AMD’s new K6-3
Last year in November AMD improved the K6-2 to the CXT-core with a feature called ‘write combining’. This feature should be known to many of us from the earlier days of Intel’s Pentium Pro processor. ‘Write combining’ has been included in all the sixth generation processor core from Intel, you can find it in the Pentium Pro, the Pentium II, the Celeron and the upcoming Pentium III. What it does is writing to memory in larger chunks and thus less often than an application may demand it. An application that e.g. wants to write only one or two bytes to memory (and this could e.g. be the frame buffer of the graphics card) would usually make the processor do a ‘store’-instruction over the memory bus right away. Especially graphic applications write a whole lot of data to the frame buffer all of the time and it is pretty slow if the CPU has to push only one or two bytes over the 64-bit wide data bus to the graphics card. Write combining waits until 64 bit of data have been collected and then transfers the data to the memory location. This results in only one memory write every four bytes, instead of four memory writes that take a lot more time. As already said, Pentium Pro was the first PC-processor equipped with this feature and many of you can certainly remember when the programm ‘fastvid’ improved Quake on a Pentium Pro significantly, because it enabled write combining to the graphics card’s frame buffer. The K6-2 with the ‘CXT’-core is now able to do the same, but the drivers of the graphics card have to be rewritten for it, which hasn’t happened in the majority of cases yet.
K6-3 adds a 256 kB on-die L2-cache to the CXT-core. This should result in a significant performance increase, because it removes one of the most important speed killers of Socket 7-systems, the on-board L2-cache that’s only running at bus clock speed, thus at either only 66 or on Super7-boards at 100 MHz. Intel’s Pentium Pro was the first CPU that had a L2-cache running at core clock and thus this L2-cache was getting faster with a faster processor core. Pentium II has a L2-cache that’s running at only half the core clock, but it’s still getting faster with a faster CPU. On Socket7 the L2-cache always ran at a fixed speed, regardless if you’ve had a K6-2 300 or K6-2 400 CPU. K6-3 does not suffer from this problem anymore, its L2-cache is ‘as fast’ as the CPU-core, so that faster K6-3s don’t have to wait longer for the L2-cache to deliver its data. The on-board L2-cache on Socket7-boards turns into L3-cache and can still improve the overall system performance by a few percent points. K6-3 is thus expected to perform significantly better than K6-2, and you can plug it into any Socket7-board that supports its voltage (2.3-2.5 V), as long as if it is upgraded to the latest BIOS.