The Magic Bus

By Team Digit Published Date
01 - Feb - 2005
| Last Updated
01 - Feb - 2005
The Magic Bus
Man's need for speed is perhaps the strongest driving force behind his innovations; blazingly fast Ferraris and supersonic jet planes bear testament to this. Computing, of course, has not been left untouched. In fact, the advances in technology in terms of processing power have been so rapid over the past 10 years that had they been replicated elsewhere, we would have already visited the most distant galaxies!

One aspect of computing-the PCI bus-has, however, remained untouched for almost a decade! We demystify the latest buzzword in the tech world-PCI express.

Everyone On The PCI Bus!   
We need to first understand how PCIe works, and why it is needed, let's first understand the basic working of a 'bus'. A bus is a bunch of conductors, and is primarily a means of transferring data from one system component to another. For example, a bus is used to transfer data from your hard disk to the printer, or from the processor to the memory, apart from other functions. PCI uses the shared bus architecture, where the devices attached to the bus share the bus' bandwidth when working.

It is interesting to note that the PCI bus is the oldest, slowest bus (or piece of technology, if you will) on your motherboard. Considering that it connects everything else-the latest video cards to the processors to the latest memory-it's a bit scary. In the infographic (on page 33), note how underserved devices connected to your USB or Firewire port or even your SATA-based hard disks are. This is the crux of the problem-the bandwidth requirement of a device is not satisfied by the PCI bus; the bus itself becomes the bottleneck that needs to be rectified for the solution.

For the second problem, consider a hard disk. Under the aegis of PCI, if this disk needs to communicate with the CPU, it will have to go via the Southbridge, stop at the Northbridge for refreshments and finally arrive at its destination, fighting latencies all along the way. "Latencies", you ask? Well, yes, notice how the USB, the FireWire and the Gigabit LAN ports are all sharing the same 133 MBps bus. Fighting for resources means system slowdown. This is problem number two in PCI-land.

The third problem with the PCI architecture is that it is a one-way lane. That is, when the CPU is talking to the Northbridge, no other device can access the CPU. This leads to a scramble for bandwidth and adds to system slowdown.

A slow bus fighting with fellow devices for resources, and clogging at the Northbridge, are a few of the problems your system faces every day. Let's find out how PCI Express will save us.

On The PCI Expressway
The most interesting thing to note is the legacy PCI bus hanging at the bottom right of the infographic (below). PCI becomes a subset of PCIe. To understand how PCIe betters this, let's first understand the basic structure of PCIe.

Unlike PCI, PCIe is a two-lane transfer system. This lets data come and go simultaneously. In PCIe, each of the dedicated connections/paths between the switch and the device is called a link. A PCIe x1 link has one lane, which can transfer data to and fro simultaneously, providing transfer speeds of 2.5 GB/s one way, such that an x1 link has a total transfer capacity of 5 GB/s.

Now, if we add another lane to this, we get an x2 link, wherein each lane can transfer 5 GB/s, making the total available bandwidth 10 GB/s. We can theoretically have x32 PCIe connections with transfer speeds of 20 GB/s.

To put things into perspective, if you could manage a x32 PCIe connection between, say, your 80 GB HDD to another 80 GB HDD, you could transfer the entire 80 GB of data in about four seconds! But how does all this bandwidth help us?

Remember how in PCI, if the processor was talking to the Northbridge, it was inaccessible to other elements of a system due to the single-channel nature of PCI? The dual-channel feature of PCIe overcomes this and allows multiple devices to communicate with each other.

PCIe would be the interconnect between various devices and bridges as well as the interconnect between the bridges themselves

PCIe will replace the entire bus architecture on a motherboard and would be the interconnect between various devices and bridges as well as the interconnect between the bridges.

PCIe is incredibly scalable and allows additional links to be provided as and when bandwidth requirements increase. For example, for a Gigabit LAN card, an x1 lane is sufficient, but for replacing your 8X AGP card, a motherboard burning an x16 link is necessary and possible.

Another aspect that has changed in PCIe is the legacy concept of PCI bus mastering. PCIe features a switch system that sits in the centre, and all the system devices connect to it via a dedicated link similar to the bus connecting the CPU to the Northbridge. This is called a 'Point-to-Point Interconnect System'.

What this switch does is route all the PCI/PCIe data through itself, allowing two devices to talk to each other-if the communication does not involve any cache-specific data or instructions, this switch will simply offer the two devices a dedicated and direct link to share with each other. This obviously makes information exchange between devices extremely fast.

The number in red express the transfer speed supported by the device. those in black are the transfer speed allowed by the bus they are connected to, in this case, PCI.
What Does This Mean?
The question now is, how well does all this translate to real-world usage. Will your games finally run enormously faster and better?

Well, not really. The performance difference between AGP 4x (1 GB/s) and AGP 8x (2 GB/s) is negligible. So, PCIe x16 cards will not make Doom 3 run any faster because there simply isn't that much data to transfer.

Immediate beneficiaries would be people into video-editing; the performance of their machines would see a huge boost due to the dual-channel capability. This is because peripheral devices such as hard disks or external FireWire devices will be able to reap the benefits that additional bandwidth and reduced latencies offer.

Our recommendation is that you should not upgrade to PCIe unless you work heavily on video editing. If, however, you are about to upgrade anyway, go ahead and get a PCIe-based motherboard-PCIe as a standard is an inevitability.

A word of caution, though: a motherboard can only support either AGP or PCIe. So if you buy a PCIe motherboard, you will have to buy a PCIe graphics card as well. On the upside, PCIe is backward-compatible with older PCI components.

If you go out to buy a PCIe motherboard, expect to be bombarded with feature-rich boards

What's On Offer?
Along with the obvious advantage of the PCIe bus, these boards offer a gamut of features previously unheard of: if you go out to buy a PCIe motherboard, expect to be bombarded with feature-rich boards. We take you through the best of what you can expect from these boards.

The latest PCIe chipsets from Intel, the 915, 925 and their variants, come with an on-board Intel Graphics Media Accelerator, which has Direct-X 9 support, and is stipulated to match the performance of the GeForce FX5200. The 915G and 915GV chipsets also have native support for HDTV and widescreen LCDs, with support for multiple monitors. The AMD solutions from nVidia do little to better this, what with the nForce4 chipset shipping without any onboard video at all!

PCI: The Evolution Path
Version Technology 
PCI Intel developed the 32-bit PCI bus, which runs at 33 MHz with a transfer speed of 133 MB/s 
PCI-X The 64-bit version of the PCI bus, running at 33 MHz with a transfer speed of 133 MB/s 
PCI-X 64-bit version running at 66MHz with speeds up to 512 MB/s 
PCI-X 64-bit version running at 133MHz with speeds up to 1 GB/s 
PCI-X 2.0 4.3 GB/s 
The reason PCI-X is not implemented in desktops is the extremely high cost of manufacture of PCI-X boards

ATi Radeon Xpress200/200P chipsets do much better, with their on-board video solution having 128 MB of memory, matching the performance of a Radeon X300. It also has built-in dual-display support (Analogue and DVI) along with SURROUNDVIEW, which is essentially support for three or more monitors.

Both Via K8T890 and K8T890PRO as well as SiS 756 and 965 chipsets do not feature any onboard video. So on this front, ATi leads the pack.

This is where the Intel chipsets emerge as clear winners. With on-board support for eight-channel Hi-Definition audio with a 192 kHz sampling rate, which can play everything from High Definition Dolby DTS to vanilla CDs without problems. It even supports up to 15 simultaneous audio streams.

The various AMD-based PCIe chipsets support eight-channel audio, but this is not High Definition. In fact, the Via K8T890 only has integrated AC-97 six-channel audio, though it does come with an eight-channel audio PCI controller.

The much-touted Soundstorm integration into nForce4 did not materialise, and we get the simple eight-channel integrated audio, which again, is not high-definition. Likewise with ATi and SiS. The Gigabyte K8NXP-9, though, improves on the nForce4 chipset and provides six programmable audio jacks. No High-Definition audio, though.

Storage And Memory
Every PCIe chipset comes with at least four SATA connections, with the Gigabyte K8NXP-9 offering eight. All offer ATA connections and most of them also have the capability to be configured in various RAID flavours: nForce4, suppors0, 1, 0 1 RAID, and they also allow you to change the RAID configuration on the fly.

Another cool piece of technology that has made its way to today's motherboards is CQ, or Native Command Queuing. This technology enables a hard disk to decide the order of reading data to minimise data throughput time-so you can watch a movie even when some encoding is going on in the background, without stressing out your hard disk.

The nForce4 and Intel chipsets support this technology; however, neither the ATi Xpress200 nor the VIA or SiS chipsets support this feature.

It should be noted here that not all these boards support DDR2. For example, the VIA K8T890 series supports only DDR memory. The Intel 915GV and 915G support both DDR and DDR2. Note that the 915GV Express chipsets support only the latest LGA775 socket Pentium 4s.

Network Support And Other Connections
All these chipsets provide a variety of connectivity options. Each of these PCIe chipsets provides at least eight USB ports and one FireWire, with the nForce4 providing 10 USB and two FireWire ports. Apart from this, it also supports Gigabit LAN.

Interestingly, the Gigabyte K8NXP-9 offers two on-board GigabitLAN ports. One is an nForce 4, and the other, added by Gigabyte, utilises a PCIe x1 link. This apart, the motherboard ships with an additional PCI Wireless LAN card as well! The card is a Gigabyte addition and is not on-board. The ASUS A8V-E Deluxe does feature an on-board wireless LAN conforming to the 802.11g standard.

For gamers who will be upgrading anyway, ATi and nVidia have released their versions of the PCIe x16 graphics cards, which are available without having to pay a premium!

Unlike ATi, though, which launched native PCIe cards, nVIDIA's solutions use a PCIe-to-AGP bridge and have merely offered bridged versions of their current AGP 8X cards. However, the performance difference due to the bridge is negligible and you can enjoy playing your favourite games on high-end cards such as the 6800 GT and the Radeon X850.

In The Future
The sheer convenience of the serial connections offered by PCIe and the bandwidth available could change the motherboard manufacturing process entirely by giving greater flexibility in terms of component placements. We might see modular PCs where the I/O system is removable from the processor and could connect to it via PCIe sockets.

This does not signal the death of USB and FireWire, though, because by the time the above scenario is realised, they would be so integrated with our lives that even the modular PCs would need to provide legacy support for them; just as PCIe boards provide support for the older generation PCI cards.

Despite some glitches like non-compatibility with AGP, PCIe technology is remarkable. It has tremendous industry support, and not without reason. The bus is well on its way.

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