Just like HyperTransport, QuickPath Interconnect provides two separate lanes for the communication between the CPU and the chipset, as you can see in Figure 3. This allows the CPU to transmit (“write”) and receive (“read”) I/O data at the same time (i.e., in parallel). On the traditional architecture using a single external bus since the external bus is used for both input and output operations reads and writes cannot be done at the same time.
Speaking of chipsets, Intel will initially launch single-chip solutions. Since on CPUs with embedded memory controllers the equivalent of the north bridge chip is embedded inside the CPU, the chipset works as the south bridge chip or “I/O Hub” or simply “IOH” on Intel’s lingo.
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Figure 3: The QuickPath Interconnect provides separated input and output datapaths.
So, how the QuickPath Interconnect works?
Each lane transfers 20 bits per time. From these 20 bits, 16 bits are used for data and the remaining 4 bits are used for a correction code called CRC (Cyclical Redundancy Check), which allows the receiver to check if the received data is intact.
The first version of the QuickPath Interconnect will work with a clock rate of 3.2 GHz transferring two data per clock cycle (a technique called DDR, Double Data Rate), making the bus to work as if it was using a 6.4 GHz clock rate (Intel uses the GT/s unit – which means giga transfers per second – to represent this). Since 16 bits are transmitted per time, we have a maximum theoretical transfer rate of 12.8 GB/s on each lane (6.4 GHz x 16 bits / 8). You will see some people saying that the QuickPath Interconnect has a maximum theoretical transfer rate of 25.6 GB/s because they simple multiply the transfer rate by two to cover the two datapaths. We don’t agree with this methodology. In brief, it is as if we said that a highway has a speed limit of 130 MPH just because there is a speed limit of 65 MPH in each direction. It makes no sense.
So compared to the front side bus QuickPath Interconnect transmits fewer bits per clock cycle but works at a far higher clock rate. Currently the fastest front side bus available on Intel processors is of 1,600 MHz (actually 400 MHz transferring four data per clock cycle, so QuickPath Interconnect works with a base clock eight times higher), meaning a maximum theoretical transfer rate of 12.8 GB/s, the same as QuickPath. QPI, however, offers 12.8 GB/s on each direction, while a 1,600 MHz front side bus provides this bandwidth for both read and write operations – and both cannot be executed at the same time on the FSB, limitation not present on QPI. Also since the front side bus transfers both memory and I/O requests, there are always more data being transferred on this bus compared to QPI, which carries only I/O requests. So QPI will work “less busy” and thus having more bandwidth available.
QuickPath Interconnect is also faster than HyperTransport. The maximum transfer rate of HyperTransport technology is 10.4 GB/s (which is already slower than QuickPath Interconnect), but current Phenom processors use a lower transfer rate of 7.2 GB/s. So Intel Core i7 CPU will have an external bus 78% faster than the one used on AMD Phenom processors. Other CPUs from AMD like Athlon (formerly known as Athlon 64) and Athlon X2 (formerly known as Athlon 64 X2) use an even lower transfer rate, 4 GB/s – QPI is 220% faster than that.
Going down to the electrical transmission, each bit is transferred using a differential pair, as shown in Figure 4 (please read this tutorial to understand how differential transmission works). So for each bit two wires are used. QuickPath Interconnect uses a total of 84 wires (including the two lanes), which is roughly half the number of wires used on the front side bus of current Intel CPUs (150 wires). So the third advantage of QuickPath Interconnect over front side bus is using less wires (in case you are wondering, the first advantage is providing separated datapaths for memory and I/O requests and the second advantage is providing separated datapaths for reads and writes).
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Figure 4: QuickPath Interconnect on the physical level.
QuickPath uses a layered architecture (i.e., similar to the architecture used on networks) with four layers: Physical, Link, Routing and Protocol.
Now let’s talk about some advanced techniques introduced on QuickPath Interconnect.