SiS 756: PCI Express for AMD Socket 939

TRas and Memory Stress Testing

Memory tRAS Recommendations

In past reviews, memory bandwidth tests established that a tRAS setting of 11 or 12 was generally best for nForce2, a tRAS of 10 was optimal for the nForce3 chipset, a tRAS of 7 was optimal for the nForce4 chipset, the ULi 1695 was best at tRAS of 10, and ATI is best at 7 to 8.

Since this is our first review of the SiS 756 chipset, tRAS timings were first tested with memtest86, a free diagnostic program with its own boot OS that will boot from either a floppy disk or optical disk. Bandwidth of OCZ PC3200 Platinum Rev. 2, based on Samsung TCCD chips, was measured from tRAS 5 to tRAS 13 to determine the best setting.

Memtest86 Bandwidth SiS 756 Reference Board with Athlon 64 4000+
5 tRAS	1954
6 tRAS	1954
7 tRAS	1995
8 tRAS	2039
9 tRAS	2039
10 tRAS	1995
11 tRAS	1995
12 tRAS	1875
13 tRAS	1875

Two things stood out in our tRAS tests. First, the SiS 756 exhibits the best bandwidth at a tRAS setting of 8 or 9. We decided to use 8 for all our testing. Second, the bandwidth numbers on the SiS 756 were lower across the tRAS tests than what we have seen with the NVIDIA nForce4 and the ATI Rx480. This points to SiS having a poorer front end to the AMD on-chip memory controller than either nForce4 or ATI Rx480 - at least at this stage of development.

Memory Stress Test

Our memory stress test measures the ability of the SiS 756 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. To make sure that memory performed properly in Dual-Channel mode, memory was only tested 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
RAS to CAS Delay:	2T
RAS Precharge:	8T
Precharge Delay:	2T
Command Rate:	1T

Using two DIMMs in Dual-Channel 128-bit mode, the memory performed in all benchmarks at the fastest 2-2-2-8 timings at default voltage, which was the only memory voltage available.

Stable DDR400 Timings - 4 DIMMs (4/4 DIMMs populated)
Clock Speed:	200MHz
CAS Latency:	2.0
RAS to CAS Delay:	2T
RAS Precharge:	8T
Precharge Delay:	2T
Command Rate:	2T

Tests with all four DIMM slots populated on the SiS 756 required a 2T Command Rate with 4 DIMMs in two dual channels. This is the pattern seen on other top-performing Socket 939 boards. Since the Athlon 64 memory controller is on the processor, there were no real surprises in the memory stress tests.