Video Memory Cache Mode (ASUS mainboards)
By definition, video memory is excluded from being cacheable since otherwise the large data chunks would fill up the L2 chache of the CPU immediately. When the CPU calculates 3D, that is, floating point operations, every data point needs to be written to the local frame buffer (on board graphics memory). Unfortunately, the CPU can either write or calculate, therefore each write cycle interrupts the floating point operations, in simple terms, the FPU unit can only work at half speed. Intel were the first to incorporate 8, so called, write combine buffers on the Pentium II, which was one of the reasons why the PII had a superior 3D performance compared to the original K6, at least at low resolutions. The reason is simple, instead of single bit writes, the data are temporarily stored in write combine buffers and released in burst mode. Therefore, depending on the size of the WC-buffers, FPU calculations are interrupted much less frequently, in other words, the graphics performance of the CPU is almost doubled.
Graphics data inherently feature high locality, therefore there is no problem pipelining these data. With the CXT revision of the K6-2 and the K6-III, AMD added two write combine buffers and, as we were the first to post, proper addressing of these buffers via software would boost graphics performance by as much as 30% within the identical hardware configuration. The Athlon family has four aggressively programmed 64 bit WC buffers buffers that can be placed over the local frame buffers for both data and hardware acceleration. Compared to the PII / III WC buffers, the novelty is being capable of executing out of order writes in ascending and descending order and thus taking write combining to a new level. This way the efficacy of the 3D floating point operations can almost be doubled. Available settings in the ASUS A7V BIOS are either uncached (UC) or uncached speculative write combining (USWC). Needless to say that USWC needs to be supported by both graphics adapter hardware and drivers but if it works, it is one sweet feature.
from
Lost Circuits
