Re: Re: Re: what's "truth"
Originally posted by AidenShaw
Huh?? Just over twice as fast?
First of all, the P4 bus is 64-bits wide, not 32-bits like the 970 bus. So, "one transfer" is 64-bits at 800 MHz, vs 32-bits at 900 MHz (assuming 1.8GHz CPU).
I don't think we know enough about the 970 bus and memory controller to compare latencies. Does the 970 have to use 2 transfers on the 32-bit bus to pass the 64-bit memory address? That will hurt latency.
Does the 970 (or the P4) immediately forward the first chunk of data up through the cache levels to the registers? Or does it wait to fill a cache line (32 to 64 bytes) before making the data available - big latency issue here.
Most likely the 970 interface is packetised, like HyperTransport is, whereas the P4 bus is the old fashioned bus type that isn't packetised. Packetisation adds to the latency of course, but means that you can have thinner busses, and variable width busses (e.g., HyperTransport can vary from 2-bits in each direction to 32-bits in each direction), and also run the bus at faster speeds.
Previous to this rumour, I thought that a dual 970 PowerMac would have 1 northbridge that would connect to the processors (one port per processor), and a dual-channel memory controller.
Now I believe that each processor has its own Northbridge, with dual-channel memory controller. This makes sense, as the same northbridge element can be used for 1 processor designs (PowerBook, iMac2004, etc) up to many-processor designs (e.g., 8-processor Xserve or something). Hence 8 slots on the motherboard. Hence the IBM 980 having this (or similar) northbridge logic integrated into the processor.
I expect that this northbridges connect to each other, and other system components, using HyperTransport - even if the IBM 970 bus is not HyperTransport itself (a standard 6.4GB/s HyperTransport configuration is 16/16 at 800MHz DDR, not 32/32 at 450MHz DDR).
However I don't really trust the rumours since a couple of MacB rumours ago ... so using them as a basis for the above reasoning is not the safest thing to do.
