The AC/DC Loadline in theory and practice
AC Loadline and DC Loadline can be set separately in the BIOS in the advanced VR settings, whereby the designation in the menu can vary somewhat from manufacturer to manufacturer (see image below). These settings always have priority over the settings of the “CPU Internal Load Line”. The values are given in 1/100 mOhm. Example: 0.8 mOhm would result in a value of 80 in our picture. An AC and a DC value of 40 (0.4 mOhm) is provided for current saving. In between there are several intermediate levels to choose from.
Let us now turn to the tasks of both values. The AC Loadline (VRM Loadline) as value 1 influences the operating voltages ONLY for AUTO voltages (not for manual overwriting), we have to consider this. So the target voltage for the VRM (which is based on the CPU’s original pre-programmed default VID at that cache and core ratio) is first boosted up to 1.520 V at full load (yes depending on the CPU, the boost at idle is lower) and then Vdroop drops the voltage for the CPU back down to a safe level.
DC Loadline as value 2 does not affect operating voltages, only power measurements and VID, but DC Loadline is best set by beginners to the same value as VRM Loadline (value 1) to get accurate VID and CPU package power reports when using <Auto> or SVID Offset (set after <Auto> is applied). In my example, both values are factory set to 80 in the BIOS.
And then there is another important fact: the tolerance band. The “DC” tolerance of the core voltage (the amount of uniform noise allowed) is specified as a so-called voltage band around the value of the load line (so-called tolerance band) and has a not insignificant influence on the stability of Vcore and thus of the system. In addition, constant overvolting (upper tolerance range) can already lead to an increase in power consumption and thus also in the thermal load (temperatures).
I already mentioned SVID Offset as well and would like to conclude by explaining that briefly as well. This is where AC loadline amplification comes in (increases VID, VR VOUT, and load voltages) to make even bad CPU samples run stably, even when they shouldn’t! SVID Offset can be used to reduce the idle voltage/VR VOUT for automatic voltages, while the load voltages for better CPU samples are then again similar to those with SVID disabled. So if you have a so-called potato chip, you could try it with Vcore set to <Auto>, LLC (Load Line Calibration) set to default, AC and DC load line set to at least 80 and then enable SVID Offset. This actually always works, but you also have to keep an eye on the temperatures (and voltages).
The following table from Intel’s internal documents shows that one could even work with values around 170 (1.7 mOhm) for DC, while MSI has pushed the default quite low, thus rather in the direction of economy. What this then means in practice and whether one can save even more, you can see on the next pages. Of course, the default settings can differ from board to board and the 80 on board A does not necessarily have to be the same 80 on board B. However, it is also important here that the corresponding LLC is selected (or it is best to leave it at <auto> or <standard> for now).
I know from internal conversations with BIOS developers that e.g. in this area of AC/DC loadline and SVID offset the manufacturers like to cheat a bit to offer more stable overclocking possibilities also for worse CPU samples. The result is well known: more stable OC for dummies who don’t have to painstakingly adjust anything else and unfortunately also higher power consumption and temperature values at the CPU. You can look at it any way you want, but it’s not pretty. Because it is also not sure if the visible menu entries are really the real value or if something is not stored completely different internally with further offsets (i.e. 80 in the AC/DC loadline menu of the UEFI and 90 then as the really used value etc.)
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