Get a copy of CPUZ, and look at the memory tab.
*******
It sounds like one DIMM is being "sub-detected".
With your 2x512MB configuration, a total of 1GB of
memory should be detected. You get to use
1024MB minus 1MB reserved by the BIOS. Or about
1023MB or so.
The BIOS on computers is cleverly designed. On
the one hand, it will probe via SMBUS, the SPD
EEPROM. SPD is a tiny chip, not shaped like the
memory chips. It is smaller than a memory chip.
There is a table stored in there, with
capacity and timing information. The parameters
are standardized by JEDEC, a standards body.
But the BIOS "doesn't trust anyone". The BIOS
will set the memory controller on the Northbridge,
according the details in the SPD. Then it does
the old-fashioned "peek n' poke" memory sizing
test. The idea is, you write a value into an
imaginary location. If there is actual memory
present at that address, the value can be read
out later and verified. Every location, they try to
write a different value - that prevents a floating
computer bus, from accidentally having the "right"
answer, at the instant the read (peek) to verify the
result is carried out. By doing a binary search,
you can rather rapidly detect the "true" value
of the DIMM capacity.
One user here, bought a DIMM, which had the wrong SPD
chip soldered to it. (We figured this out later, using
CPUZ.) The SPD chip was basically telling
a lie about the memory capacity. Yet, the computer
still ran. It did not crash. Because of "peek n' poke",
the BIOS figured this out, all by itself. The BIOS
knew the actual DIMM, was half the size declared
in the SPD.
The evidence comes, when the user checks the claimed
amount of physical memory, and it isn't right. That's when
you begin to suspect, the BIOS has saved you from a
"surprise".
Try the non-install version of CPUZ, and verify the
memory details.
http://www.cpuid.com/softwares/cpu-z.html
1.67.1 zip, english
(no installation, includes 32 and 64-bit binaries)
When you click the "Memory" tab, it shows
the total memory detected by the computer.
The BIOS helped work out all the "safe bits"
of memory, so the computer will not crash.
The "SPD" tab, on the other hand, displays
the declaration soldered to the DIMM.
If there is a difference, between the value
seen in the Memory section, versus the sum
total of the SPD declarations (all slots
totaled together), it could be
one of those "surprise" situations.
For some reason, the DIMM is sub-detected.
*******
Shut down the computer. Turn off the power.
Unplug the computer. Wait around 60 seconds
for +5VSB to drain. On an Asus motherboard,
wait for the green LED to go out, on the motherboard.
It connects directly to +5VSB.
It's important for +5VSB to drain, as you want
all the sockets on the motherboard, to not
have any power floating on them. A DIMM could
get damaged, if you don't wait.
With all power drained, and with an anti-static
strap attached between your wrist and the computer
chassis, you can pull the DIMMs and reseat them.
Due to the usage of 10 micron gold plating, with
pin-holes in the finish, sometimes the electrical
contacts don't "make" as well as they should. There
is quite a strong incentive to "cheat" on the
gold finish on the pins of a DIMM. A simple
re-insertion may fix your problem - for now.
I do not recommend pencil erasers or abrasives
for cleaning DIMMs. That only makes potential
future contact problems, worse. If there is dirt
on the DIMM, you can use isopropyl alcohol. Some
of those simple alcohols, are used for washing
at the factory, and are safe on plastics. Acetone
and gasoline, are not safe.
Other than that, the insertion of the DIMM, has sufficient
wiping action for the purpose of making a
fresh contact.
Gold on gold, wipes or slides. Tin on tin, "bites".
The two metalizations should not be mixed. It's less than
desirable to use a tin socket, with a gold contact.
Tin works best with tin, gold works well with gold.
Gold on gold is a low friction fit, so the contact
friction would be lower than say, tin on tin. With
tin on tin, the oxides scrape over one another, until
fresh metal touches. Gold has no oxide to scrape off.
HTH,
Paul