Found this
"'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.
Now to the problem. A memory 'slot' is usually designed for 'standard'
x8 chips and memory sticks containing 2 ranks. That is how the
'capacity' will be described. I.E. the K7S5A says it can handle up to
"1 gig" with two DDR slots, and that means 512Meg per slot, which
means 256 Meg PER RANK. (2 ranks per slot, 2 slots, etc)
But a 'high density' memory stick crams the 512 Meg 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.
So, a 512 Meg 'high density' stick puts 512 Meg in ONE RANK and a
'standard' density 512 Meg stick is two 256 Meg Ranks. Same size, same
'density'. It's the RANK organization that's different.
Since the memory slot on a K7S5A is two 256 Meg Ranks, a standard
density 512 Meg stick will work but a 'high density' 512 Meg stick
will not.
However, a 'high density' 256 Meg stick probably will since it crams
the 256 Meg into ONE RANK and the K7S5A can handle 256 Meg ranks.
For example, I'm running SDRAM on my K7S5A and, as it turns out, one
is 'standard' x8 density and the other is 'high density x4. But
they're both 256 Meg sticks and they both work. Damn irritating,
however, as they were both bought at the same time inside the same
package label, and same part number, with 16 chips on each so they
look identical (Kingston) but they are NOT the same thing. Doesn't
matter to my K7S5A but it means I can not use the 'high density' one
in my other motherboards because they support 128 Meg ranks (256 Meg
per slot but NOT in 'high density')."
