ATI Radeon Xpress 200: Performance, PCI Express & DX9 for Athlon 64

Memory Stress Testing: ATI Bullhead

The memory stress test is very basic. It simply tests the ability of the ATI RX480 to operate at its officially supported memory frequency (400MHz DDR), at the lowest memory timings that OCZ PC3200 Platinum Rev. 2 modules will support. All DIMMs used for stress testing were 512MB double-sided (or double-bank) memory. Memory was only tested in Dual-Channel mode using either one dual-channel (2 DIMMs) or 2 dual-channels (4 DIMMs).

Stable DDR400 Timings - One Dual-Channel (2/4 DIMMs populated)
Clock Speed:	200MHz
CAS Latency:	2.0
RAS to CAS Delay:	2T
RAS Precharge:	10T*
Precharge Delay:	2T
Command Rate:	1T

*Several memory tests have shown that memory performs fastest on the nVidia nForce chipsets at a TRas (RAS Precharge) settings in the 9 to 13 range. We ran our own Memory Bandwidth tests with memtest86 with TRas settings from 5 to 15 at a wide range of different memory speeds. The best bandwidth was consistently at 9 to 11 at every speed, with TRas 10 always in the best range at every speed. The performance improvement at TRas 10 was only 2% to 4% over TRas 5 and 6 depending on the speed, but the performance advantage was consistent across all tests. All benchmarks were run at a TRas setting of 10.

Using two DIMMs in Dual-Channel 128-bit mode, the memory performed in all benchmarks at the fastest 2-2-2-10 timings at 2.6V.

Stable DDR400 Timings - Two Dual-Channel (4/4 DIMMs populated)
Clock Speed:	166MHz
CAS Latency:	2.0
RAS to CAS Delay:	2T
RAS Precharge:	10T*
Precharge Delay:	2T
Command Rate:	2T

Tests with all four DIMM slots populated on the ATI Bullhead required a 2T Command Rate with 4 DIMMs in two dual channels. This is the pattern seen on other top-performing Socket 939 boards. This should not be a surprise, since the A64 memory controller is located on the CPU and not a part of the chipset itself. On the Auto setting, the ATI RX480 set the memory speed to 166, and a manual override to 200 did not work with 4 DIMMs. ATI specifies 4 DIMMs to be set to DDR166 at 2T Command Rate, but most of the better Socket 939 boards that we have tested run fine at DDR400 at 2T. ATI tells us that the override not working is an issue in current BIOS code, which they expect to be fixed with production boards.

UPDATE 11/11/2004: ATI has provided an updated BIOS which corrects the issues of 333 timings with 4 double-sided dimms. With the new BIOS we were able to run 4X512MB DS OCZ 3200 Platinum Rev.2 at 2-2-2-10 timings at DDR400 with a 2T Command Rate. This performance matches the best we have seen with 4 DS dimms on an Athlon 64 motherboard.