PCI Express Replaces AGP 8X
PCI Express (not to be confused with PCI-X) is the replacement for both PCI and AGP. While the classic PCI Bus is based upon a parallel architecture, PCI Express is serial based, drastically reducing pin count. It is a point-to-point protocol much like AGP. Devices do not share bandwidth. PCI Express means X16 next generation graphics delivering 4GBs per second per direction.
To clarify, there are four different types of PCI, the original PCI, 66MHz PCI, PCI-X at 64bit/133MHz, and the introduced PCI-X 2.0. Due to the complexity and cost, most of these technologies will probably not be adopted by desktop motherboard manufacturers. PCI-X, for example, requires a controller for every slot and that is just too expensive.
So that everyone wins, Intel has proposed a solution to the cost and complexity problem. What they have done is to take a technology formally known as 3GIO or 3rd Generation I/O, and sell it to the industry as PCI Express. To accomplish this, Intel has strategically partnered with IBM, Dell, Compaq, HP, Microsoft and some other companies, which is a wise move to help forward industry acceptance.
PCI Express is an open-standards PCI replacement. Unlike PCI and PCI-X, which are based on 32- and 64-bit parallel buses, PCI Express uses high-speed serial link technology similar to that found in Gigabit1 Ethernet, Serial ATA (SATA), and Serial-Attached SCSI (SAS). PCI Express reflects an industry trend to replace legacy shared parallel buses with high-speed point-to-point serial buses.
PCI Express will replace the PCI, PCI-X, and AGP parallel buses gradually over the next few years. It will initially replace buses that need the additional performance or features. For instance, PCI Express will initially be deployed as a replacement for the AGP8X graphics bus in client systems, providing high bandwidth and support for multimedia traffic. It will also coexist with and ultimately replace the PCI-X bus in server systems.
PCI Express has the following advantages over PCI:
- Serial technology providing scalable performance.
- High bandwidth—Initially, 5-80 gigabits per second (Gbps) peak theoretical bandwidth, depending on the implementation.
- Point-to-point link dedicated to each device, instead of the PCI shared bus.
- Opportunities for lower latency (or delay) in server architectures, because PCI Express provides a more direct connection to the chip set Northbridge (see Note 2) than PCI-X.
- Small connectors and, in many cases, easier implementation for system designers.
- Advanced features—Quality of service (QoS) via isochronous channels for guaranteed bandwidth delivery when required, advanced power management, and native hot plug/hot swap support.
Bandwidth of PCI, PCI-X, and AGP Buses Bus and Frequency Peak 32-Bit Transfer Rate Peak 64-Bit Transfer Rate 33-MHz PCI 133 MB/sec 266 MB/sec 66-MHz PCI 266 MB/sec 532 MB/sec 100-MHz PCI-X Not applicable 800 MB/sec 133-MHz PCI-X Not applicable 1 GB/sec AGP8X 2.1 GB/sec Not applicable
There are two main reasons to care about PCI-Expres: 1) PCI is now an old standard dating back to the early 90’s and no longer fits our needs in terms of speed/performance. 2) AGP also is in a similar position as PCI now, and chipset manufacturers are killing AGP motherboard support in favor of the much faster PCI Express interface. This means you are looking at a forced transition in the graphic sector, thus you really don’t have a lot of choice in the coming years.
A 16x PCI Express connection is at least 190% Faster than AGP 8x but this is the connection between the system and the video card. You use the connection the most when your video card is low on memory or when the game you are using uses a Direct X or Open GL feature that isn’t supported in hardware.
In an upcoming article we will be discussing the new power supplies available today. The need for a new power supply is partly due to the introduction of PCI Express. Although the PCI-E spec calls for a PCI Express power connector, many PCI-E cards don’t currently use it. Cards based on the X600, X700, X300, or 6600 series graphic chipsets also rarely use the new extra power supply connector. If you are in a situation where you need a PCI Express power connector but the power supply doesn’t have one you can always just use a PCI Express Power Adapter that converts a 4-pin molex connector to PCI-E Power.
Even Apple is making the move. The new Power Mac G5 introduces a modern PCI Express architecture to the Mac platform. So if you are planning on building a new system, you really should go with PCI Express. Both ATI and Nvidia are offering high performance PCI Express x16 cards.
trc contributor: Derek Winterstien