Greg, OMG! I am so behind. Very nice explanation. I appreciate this. How do you keep on top of this? Thank you. Duncan
At 23:00 11/10/2008 -0600, you wrote:
> Greg, > Could you be a bit more expansive? I am still confused by your > explanation. Perhaps we are just talking "bench racing" again? > I have ram that is Intel-rated PC66, PC100, and PC133. I know that > these > numbers > follow the Intel FSB freq. > > Then it gets confusing to me because then I jump to ram for AMD that is > PC3200 and PC3500. > PC66, PC100, etc. were used to refer to the memory clock speed for Single Data Rate (SDR) "original" SDRAM. With the advent of DDR, the PCxxxx designation has been used to refer to the memory's maximum theoretical throughput in MB/s. DDR-266 is PC-2100 DDR2-800 is PC2-6400 DDR3-1333 is PC3-10600 The DDR[2/3]-xxxx is used to refer to the number of transfers per second. DDR added the ability to transfer data on both the rise and the fall of the i/o bus clock cycle. Whereas PC-133 transfers 133 million times per second on a 133MHz bus, DDR-266 transfers 266 million times per second, on the same 133MHz bus. Since the memory bus isn't actually running at a 266MHz actual clock speed, but that's the effective clock speed, the proper term is 266MT/s (Million Transfers/second). 133MHz clock rate * 2 transfers per clock = 266 million transfers per second. 266MT/s * 64 bits (bus width in bits) / 8 (bits in a byte) = 2128MB/s...rounded to 2100MB/s. Hence the PC-2100 designation. (Note that there is some additional rounding here on my part, since a 133Mhz bus is actually 133,333,333Hz) Your DDR3-1333, PC3-10600 memory operates at a 667MHz i/o bus, 1333MT/s, and is capable of 10,600MB/s of throughput per stick. 667MHz clock * 2 = 1333MT/s, 1333MT/s * 64 bits bus width / 8 bits in a byte = ~10600MB/s. This has nothing to do with AMD's performance rating system (ie: Athlon XP 2500+, Athlon 64 3000+).
