crazijoe

There seems to be a lot of confusion with understanding high density and low density memory modules. I will try and clear this up a little.
A lot of people can obtain high density memory for a ‘cheap’ cost. These as usually branded as generic or do not have a brand name on them. While they save money on this memory, it may not work with their specific motherboard. A majority of motherboards do not accept high density modules. They usually show or register as half of what they spec out or advertised to be (i.e. bought a 1GB module and it only shows as 512MB), or they don’t work at all. This is common on all size modules. DDR and standard SDRAM included.

First rule, Double sided memory is not always low density modules. High density modules can have 16 chips (8 on each side). 'Standard' RAM chips are organized a DEPTH x 8 Bits. E.g. 32x8, 64x8, etc. That means 8 of the chips make up a 64 bit wide rank (memory bus is 64 bits wide). "Double Sided" is an old term to describe a stick with 16 chips, 8 on each side. And with DEPTH x 8 chips that makes for 2 ranks (or, again in an older style terminology: 2 'banks').

'High Density' chips are DEPTH x 4 bits so it takes 16 to make a 64 bit wide memory rank. And this is why "double sided" is no longer the favored description because when x8 chips are used "Double sided" means "double rank" but with x4 chips it takes both sides and 16 chips to make the ONE, single, rank.

Using a 1GB memory module, the 'High density' memory stick crams the 1GB into ONE RANK by using 16 deeper (twice as deep) x4 bit width chips. And this is how they arrive at the confusing term 'high density'. The chips themselves are no higher in density than the x8 chips but since they are organized as x4 they can cram more 'bytes' into a single rank because 16 chips make up a rank rather than 8, not that it helps make the stick itself any 'higher' in 'density' because you can still only get 16 chips mounted on the thing.

A memory 'slot' is usually designed for 'standard' x8 chips and memory sticks containing 2 ranks. That is how the 'capacity' will be described.
For example, a motherboard that will accept 3GB of memory with 3 slots. Each slot will accept a “double rank” module. Never exceeding 512MB per rank.
Next example, a motherboard that will accept 2GB of memory with 3 slots. This would be tricky and would require reading the owners manual of the system board. Typically on these boards, the first slot will accept a double rank module and the combination of the second and third will accept 2 ranks never exceeding 512MB per rank. I.E.
first slot – double rank module
second slot – single rank
third slot – single rank
or
first slot – double rank
second OR third slot – double rank leaving one of the slots empty.

So, a 1GB 'high density' stick puts 1GB in ONE RANK and a 'standard' (low) density 1GB stick is two 512MB Ranks. Same size, same 'density'. It's the RANK organization that's different.

This still doesn’t mean you cannot use a ‘high density’ module in a motherboard. If the slot will accept a double rank module, never exceeding 512MB per rank, you could still use a 512MB ‘high density’ stick of memory, because it is 512MB in a single rank module.

In conclusion, If you don’t know if your motherboard will accept ‘high density’ modules, buy the more expensive ‘low density’ or what they call ‘100% compatible’ modules. The more money you spent, will give you more assurance and less of a headache later.

crazijoe

New update to add to the example.
I just recieved an email ad for some inexpensive memory. 1GB of PC3200 memory for $59.49 a stick. Excellent deal however look closely and the clue is in the first part of the description:

NCP DDR 400 PC3200 1GB (1024MB) 128x4 16C - NEW
Model: NCP-1GB
Manufacturer:
Weight: 0.59 lbs
Price: $59.49

The chips are organized in a 128x4 configuration. They are squeezing the whole 1GB into a single rank. Therfore this would be considered 'high density'.

crazijoe

4GB of memory installed but only 3GB is showing in Windows.
Does this sound familiar?

Many people have noticed that when they install large amounts of memory in their systems, that Windows will show much lower then the actual physical amount. This is basically a limit of the 32 bit OS. Example: you install 4GB of memory. The system BIOS will show 4 GB of memory but the Windows OS only shows 3.25GB of memory.

Windows XP 32 bit version will accept up to 4GB of memory. However Windows will reserve the address space 3.5GB to 4GB to hardware devices. If these devices need more memory, Windows will allocate more of it to accommodate it. It is rare but not uncommon to see a system with 4GB of memory installed and only 2.75GB that is usable. Another note PCI-E 16x usually will gobble up 256MB of the address space. So if you have an SLI configuration, the 2 PCI-E 16x cards will chew up 512MB on top of the other devices that need the address space. It is possible to tweak the BIOS to reduce the amount of memory these devices use but often the result is negligible.

This is a note taken from an Intel MB manual:
ftp://download.intel.com/support/mot.../sp2gp2tps.pdf

Note: Memory between 4GB and 4GB minus 512MB will not
be accessible for use by the operating system and may be lost
to the user, because this area is reserved for BIOS, APIC
configuration space, PCI adapter interface, and virtual video
memory space. This means that if 4GB of memory is installed,
3.5GB of this memory is usable. The chipset should allow the
remapping of unused memory above the 4GB address, but
this memory may not be accessible to an operating system that
has a 4GB memory limit.

You can have more than 4GB on Windows XP 32 bit by using the PAE or Physical Address Extension switch and it may show in Windows, however the OS is still limited by 4GB and any memory installed above that will not be usable.

One must realize this is not a hardware issue but an operating system issue. These limits are not in the 64 bit versions of XP or Vista. Even the 32 bit version of Vista will have this limitation.

Also note that any application running in Windows XP 32bit is limited to 2GB and will not address anything above 2GB. This is lifted in 64bit, however if you are running a 32bit application in a 64bit OS, you still have the 2GB limit.

More information.

http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf

crazijoe

More additions.
When trying to identify a High Density or Low Density module.
This can be very tough. Most the time the chips will be individually marked "16x8" or "32x4" etc.. A **x4 will guaranty a High Density module. A **x8 single sided module will also guaranty it's High Density. I have also seen a double sided module that was configured with 4 chips on each side. Each chip was 128x8. This was a 1GB High Density module.

When buying memory, if the seller does not know if the module is high density or low density, ask them if they guaranty it will work in your specific machine. Or ask them if it does not work if you can receive a full refund. If they do not, then do not buy. Like I've said before. It's better to spend more and get the correct product than spend a little and hope it works. If you want a module that is guaranty to work in your machine, buy from memory experts like Crucial or 4AllMemory. They may charge a little more but the extra cost is better then the headache of getting the wrong memory.
As far as what your motherboard supports, read the manual.

crazijoe

Ok, I'll try a little more clarification.

A memory bus is 64 bit wide. Chips on a memory module are labled 32x4, 64x8, 32x16, etc. The 16 on a 32x 16 module designates the bit.
The 32 on a 32x16 module designates the size of the chip in MB.
Let's do an example:

You have a 512MB memory module that has 16 chips on it. 8 on each side. Each chip is labeled 32x8.
We need enough chips to make the 64 bit memory bus. So since each chip is 8 bit, we need 8 chips. And since this module has 16 of these chips, and only 8 of them are required to make the 64 bit wide bus, this is considered a double rank module or Low Density module.
When you take the size of each chip (32) times the number it takes to make the 64 bit bus (8), this would be how much memory is in the rank (256MB).
Now since this is a double rank module, the total amount of the stick is (256x256) 512MB.
You motherboard will need to accept at least 256MB in each rank.

Next example.

1GB memory module. This has 16 chips on it, all labeled as 64x4. Because it is designated x 4, each chip is only 4 bit wide. You will need all 16 chips to make the 64 bit wide memory bus. Therefore this is a single rank module or High Density.

Next example.

512MB memory module. It has 8 chips on it, all on one side. Each chip is labled 64x16. Now since each chip is 16 bit wide, it only takes 4 chips to make the 64 bit wide memory bus. And becasue there are 8 chips on this module, this is considered a double rank module or Low Density.
Imagine that. A single sided module that is low density.
It doesn't matter how the chips are arranged on the module but what type of chips are being used.

Now back to the motherboard.
What will work in your motherboard?
Read the manual.
If your motherboard has 2 slots, each slot is double ranked, and each slot will accept 1GB. This means that each slot will accept up to a 1GB double ranked or Low Density module or 512MB single rank or High Density module.

TheMatt

To further CJ's clarification of high density and lo density RAM:

While taking out the trash at the shop I work at, I noticed a stick of 184-pin DDR RAM in the trash pile among some dead power supplies. I figured it was worth testing it out before letting it go. I took it home and was about to test it when I realized that it had only chips on one side.

There is a quick way to test if a module is high or low density:

For 168-pin SDRAM, 184-pin DDR SDRAM, and 240-pin DDR2 SDRAM modules greater than 256 MB:

• Modules with chips on only one side (8 per side) are high density.
• Modules with chips on both sides (4 per side) are high density.
• Modules with chips on only one side (4 per side) are high density.

So basically, modules with 8 chips total are high density. This does not mean that if you have a module with 16 total chips you get a free pass though! It gets sticky when you get to 1 GB modules because high density modules do have 16 chips total.

This is where you have to look at the configuration. High density 1 GB modules have a 128MBx4 configuration whereas a low density 1 GB modules use a 64MBx8 configuration.

What did I do that high density module I brought home? I used it as a desk ornament.