16GB on a Skulltrail workstation?

First impressions: Hynix 4 GBFB-DIMMs

FB-DIMMS, even in their FBD-800 speed grade, were never really considered performance champions. Yes, with four channels on the Seaburg chipset, these babies do give you 25.6GB/s theoretical bandwidth, but even the best Sandra scores never went above 9GB/s sustained test results.

On the other hand, use of reliable, high-capacity server modules did let Intel's Skulltrail dual-socket mainboard support some nasty densities fairly early for the desktop. In this case, 16GB in four sockets would have been a reality - BIOS support quirks aside - far earlier than on any uniprocessor desktops.

But to overclock those? Get real - even Kingston's 1GB heat-piped FB-DIMMs couldn't go above 4-4-3-9 latency settings at FBD-800. And the last parameter, Tras min, was conveniently still preset at 12 by the BIOS no matter what you manually set it to.

Well, maybe all's not that bad - this week we got hold of a quadruplet of Hynix 4GB FBD-800 modules, with an attractive sexy-named HYMP 151F72CP4D5-S5-C. Each DIMM contains 36 pcs 1Gbit memory dies plus the usual hot Advanced Memory Buffer chip. The modules have a standard defined CL 5-5-5-18 SPD latency, and we didn't expect them to perform much better anyway.

Wrong - without changing any voltages, these were booting at 4-4-3-9 all the way into Windows Vista64. Interestingly, the previous BIOS 1140 didn't recognise 16GB properly, showing the configuration as 8 or 12GB depending on the luck. The upgrade to the newest BIOS 1149 solved the problem.

Ain't that a pretty sight...

Now we had 16GB on Windows Vista - however, the memory benchmarks, including Sandra, couldn't complete until I changed the DRAM die voltage to 1.92 volts (the AMB chip voltage stays at 1.5 volts all the while). The 4-4-4-9 is what at the end passed all the bandwidth, latency and stress tests well.