Overclocking A64 Processors

Introduction - AMD Athlon Overclocking

This is a guide designed to walk through the basic process of overclocking an AMD Athlon 64 family processor that comes with an on-die memory controler. These processors include the Athlon 64, Athlon X2 and FX series chips. As the AMD Opteron socket 940 and Opteron 939 processors use the same core as the FX series and some A64 series CPUs, this guide holds true for them as well.

This guide works for the processors listed in Athlon 64 as well as Athlon 64 X2.

Basic Architecture

Older processors, and current model Intel prcessors use the tried and true method of organising a motherboard, by having a northbridge control the interaction of the memory and the processor, while the processor is also attached to the southbridge, which is connected to devices such as the PCI slots and harddiscs. Originally, to overclock this style of architecture, you would raise the FSB - front side bus - rate of operation. Usually, but not always, this figure is around 200Mhz.

However, AMD architecture has changed somewhat. Now, rather than have a seperate memory controler (the northbridge) on the motherboard, they moved it to the die (sharing the same piece of silicon as the rest of the processor). Now, between the CPU unit (containing the memory controller) and the RAM is nothing. this path is called the LDT bus, which I will describe in more depth later.

As with most current setups using both styles of arrangement, the base, stock operating frequency is 200Mz.

The LDT - Lightning Data Transfer - bus method of arangement has the advantage of more bandwidth between the memory, as well as reducing the time required by the processor to talk to the memory controler (seeing how its just next door on the die). The LDT of these processors is currently 2000Mbit/s, which is calculated by multiplying the following figures together:

• 200Mhz (base operating frequency - HTT)
• x2 (for the DDR (double dynamic) RAM speed)
• x1 - x5 (for the LDT multi, set at x5)

The actual CPU portion of the is controlled by a frequency called the HTT (HyperTransport Technology). This operates at the base operating frequency and, when modified by the CPU multi, produces the clockspeed of the processor. The stock value for these multipliers vary from 14x of the FX-57 CPUs down to 9x of the 3000+. With the exception of the FX series CPUs, the HTT multi is upward locked, meaning that you can adjust the multiplier to make the CPU go slower, but not faster than its stock rated speed.

With this finer, 90nm architecture, the voltage requirements have dropped from previous core designs. The stock voltage has been dropped to 1.35v. According to AMD, the core is rated to 1.7v, however, running at this voltage is not conducive to a long processor life.

DISCLAIMER

Firstly, this is my method, based upon readings Ive done across various fora. This method, while having worked well for me, may be both ineffective and dangerous for you and your equipment. As always with overclocking, acknowledge that this process voids all warranties and can damage and distroy the equipment you are running. If you cant afford to replace it, dont do it.

Also, all of these settings and methods were carried out on a DFI LANparty nF4 Ultra-D, Athlon 64 4000+ and OCZ performance series RAM.

Overclocking the CPU

Im of the preference of overclocking the CPU first, then worrying about the RAM later. Thus, with this in mind, the first thing I do is make sure the RAM clock is not going to interfere with my CPU improvements. To do this, i use a RAM divider. Usually, the first value ill set to is a 5/6 division, which takes DDR400 and slows it to DDR333 speeds. this means that the RAM is operating at 166Mhz, plus whatever HTT increase you use. So if I added 10 to the HTT, the RAM speed becomes 176Mhz, which equates to 352Mhz DDR. Well under 400Mhz DDR and thus, well out of the way.

Before you start cranking the HTT up, there is one other factor to consider. the LDT reacts badly to overclocking, and it doesnt really suffer from underclocking as the A64 barely makes use of the whole bandwidth, in most cases. As mentioned above, the stock frequency is 200Mhz and the LDT multi default value is 5x. The first change I would make here is to drop the LDT multi to x4. This produces an LDT bandwidth of 1600Mbit/s, which is fine for the purposes of finding the CPU limit.

To prepare for my overclock, I got a notepad and drew up a rough spreadsheet. In the columns I had HTT, CPU Multi, CPU Clock, RAM Clock, LDT Multi, LDT CLock, vCore and Success? With this sheet, i kept track of the changes i made throughout the process.

So, with the RAM and the LDT out of the way, its time to start winding it up. Start with making relatively large jumps - most processors will probably stand to go straight to 210HTT straight away. After that, start going up in steps of 5HTT Mhz. After every step, fill in yourr chart, check that your RAM isnt running over DDR400 speeds and check that the LDT hasnt strayed over 2000Mbit/s.

The checkpoints where this will happen will be at 250Mhz HTT (which will limit the LDT) and 240Mhz RAM (using the 5:6 divider). In both cases, keep the LDT and RAM out of the picture by dropping the LDT multi to 3.5x or lower (as required) or raising the RAM divider to 3:4.

Everytime you increase the HTT, and adjust other settings around it, boot into windows and submit the CPU to some benchmarking programs, such as Prime95 or SuperPI, to push the CPU to 100% load for at least 10 minutes. If it survives this long without any signs of giving up, then reboot into BIOS and push the HTT up some more. Keep pushing up, keep the RAM and The LDT out of the way and eventually, the CPU will start showing signs of being clocked too hard. Signs that you are close include BSoD's and random reboots and if you have pushed the HTT too far, it wont POST on rebooting the computer. In this event, youll have to reset your BIOS (check your motherboard manual) and re-enter all the relevant settings again, minus some HTT Mhz.

Eventually you will reach a point where the CPU seems happy to operate, but wont go any higher. At this point, you have 3 options. Firstly, you can call it a day at that point, happy that you are running X Ghz at stock voltage. Secondly, you can improve the circumstances of the CPU to give it a little more of an edge - buying a better heatsink / cooling setup, better case cooling, etc. Finally, you can choose to raise the voltage to give your processor more room to move.

Voltage Adjustments

As mentioned above, you can increase the voltage to the processor to make it more stable while on a hard overclock. This adjustment, if avaliable, will be found in the BIOS setup of your motherboard, and usually goes under the name "vCore" or "CPU Vid". Check with your motherboard manual to be sure.

Most current AMD processors that this guide refer to are built with the 90nm SOI process. The stock vCore for these cores is 1.35v, which is enough for a small overclock (I got my 4000+ from 2.4Ghz to 2.78Ghz on 1.35v). This made for a small increase in heat, due to the extra power (in watts, which is volts x amps).

Before you start raising the voltages supplied to your processor, you have to decide how badly you want more performance. Bear in mind that raising the voltage is really pushing the processor out of its intended use, and will eventually result in the demise of your processor. How quickly small voltage changes can affect your processor isnt widely known and can vary with the production week and grade of your processors core. Large voltage increases, generally above 1.7v, will lead the your processor dying fairly quickly. To make the decision, you should also see how much voltage people are getting away with on processors of the same production. Research is most easily done with venice core CPUs, found in the A64 3000, 3200 and 3500+ processors and plenty of people are working Opterons (with san diego cores) very hard, but these results are not useful to people with 3700+ and 4000+ (san diego) A64's as the Opteron core selection criteria is much more stringent. As with all overclocking, read up on what people are doing with similar equipment to get a guide for what to do and expect.

Begin changing voltages gradually to see how large the effect will be. Start out doing up .1v to see how large an increase in HTT you can manage and then decide how high you wish to send the voltage. Remember to keep filling out your chart to make sure that the RAM and LDT isnt becoming an issue. If you have decided that processor life isnt much of an issue to you, then increase the voltage gradually while raising the HTT. Eventually, you will reach a point where the voltage increases are not leading to any increase in HTT. Other voltages to investigate include the chipset voltages (rarely effective, but has been seen to gain another few Mhz) or DRAM voltages (often very beneficial to RAM overclocking).

RAM Overclocking

RAM overclocking is a much more complex process than overclocking a processor, in that you have to balance latencies with the DRAM frequency. There are other, more specialised and better written guides for this process, and reading up on what equipment you own is also very helpful. In my personal opinion, the best place for memory overclocking advice is through i4memory.com.

Essentially, it is the same as CPU overclocking, but in reverse. instead of using RAM dividers to keep the RAM clock below DDR400, you want to use the CPU multiplier to back the CPU to slower than stock speeds. Again, keep the LDT multi at a level that keeps the LDT bandwidth near 2000Mbit/s, but not over.

Matching the Whole Package

Once you have found the limit of your CPU and you have found the maximum clock of the RAM, now is when you can clock them both to make the pair work together.

The best way to describe this process is to use an example. The case in point was with the equipment mentioned in the introduction.

I Had discovered that the maximum clock that the CPU can hold, at the voltage i had decided upon, was about 2810Mhz, whilst the RAM was limited at 257Mhz. To see what options were possible, i wrote column of HTT's and a column of HTT multis that I had avaliable, so that the product of the two was as close as possible to my maximum overclock. With my equipment, this looked as such:

• HTT 234 * 12 = 2808Mhz
• HTT 255 * 11 = 2805Mhz
• HTT 267 * 10.5 = 2803Mhz
• HTT 281 * 10 = 2810Mhz

And so on.

With this chart, you can see that the second option was the closest i had to reaching both CPU and RAM limit, being only a few Mhz shy of what i found to be the limit. With this figure in mind, I inputed the HTT of 255 into the BIOS, adjusted the LDT Multi to 3.5x, set the HTT Multi to 11x and the RAM divider back to 1:1.

Another option I could have used was to set the HTT to 281, the HTT Multi to 10x, LDT to 3.5x and used a 9:10 RAM divider to get the RAM clock to 253Mhz, which would net approximately the same results.

Conclusion

Overclocking A64 family processors is a similar process to that of FSB styled processors, with the major difference being the location of the memory controler. Overclocking these processors can be quite rewarding, with big gains to be had in benchmarks as well as in real world situations such as games and encoding processes.