[Pc_Support] Re: Memory Technology and Chip Select -- why some DIMMs only come up as only half ...

[Bryan J. Smith]

On 9/22/06, Bryan J. Smith <thebs413 at gmail.com> wrote:
> I only need 17 pins on the SDRAM ICs themselves for addressing,

If you've ever wondered why you only saw half of the memory when you
stuck in a memory module, now you know why.  If the IC technology is
newer than the chipset's supported IC technology, you're IC has one
more address pin than your chipset supports.  In the case of some
non-standard 32 chip x4 (128-bit) designs back in the PC133 days, you
could even see a quarter.

My personal favorite, which I tried to bitch-smack into a former
Leapster who argued the point with me after a reseller tried to tell
him otherwise (yeah, don't listen to me, the engineer who has actually
designed memory controllers ;), was the every popular Intel i440BX v.
the popular Intel i815.

The Intel i440BX was a 128Mib technology, but could support registered
32 chip x4 (128-bit -- not looking at the 36 chip x4, 144-bit ECC
option).  The i440BX did support x8 and x16, but only in 8 and 4 chip
unbuffered (not 16 and 8 chip, respectively, registered).

So that meant for the i440BX, the 256MiB DIMM was 32 x4 registered,
and the 128MiB was 8 x8 or 4 x16.

The Intel i815 was a 256Mib technology, but it could only support
unbuffered 8 chip x8 or 4 chip x16.  It supported no x4 option at all,
not even 16 x4, much less a 32 x4 -- not even for older 128Mib
technology.

So that meant for the i815, the 256MiB DIMM was 8 x8 or 4 x16
unbuffered, and the 128MiB was 8 x8 or 4 x16.

So while the i440BX and i815 could use the same 128MiB DIMM, they
could _not_ use the same 256MiB DIMM -- even though both chipsets were
PC100/133 SDRAM (Intel unofficially supported 133MHz FSB on the
i440BX, it had an undocumented 1/4th divider).

SIDE NOTES (for completeness):

A.  The i815 was preceded by the i810, which only supported 128Mib
SDRAM technology.  So back then, it only supported a 128MiB DIMM,
which was the same as the i440BX.  It was once the i815 was
introduced, and the new 256MiB unbuffered DIMMs became available, that
people started buying them and seeing only 128MiB when inserted in the
i440BX.

B.  The i440GX was limitedly introduced for low-end servers (along
with the i450NX for higher-end servers) around the time of the i815
(Intel was also flirting with Rambus too, long story), and it finally
added 256Mib technology support to the i440 line.  It could not only
use the same 256MiB DIMMs as the i815, but it also offered support for
a 512MiB DIMM (using 256Mib technology in a 32 x4 registered module).
These DIMMs were very rare and damn expensive.