Ivy Bridge Memory Controller
Memory controllers integrated into Intel CPUs have gone a long and winding way but the engineers seem to have finally found the optimal solution, so the Ivy Bridge memory controller has no significant differences from its predecessor. It is still based on an internal ring bus introduced back in the Sandy Bridge microarchitecture. The bus provides each execution and graphics core a quick and equal access both to the L3 cache and system memory. As a result, the peak data-transfer rate increases considerably and the Core CPUs for LGA1155 platforms are faster than their opponents in memory subsystem benchmarks.
So, it is no wonder that the same principle of interaction between the CPU and memory controller is implemented in the Ivy Bridge microarchitecture. Moreover, the engineers didn’t even try to revise anything in the controller’s internals. There are but minor improvements, especially as the Ivy Bridge series are perfectly compatible with the existing LGA1155 platform. It is still a dual-channel controller for DDR3 SDRAM but it supports higher clock rates than its Sandy Bridge predecessor: DDR3-1333 and DDR3-1600. The XMP technology has been upgraded to version 1.3. Both improvements can hardly be taken seriously because they do not mean much in practical terms.
Like its predecessor, the Ivy Bridge memory controller can work in symmetry mode (when the amount, clock rate and timings of memory modules in both channels coincide) as well as in compatibility mode which is referred to as Intel Flex Memory Technology. The latter’s point is in dividing the whole memory array into two parts, basing on the modules' specs: one with symmetric access mode and another with asymmetric single-channel mode. As a result, LGA1155 systems can be equipped with different memory modules without a catastrophic performance hit.
Each controller channel can work with one or two DDR3 SDRAM modules, either single- or dual-sided, so the maximum amount of system memory supported by today's LGA1155 systems is 32 gigabytes. For users who need more memory, Intel offers the higher-class LGA2011 platform.
Everything we’ve said so far in this section of our review is but a description of standard properties of Intel’s memory controller which applies to both Ivy Bridge and Sandy Bridge series. The new CPU design does have something new in terms of the memory controller, though.
There are two such things we want to note here. First, the frequency multiplier has become more flexible. With Sandy Bridge, DDR3-2400 was the highest memory mode possible whereas Ivy Bridge CPUs can clock system memory at frequencies up to 3200 MHz. And second, an additional variable multiplier has been introduced into the memory frequency formula that allows changing the clock rate with a step of 200 MHz besides the conventional step of 266 MHz.
All of this makes memory clocking much more flexible than before. Considering that LGA1155 systems do not allow changing the base clock rate, there are quite a lot of memory frequency options available now. Here is the full list for DDR3 SDRAM on an LGA1155 platform with an Ivy Bridge CPU:
It must be noted that this screenshot was captured on a Z77-based mainboard with a Core i5-3570K. Mainboards with H series chipsets are not that flexible when it comes to clocking DDR3 SDRAM. They are limited to the standard values of DDR3-1333 and DDR3-1600. Another limitation concerns Ivy Bridge CPUs other than the overclocker-friendly K series. They can only increase the memory clock rate to 2400 MHz.
So, it is with Core i5-3570K and Core i7-3770K processors that you can get as much flexibility in memory configuring as possible. Our experiments suggest that high-speed memory modes are perfectly functional and do not even require any tricks like fine-tuning secondary voltages. For example, it only took a small (by a mere 50 millivolts) increase in memory controller voltage for our Ivy Bridge CPU to work faultlessly with DDR3-2667 SDRAM.
As a matter of fact, Intel emphasizes the fact that it's extremely easy to reach high memory clock rates now. And you can keep your system stable by changing two voltages only. These are VDDQ, which is applied directly to the modules, and VCCSA, which powers the system agent and memory controller. It is not recommended to increase the former above 1.65 volts to safeguard the CPU against damage or degradation. The latter voltage is 0.925 volts by default and you can increase it a little to make your system more stable at high DDR3 clock rates.
Thus, the innovations in the Ivy Bridge memory controller seem to be targeted at overclockers who prefer running their DDR3 SDRAM in nonstandard high-frequency mode. As for the controller’s operation in its default mode, Intel doesn’t promise any changes compared to Sandy Bridge.
Anyway, we carried out a practical test to compare the Sandy Bridge and Ivy Bridge memory controllers as they worked with regular dual-channel DDR3-1600 at standard timings of 9-9-9-27-1N. We used the same LGA1155 platform, changing the CPUs only. For the clock rate not to affect the performance of the integrated memory controllers, the 22nm and 32nm CPUs were both overclocked to the same frequency of 4.5 GHz. Every power-saving technology, and Turbo Boost too, was turned off. Considering that the frequency of 1600 MHz can be reached in two ways with Ivy Bridge CPUs (1600 MHz DDR = 100 MHz x 1.33 x 6 or 100 MHz x 1.00 x 8), we checked out both of them.
The table below shows the results we obtained:
First off, we can note that the Ivy Bridge memory controller’s performance does not depend on the additional multiplier. We get the same results irrespective of whether we use a step of 200 or 266 MHz to build the memory clock rate. The difference amounts to a tenth of percent, which may just as well be due to some measurement inaccuracies.
Comparing the memory controllers of the Sandy Bridge and Ivy Bridge CPUs, we can see that they don’t seem to be identical. While the memory bandwidth is almost the same, the practical latencies can differ by a few percent. And it’s absolutely impossible to predict which controller is going to be faster in a particular test, so we can conclude that neither the Sandy Bridge nor the Ivy Bridge CPU offers specific benefits in terms of system memory performance.