg00, and will bet that the two RAM sticks are different speeds (eg, PC 3200 and PC 2700) even though they are the same size (512Mg). Ideally, I suggest that both sticks be identical: same manufacturer, size, and speed. Note that's a recommendation rather than an absolute requirement, but mixing different speed RAM together is almost always a sure-fire way to cause problems, because the effective speed of your memory will be limited to the slower stick. In other words, it's sort of like pairing a champion sprinter with an overweight couch potato in a three-legged race - you'd be a lot better matching runners of equal speed with one another.

Anyway, assuming that the RAM sticks have labels, what do they show? If they indeed are different speeds, then if I were you I'd try to return the new stick for one that matches the previous, existing stick, or to just buy a second new stick at the faster speed.

Other comments:

1. Does your mobo support the speed of your fastest RAM stick? According to what I see about your laptop, it can support 266-MHz SDRAM DDR, which is "PC 2100" RAM. If one of the sticks is faster than that, it's not compatible with your laptop.

2. It is possible that the RAM stick is defective. Thy running memtest to inspect your RAM. The memtest option is often one of the boot options on most distros, so you might not even need to download anything. Just reboot and check out the various boot options, or use a Live CD such as Knoppix, which has it

3. Lastly, it's possible that one of the sticks just isn't seated properly. Recheck them
Good luck with it either way

Thanks for the thorough replies.

I realize that mixing RAM is always dangerous, but on the other hand, HP (my laptop is an HP Compaq NC4000) doesn't tell exactly which manufacturer they used for the particular production lot to which my laptop belongs, and as the other onboard RAM-chip is not visible (or even "built-in"), I have no way of seeing without picking the entire laptop into pieces, which is also highly recommended against.

However, both HP and the Crucial's "expansion memory guides" have picked out this particular memory chip to fit my computer, based on the serial number of my PC, so it *should* definitely match both speed and other settings.

I downloaded and ran memtest last night, and in the process noticed that my system is significantly slower with 1GB RAM than 512MB RAM, and furthermore, the system "hard froze" after only 12 minutes of memtest with the additional chip, while it happily tested the original base 512MB...

So my new question is - on a laptop - how do I tell exactly what memory I have (except for trusting the laptop and memory manufacturer's online guides) without risking breaking my system by opening all the "will-void-warranty" pieces like taking off the keyboard and disassembling the motherboard ?

-Y1