13.4MB of memory is missing!

[Aloone_Jonez]

I've just upgraded to 512MB but Linux is only reporting 490.6MB.

I know I have an on board video adaptor which is configured to use 8MB which makes 504MB, so where's the remaining 13.4MB?

I know that this is an OS problem, not a BIOS or hardware problem because Windows reports 504MB.

I've tried removing one of the RAM modules and I still get the same problem so the upgrade isn't the problem, it's just what's caused me to notice it.

I know 13.4MB of memory isn't much, it would just be nice to know what's happened to it.

[davidnix71]

Bit and bytes, maybe. 512 divided by 1.024 equals exactly 500.

Maybe your ram is off-spec and the flavor of Linux you use does a ram check at boot and only found 490.6.

[Aloone_Jonez]

I know it isn't the RAM module because the amount of memory is reported correctly by  Windows and the BIOS.

As far as the rounding error is concerned: I don't see why it should be rounded like that. As far as I'm aware, RAM is normally specified in MiB, i.e. 1MiB = 1024KiB = 1,048,576 bytes, so to be pedantic I really have 536.9MB but 8.38MB is being used for video so it should report 528.5MB (more) not 490.6MB (less).

EDIT:
Even GRUB seems to be reporting the amount of memory correctly, 639KiB of conventional + 515,008KiB of extended memory which works out at 503.56MiB and is probably a rounding error.

[davidnix71]

If your ram maker lied like hard drive makers did before they got sued, then 512 million bites would be 488.28 MB
(512 divided by 1.048576)

If you read the fine print on some hard drives, the number may be "one terabyte" meaning 1 trillion bites (which is a lie, of course), but that's actually the minimum number of good sectors they will ship the drive with. The real number is always a little larger than that. When that ram module was made, it could have came in at 514.4 million bites or so, which is really 490.6 MB

Your ram module firmware may report it's size to Windows in a rounded fashion "I'm a 512MB DDR2-444 SO-DIMM module", but that's not the real usuable number.

I would trust Linux to check the real usable number and pass that on to you.

[Aloone_Jonez]

But the BIOS memory check actually scans the memory at boot up and reports the memory in KB which I divided by 1024 to give exactly 504MB of RAM.

I doubt the hard drive companies were actually sued because they were right, the International Standard System of Units lists 1MB as 1,000,000 bytes not 1,048,576 bytes which is really 1MiB.

http://en.wikipedia.org/wiki/Megabyte
http://en.wikipedia.org/wiki/Binary_prefix

I think more investigation is required, I'll try removing one of the of the modules and see if the amount of RAM missing stays the same, if it does then I'll replace the module and remove the other, to check they're both the same capacity.

[Aloone_Jonez]

I removed the new module and Linux reported 238MB of RAM. I swapped the old module with the new and it still reported 238MB.


Let's see if it's a rounding error as you descried above:
238

[Lead Head]

RAM has always been in MiB, and still is (as far as I know), hence why 1GB of ram is advertised as 1024MB

[Aloone_Jonez]

That's what I thought.

MiB makes sense when you think of the addressing system on the memory IC.

I don't know though, what else could explain this mystery?

I ran a memtest and it reported 504MB so I assume it's a problem with the kernel as it's the only thing that seems to report a lower figure.

[Lead Head]

Do you have an onboard modem/ethernet/sound that could be using some of that ram?

[Aloone_Jonez]

I don't know, I would've thought that the BIOS would have taken that into account like it did for my on-board graphics.

I've installed a graphics card now so I'll have a look at the memory reported under Linux next time I reboot.

Windows: Control Panel -> System says 512MB and task manager says 523,760KB which works out just under 512MB but it's an acceptable rounding error.

[Lead Head]

Was just messing around on my Ubuntu rig, with a dedicated video card and 512MB of ram, "Sysinfo" reports 497MB. Odd.

[Aloone_Jonez]

I'll try installing a Linux distro in a VM with a known amount of RAM and see what it reports.

[Aloone_Jonez]

I've just tried that with Fedora, I set up a VM with 512MB RAM and it said 245MB.

Perhaps this is a question for the kernel developers?

[Lead Head]

Interesting issue. Are you sure in Fedora it wasn't reporting useable ram, instead of say total installed?

[Aloone_Jonez]

Sorry I ment to say the VM had 256MB RAM and it said 245MB.

Why should there be a difference between usable and total RAM?

I told VirtualBox to allocate 256MB for the VM so I don't see why any less than 256MB should be available to the VM.