P4B266 and memory issues

I have a P4B266 w/ latest BIOS running a 2.-GHz Pentium 4 (400 MHz FSB) and
a single stick of 512 MB PC2100 "Mushkin Black" (premium high-perf 2-2-2
blah blah). I've been running this config for over a year now.

A few days ago, I was able to obtain two 256-MB sticks of the **very same**
memory. Both 256-MB DIMMs are "known good" (flawless, in fact), as I tested
them w/ MemTest86 on two P4 motherboards in my workshop. I wanted to use
them in my own work system and bring it up to a full 1 GB, so I popped them
into the two remaining free memory slots. Unfortunately, my system would
only give me long beeps and would not POST. If I removed one of them (from
the third slot), the system would POST fine w/ 768 MB of memory.

Any idea what the problem may be? Does the i845D chipset not like memory in
the third slot or something?

This motherboard only supports 4 banks of memory. If double sided DIMMs
are used the 3rd socket is disabled.

Hackworth wrote:
> I have a P4B266 w/ latest BIOS running a 2.-GHz Pentium 4 (400 MHz FSB) and
> a single stick of 512 MB PC2100 "Mushkin Black" (premium high-perf 2-2-2
> blah blah). I've been running this config for over a year now.
>
> A few days ago, I was able to obtain two 256-MB sticks of the **very same**
> memory. Both 256-MB DIMMs are "known good" (flawless, in fact), as I tested
> them w/ MemTest86 on two P4 motherboards in my workshop. I wanted to use
> them in my own work system and bring it up to a full 1 GB, so I popped them
> into the two remaining free memory slots. Unfortunately, my system would
> only give me long beeps and would not POST. If I removed one of them (from
> the third slot), the system would POST fine w/ 768 MB of memory.
>
> Any idea what the problem may be? Does the i845D chipset not like memory in
> the third slot or something?
>
>