ZDNet Clock is easy to install. After downloading, you simply click on the ZDNet-Clock icon to start. Then the chipset and the clock chip are detected. The superuser password has to be entered at this point. The only computers currently supported are those equipped with 5000 or Intel 5400 chipsets. That means all Mac-Pro and Xserve models with Intel processors. If the computer is not recognised the program closes with an error message.
The console can be used for diagnosing problems. It is to be found under "Applications – Utilities – Console". The ZDNet-Clock messages can be found in the "All Messages" window, see figure 3. When the chipset and clock chip have been detected the application appears as shown in figure 1 and figure 2.
The main control is the slider control, with which the frequency is set. The left-hand stop is always the standard frequency of the clock chip. "Underclocking" cannot be achieved with the GUI. Three frequencies are displayed above the slider control. "Bus Clock" displays the frequency that is actually generated by the clock chip. The front-side bus is also clocked with this frequency. As current FSBs are four times as wide as the original Intel FSB, Intel always indicates the quadruple frequency. ZDNet Clock calls this frequency the "Marketing Clock".
If you move the slider control completely to the left, 1333 MHz or 1600 MHz should be displayed there, depending on the model. The "CPU Clock" value should show the standard frequency of the CPU. To raise the frequency you slide the control to the right. You should not overdo it when overclocking for the first time. A good first move is to increase the "CPU Clock" by 10 MHz.
If you push the slider all the way to the right, to the maximum frequency, and click on the "Apply" button, the computer will certainly crash. The optimum CPU frequency with which stable operation is possible has to be worked out individually for each computer.
Increasing the front-side bus frequency raises the clock speed of the CPU without having to change the multiplier. The frequency of the Northbridge (MCH) is raised at the same time. So the memory is always overclocked as well. In the case of the Mac Pro, the CPU will normally tolerate higher overclocking than the MCH. As the Mac Pro uses fully buffered DDR2 memory modules (FB-DIMMs), which have error checking and correction (ECC), it is easily possible to feel your way up to the maximum stable overclocking frequency.
The console is used for that purpose. The "All Messages" window contains not only the messages from ZDNet Clock but also entries relating to corrected memory errors. Immediately after changing the frequency, entries relating to corrected errors will mainly appear, that are designated as a "correctable fbd error" or as "recoverable parity errors". They are caused by the switching of the frequency. The CPU and MCH briefly encounter synchronisation problems. This is normal, and there is no need to worry about it.
If further memory errors appear in the console after successful overclocking, this is an indication that you have exceeded a stable threshold with the overclocking. It makes sense to carry out a test after switching clock speed. Longer running benchmarks can be considered in this context, such as the prime number search program mprime. If no errors occur for several hours, you can assume secure operation.
In order to keep the effect of memory errors resulting from frequency switching to a minimum, ZDNet Clock overclocks the computer in steps of 1MHz (FSB). Tests have shown that an increase in the FSB by a large amount and the associated increase in the CPU frequency, for example raising the CPU clock speed from 2800 MHz to 3200 MHz, often puts the RAM modules so seriously out of synchronisation that more bits ‚topple‘ than error correction can pick up. This leads to a controlled crash. The memory modules report the error via NMI, the kernel acknowledges it with panic, see figure 4.
The result of step-by-step switching is that it takes some time for confirmation to appear after clicking on the Apply button. If you prefer direct switching, this can be selected by ticking the "Use Rapid Overclocking" checkbox.
Whether the switching can be successfully carried out depends very much on the RAM modules used. The first-generation tester (Mac Pro 1.1) is equipped by Positivo Informática. In this case overclocking only works in very slow steps. The 2008 model (Mac Pro 3.1) comes with preinstalled Hynix modules. In this case, switching also works with "Rapid Overclocking" turned on.with modules from
ZDNet has also tested modules that do not come from Apple. Tests with modules from CSX equipped with memory chips from Elpida are disappointing. They run the Mac Pro clocked at 2800 MHz at a maximum of 2835 MHz. On the other hand, marginally more expensive proprietary modules from Kingston and Transcend (TS2GAPMACP8U-T) display no problems.