The life and times of the modern motherboard

Posted at: Tuesday 27th November 2007 by Alex Watson

Ever wondered how a motherboard and its BIOS are designed and made? What makes one a brilliant overclocker and another as stable as a plate of jelly on a bouncy castle? Alex Watson investigates.

One of the most important aspects of the BIOS for an Intel board is the Memory Reference Code (MRC). 'The MRC is part of reference BIOS code, which relates to memory initialisation in the BIOS. It includes information about memory settings, frequency, timing, driving and detailed operations of the memory controller. The MRC is written in a C-language code, which can be edited and compiled by board makers. It provides a space to develop advanced features, and the ability to tune memory. 'We try to look into the Memory Reference Code to try to understand its behaviour,' continues Chen. 'Sometimes, there are some remarks inside the source code, while on other occasions, we just try to change the arguments to see what effect it has. If we take the time to understand the MRC then we can get more out of it. If we put in more effort than our competitors, we can find more information.'

'The MRC usually only provides support for industry-standard memory configurations. For instance, under a 1,066MHz FSB, the only choices regarding memory speed in the MRC are DDR2-667 and DDR2-800. We have to provide additional choices. For people who want higher memory frequency, we used the setting of 800MHz FSB:DDR2-800 in MRC, but overclocked it to work with a 1,066MHz FSB, so we could implement support for DDR2-1066. For people who want to overclock the FSB, we used the lowest ratio of memory, 1,333MHz FSB (DDR2-667), in the MRC to allow the highest possible FSB to be reached.'

At reference speeds, the timing of every device on the board has been tested and set at a certain level. When you're overclocking, you're literally running clocks faster than normal. Working out the effect of this on the various different components on the board is a task that a good BIOS must be able to handle.

'Overclocking means every timing is shrunk,' explains Chen. 'If you leave anything running with normal lead times when parts of the system are overclocked, the timings will be too tight to fulfil the transaction.'

He offers this example: 'A CAS latency timing is defined in nanoseconds in the RAM's SPD, but not in terms of clock cycles, which is the timing system the motherboard uses. If CAS latency is defined as 12.5ns, under a DDR2-800 memory system, the CAS latency should be set as 12.5ns divided by 2.5ns, as this is the period of a clock cycle at this speed. You can see that from 12.5/2.5, you end up with 5T. If memory is overclocked to DDR2-1000, the period of a clock cycle becomes something like 2ns; in this case, the CL should be set as 12.5/2= 6.25T, which would be rounded off to 7T. A good BIOS designed for overclocking should automatically consider timing settings such as these for higher-frequency operation. If a timing is missed, or mistaken under high-frequency operation, it will restrict overclock margins or cause the system to crash.'

One tweak that Asus has been using recently is its 'AI clock twister'. Chen describes this as focusing on the internal clock-crossing mechanism between the FSB and the memory: 'Clock crossing refers to the hand-shaking method for data transaction between the FSB and the memory. When data is written from the FSB to memory, the data delivering period is counted in FSB clock cycles. At the same time, the memory controller also counts how many memory clock cycles there will be before it receives data. The clock cycle counting figures on both sides will determine data transaction stability, but it's a delay. This total delay is an "overhead" for the data transaction. We found that you can alter this overhead, which is what our clock twister does. It has two settings - using "strong", we adjust the clock crossing method to shorten the overhead. This improves performance, but stability becomes worse. The "light" setting is the reverse; it increases stability, but performance gets worse. If your PC can take it, the strong setting seems to add around 1-1.5 per cent to performance.'