Memory Interface--Tri-Channel And DDR3-1600
The Integrated Memory Controller
One of the most widely publicized changes that Intel is making with the Core i7 is moving the memory controller into the CPU itself. Its memory interface supports DDR3 exclusively, and can group three memory modules into a tri-channel array. Depending on the processor, different memory speeds will be supported ; for example, the Core i7 920 and 940 will offer the two memory multipliers, 6 and 8. Together with the internal chip base clock of 133 MHz, that gives us memory speeds of DDR3-800 and DDR3-1066. The Core i7 965 will support the multipliers 10 and 12, giving user the option of installing faster DDR3-1333 or DDR3-1600 RAM.
If you want to run your memory at faster speeds on the lower-end Core i7 models, you’ll have to overclock the CPU’s FSB. In our synthetic tests, the integrated memory controller proved to be much faster than its Core 2 counterpart in the northbridge. It is unclear at the moment whether Intel has given itself the option to use DDR2 in a future revision or lower-cost chip—right now, DDR3 is still more than twice as expensive as DDR2. The Core i7 processor can run in single, dual or tri-channel mode, and it still supports the XMP functionality that can make use of the pre-configured timing profiles offered by some memory modules.
| Processor | Channel | Read | Write | Copy | Latency |
|---|---|---|---|---|---|
| Core i7 | Tri DDR3-1333 | 15.5 GB/s | 13.8 GB/s | 19.4 GB/s | 34.3 ns |
| Core i7 | Dual DDR3-1333 | 14.7 GB/s | 13.8 GB/s | 18.9 GB/s | 29.6 ns |
| Core i7 | Single DDR3-1333 | 10.3 GB/s | 10.3 GB/s | 14.3 GB/s | 28.5 ns |
| Core 2 | Dual DDR3-1333 | 8.1 GB/s | 8.5 GB/s | 7.1 GB/s | 68.8 ns |
| Phenom X4 | Dual DDR2-1066 | 7.6 GB/s | 5.0 GB/s | 8.6 GB/s | 56.3 ns |
We measured the memory bandwidth of the Core i7, and compared it to the fastest Core 2 Extreme QX9770, as well as the Phenom X4 9550 Black Edition. At the same CPU and memory speeds, running in a dual-channel configuration, the Core i7 offers a bandwidth that is on average twice that of the Core 2 ; AMD’s integrated DDR2-1066 memory interface on the Phenom X4 can’t keep up. Running in a tri-channel configuration, the Core i7 reaches a peak bandwidth of 19.4 GB/s.
We can see that the performance increase when going from dual- to tri-channel mode is much lower than when switching from single- to dual-channel mode. It is possible that the Core i7 running at 3.20 GHz is not fast enough yet to reap any real benefit from the added bandwidth of the tri-channel memory interface. Perhaps only future systems or multi-processor server setups will be able to realize performance gains.
Compared to the Core 2 and AMD’s Phenom X4, the memory interface of the Core i7 has the advantage of giving the system a measurable performance boost. At 34 ns, its latency is also much lower than that of the Core 2 (69 ns) and the Phenom X4 (56 ns). Memory-intensive applications run a good deal faster, in particular. Memory company A-DATA is already offering a tri-channel kit that comes in sizes of 3 GB or 6 GB. We also received 6 GB kits from Corsair and OCZ supporting 1,333 and 1,600 MHz frequencies at Intel’s recommended 1.65V ceiling.