Memory Timings

Memory Timings

Memory Timings Explored

Decoding the Bank X/Y DRAM Timing setting on the ABIT KT7 Motherboard



Ever wondered what on earth the difference between the Bank X/Y DRAM Timing setting was, and the DRAM Clock setting? Confused by the difference between 8-10ns, Normal, Medium, Fast and Turbo? Thought Medium was faster than Normal, and Fast faster than Normal? Think again.

This is one of the most confusing of all the BIOS settings, and also one of the most inappropriately named. The selection of options (8/10ns, Normal. Medium. Fast and Turbo) is historical, and actually bears no resemblance to what the settings actually do. As we will see, there are a number of surprises: So we really only have four options - and these four options have been horribly misnamed!

What the setting alters

This setting actually adjusts a number of distinct SDRAM settings - the Bank Cycle Time (tRAS), RAS Precharge time (tRP) and RAS-to-CAS delay (tRCD). Using H Oda's excellent WPCREDIT program, we can dump the hex contents of the CMOS and reverse engineer what these DRAM Timings actually do. I also used the KX133 PCR file to decode the hexadecimal CMOS values into a meaningful value - although the KT7 uses the KT133 chipset, the meaning of these settings is

the same. I can't claim credit for the idea of this - see the excellent BIOS Guide at LostCircuits for the original analysis. This also provides a detailed description of what tRAS, tRP and tRCD actually mean. However, the results presented there are not completely the same on the KT7, and hence this discussion.

The Method

To determine the affect of these BIOS settings, start up WPCREDIT and load the KX133 PCR file. Your screen should look something like the following:

BIOS settings This represents a hexadecimal dump of your CMOS. Row 60 contains details of your SDRAM settings. By loading the KX133 PCR file, the right hand side of the window can provide a textual decode of what the hex means. Columns 04, 05 and 06 of row 60 provide details of RAM settings for Banks 0/1, 2/3 and 4/5, respectively. Column 07 represents Bank 6/7, which doesn't exist on the KT7. It is here that we can observe the effect of the Bank X/Y DRAM Setting.

Having an hour to spend, I patiently went through each BIOS setting, decoded the hex and recorded the Sandra memory score. The results are shown below.

The Results

For the purposes of this test I disabled 4-way interleave, and set the CAS Latency as 2 (if you have CAS3 SDRAM, you'll have to use 3 - it doesn't affect the rest of this discussion). I used one 128MB stick of Mushkin PC133 Rev 2 SDRAM, with the memory bus set at the default 133MHz. I then used Sandra Millennium 2001 to provide benchmark scores. It is worth noting that there is considerable variance between Sandra scores - thus two benchmarks with identical settings can give significantly different scores. Bear this in mind when claiming certain settings give better results!!

Setting tRAS tRP tRCD Bank Interleave Sandra Score
8-10ns 6 3 3 Off 390/420
Normal 5 2 3 Off 414/469
Medium 6 3 3 Off 390/420
Fast 6 2 3 Off 416/452
Turbo 6 3 2 Off 414/449


Firstly, note how 8/10ns and Medium are identical. On my machine at least, there was little difference in perfromance between Normal, Fast and Turbo, although the timings are slightly different. Normal, however, is the most aggressive of the DRAM timings - and this is reflected in its slightly higher Sandra score. User results in "What should my Sandra Memory Benchmarks be like?" in the memory section of the KT7 FAQ indicate little difference between Fast and Turbo - although no-one has posted results for Normal. This is probably because they thought Fast and Turbo would be better!

The Normal, Fast and Turbo settings can impact on stability, however. Some people find that they can boot in Fast and Turbo modes, but not Normal. This is due to the more aggressive tRAS=5T setting - all other modes use 6T. Other people cannot boot in Turbo mode as this is the only mode with tRCD=2T. It seems, therefore, that the principal difference between Normal, Fast and Turbo is not so much speed, but stability. You'll have to benchmark on your machine to see which works best. Note, of course, these scores can be considerably improved by enabling 4-Way Interleave, but that is a different setting.

Of course, you can gain a vast performance improvement with PC133 RAM by setting the DRAM clock to HCLK+PCICLK which sets the memory bus at 133MHz. For PC100 RAM use Host CLK which sets the memory bus at 100MHz. Conclusions

So, in conclusion, "Bank X/Y DRAM Timing" enables you to fine tune the timings used by the RAM circuitry. In simple terms, DRAM clock sets the speed (or frequency) of the timing signal, and Bank X/Y DRAM Timing determines how many clock ticks are used for each operation in the RAM.

8-10ns and Medium modes are identical. Normal is more aggressive than Fast and Turbo. Fast and Turbo are just different. In terms of speed, 8-10ns/Medium is consistently the slowest and most stable option. Normal, Fast and Turbo are usually much the same in terms of speed and stability, although Turbo can cause stability problems because of its more aggressive tRCD setting, and Normal can cause stability problems because of it more aggresive tRAS setting.

User results in the memory section of the KT7 FAQ have shown fairly conclusively that on average there is little performance difference between Fast and Turbo - you'll have to benchmark your own configuration and see what works best for you. But remember, Sandra scores have a high variance, and you'll need to find the average of a number of scores.

One is still left asking why AWARD insist on using these outdated descriptions for this BIOS setting! I guess the answer must be that they can't think of anything more appropriate! Perhaps Apu's Hardware should hold a competition. Answers on a postcard please...
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