Core i7 Up Close

Core i7 Up Close

Tick tock? More like ding-dong, mutha—shut your mouth. What baby? We’re talkin’ about Core i7.

Our apologies to Isaac Hayes, but if he were alive, we’re almost certain he would have been tapped to hammer out a theme song for Intel’s most significant CPU launch in, well, ever.

Why is this CPU more significant than the 8088, Pentium, or Pentium M? As the second new chip produced after a series of embarrassing losses to archrival AMD, the Core i7 will answer for the world whether Intel is prepared to ride the momentum of its Core 2 launch with another winning chip or if it’s content to rest on its laurels, as it did with the Pentium 4.

Core i7 also represents a major new direction for Intel, which has stubbornly clung to the ancient front-side-bus architecture and discrete memory controller for years. Indeed, with its triple-channel integrated DDR3 memory controller and chip-to-chip interconnect, the block map of a Core i7 looks more like an Athlon 64 than a Core 2 chip.

Intel actually has three quad-core Core i7 CPUs ready: the top-end 3.2GHz Core i7-965 Extreme Edition, the performance-oriented 2.93GHz Core i7-940, and the midrange 2.66GHz Core i7-920. For the most part, all three are exactly the same except for clock speeds, multiplier locking (only the Extreme is unlocked), and QuickPath Interconnect speed. See the chart on page 42 for details.

The bigger issue is how Core i7 performs. To find out, we ran the Extreme 965 against AMD’s fastest proc as well as Intel’s previous top gun in a gauntlet of benchmarks. Read on for the results.

Intel takes a bold approach to processor architecture, multi-core computing

As a buttoned-down company, Intel rarely likes to make sweeping changes, but its upcoming Core i7 CPU is a major break from the past. Gone is the ancient front-side bus that connects all of the current-gen CPU cores. Instead, cores will communicate via a high-speed crossbar switch, and different CPUs will communicate via a high-speed interconnect.

Also on the outs is the need for an external memory controller. Intel, which has relied on gluing two dual-core chips together under the heat spreader to make its quad-core CPUs, is now placing all four cores on a single die.

Even overclocking, which was once verboten to even talk about within 10 miles of Intel’s HQ, is now automatically supported. Intrigued? You should be. Intel’s Core i7 is the most radical new design the company has taken in decades.

An Inside Job

One of Core i7’s most significant changes is the inclusion of an integrated memory controller. Instead of memory accesses going from the CPU across a relatively slow front-side bus to the motherboard chipset and finally to the RAM, an IMC will eliminate the need for a front-side bus and external memory controller. The result is dramatically lower latency than was found in the Core 2 and Pentium 4 CPUs.

Why can’t the memory controller on the motherboard simply be pushed to higher speeds to match an IMC? Remember, when you’re talking about a memory controller residing directly in the core, the signals have to travel mere millimeters across silicon that’s running at several gigahertz. With an external design, the signals have to travel out of the CPU to a memory controller in the chipset an inch or so away. It’s not just distance, either—the data is traveling across a PCB at far, far slower speeds than it would if it were within the CPU. In essence, it’s like having to go from an interstate to an unpaved, bumpy road.

Of course, if you’re an AMD loyalist, you’re probably bristling at the thought of Intel calling an IMC an innovation. After all, AMD did it first. So doesn’t that make AMD the pioneer? We asked Intel the same question. The company’s response: One: An IMC isn’t an AMD invention and, in fact, Intel had both an IMC and graphics core planned for its never-released Timna CPU years before the Athlon 64. Two: If AMD’s IMC design is so great, why does the Core 2 so thoroughly trash it with an external controller design? In short, Intel’s message to the AMD fanboys is nyah, nyah!

Naturally, you’re probably wondering why Intel thinks it needs an IMC now. Intel says the more efficient, faster execution engine of the Core i7 chip benefits from the internal controller more than previous designs. The new design demands boatloads of bandwidth and low latency to keep it from starving as it waits for data.

Memory a Trois

The Core i7 CPU is designed to be a very wide chip capable of executing instructions with far more parallelism than previous designs. But keeping the chip fed requires tons of bandwidth. To achieve that goal, the top-end Core i7 CPUs will feature an integrated tri-channel DDR3 controller. Just as you had to populate both independent channels in a dual-channel motherboard, you’ll have to run three sticks of memory to give the chip the most bandwidth possible. This does present some problems for board vendors though, as standard consumer mobos have limited real estate.

Most performance boards will feature six memory slots jammed onto the PCB, but some will feature only four. On these four-slot boards, you’ll plug in three sticks of RAM and use the fourth only if you absolutely have to, as populating the last slot will actually reduce the bandwidth of the system. Intel, in fact, recommends the fourth slot only for people who need more RAM than bandwidth. With three 2GB DIMMs, though, most enthusiast systems will feature 6GB of RAM as standard.

Although it may change, Core i7 will support DDR3/1066, with higher unofficial speeds supported through overclocking. Folks hoping to reuse DDR2 RAM with Intel’s budget chips next year can forget about it. Intel has no plans to support DDR2 with a Core i7 chip at this point, and with DDR3 prices getting far friendlier to the wallet, we don’t expect the company to change its mind.

Hyper-Threading Revisited

A CPU core can execute only one instruction thread at a time. Since that thread will touch on only some portions of the CPU, resources that are not used sit idle. To address that, Intel introduced consumers to Hyper-Threading with its 3.06GHz Pentium 4 chip. Hyper-Threading, more commonly called simultaneous multi-threading, partitioned the CPU’s resources so that multiple threads could be executed simultaneously. In essence, a single-core Pentium 4 appeared as two CPUs to the OS. Because it was actually just one core dividing its resources, you didn’t get the same performance boost you would receive from adding a second core, but Hyper-Threading did generally smooth out multitasking, and in applications that were optimized for multi-threading, you would see a modest performance advantage.

The 45nm-based Core i7 will pack all four cores on a single die. The cores will communicate via a high-speed crossbar switch. An integrated memory controller and Quick Path Interconnect links to other CPUs also make the Core i7 very AMD-like.

The problem was that very few applications were coded for Hyper-Threading when it was released and performance could actually be hindered. Hyper-Threading went away with the Core 2 series of CPUs, but Intel has dusted off the concept for the new Core i7 series because the transistor cost is minimal and the performance benefits stand to be far better than what the Pentium 4 could ever achieve.

Intel toyed with the idea of redubbing the feature Hyper-Threading 2 but decided against it, as the essential technology is unchanged. So why should we expect Hyper-Threading to be more successful this go around? Intel says it’s due to Core i7’s huge advantage over the Pentium 4 in bandwidth, parallelism, cache sizes, and performance. Depending on the application, the company says you can expect from 10 to 30 percent more performance with Hyper-Threading enabled. Still, Intel doesn’t force it down your throat because it knows many people still have mixed feelings about the feature. The company recommends that you give it a spin with your apps. If you don’t like it, you can just switch it off in the BIOS. Intel’s pretty confident, however, that you’ll leave it on.