 |
 |
 |
 |
|
|
| |
| |
| |
|
||||||||||||
Tech Tip 66 - Take the PCI Express for Great Video
|
||||||||||||
|
One of the hot new technologies is something called PCI Express, a new I/O (Input/Output) bus architecture that is the first big step in this direction in at least a decade. The most evident performance measure of our computers is the speed that detailed graphics are updated. PCI Express, also known as PCIe, gives this performance a boost way past anything that was available before.1. What’s an I/O Bus?
Most of the hardware that makes your computer a computer is actually on a single Motherboard. It has the CPU (Central Processing Unit) in the form of a single chip microprocessor such as an Athlon 64 and the main memory that we usually call RAM for Random Access Memory. These two items handle most of the actual computing functions and have a very high-speed bus between them. But, raw computing isn’t much fun, especially when it comes to video games and simulations. You need some eye-dazzling color graphics to show off what your computer can do. The circuits that generate those graphics are usually on a plug-in gadget called a graphics adapter or Video Card. The video card connects to the motherboard through a connector with lots of pins. It takes all those pins to carry the I/O bus with many data bits and address lines. Generally, the CPU puts an address on the address part of the I/O bus to point at a particular I/O address location. These addresses are a lot like memory addresses except that instead of a RAM device, it’s an input or output device that is being fingered. Then, the CPU either sends data down the data bus part of the I/O bus or requests that the I/O device drive data back from that direction. Data goes out and data comes in – that’s why it’s called an Input/Output bus. |
2. A Little History of the PC I/O Bus
At the dawn of the PC era, we had the ISA (Industry Standard Architecture) bus that moved 16 bits of data at a time and was clocked at 8.33 Megahertz. The CPUs back then were 16 bit affairs, so the ISA bus was just perfect. The connector had fewer connections. Life was simple back then, but our computers were slow. In the early nineties, CPUs expanded to 32 bits, so the I/O bus had to grow up, too. The PCI (Peripheral Component Interconnect) |
bus has twice as many address and data pins, which made the connector a lot more complex with the pins squeezed closer together. The clock frequency took a leap to 33 Megahertz, so circuit routing was trickier and early PCI boards worked only with particular motherboards.3. The AGP Video Bus Makes 3-D Possible
At the end of the nineties, the AGP (Accelerated Graphics Port, also called Advanced Graphics Port) special purpose video bus came about. It’s a 32 bit bus like its predecessor. The PCI bus was still used as the basic control interface to the video card, but the AGP handled most of the data transfers for 3-D video processing.
Since the AGP was dedicated to video processing, they took liberties to change it often and it was difficult keeping up with which video card worked with which motherboard. The first version of AGP is known as AGP 1X. It doubled the PCI clock rate and moved 266 Megabytes per second with 3.3 Volt digital signals. AGP proceeded through AGP 2X, 4X, and even 8X. The data rate stepped up to 533 MB/sec, then 1066 MB/sec and finally 2133 Megabytes per second. As the speed went up, the digital signals when from 3.3 Volts, to 1.5 Volts to 0.8 Volts. There was even a 64 bit version called AGP 64. |
4. PCI Express Combines and Simplifies
As I mentioned, when AGP came along, it did not do away with the PCI connection to the video card. AGP was used for high-bandwidth data movement while the PCI bus pulled the strings to control the video card. The need for speed was not the only reason for PCI Express, though it did immediately double what AGP 8X could do performance-wise. Computer users want full-screen, full-motion video with 3-D rendering, and PCI Express has the bandwidth to pull it off. |
5. Lines Turn Into Lanes
All of the PC data busses before PCI Express used a standard parallel data bit design. The ISA bus had 16 bits of data and 16 bits of address. That meant there were 16 wires carrying data and 16 more wires handling the address bits. When the I/O bus grew to 32 bits, the number of wires and the associated pins on the connectors doubled.
PCI Express replaced the parallel bus with a series of serial buses. Instead of 32 data bits all clocked at the rate of the slowest bit, it has up to 32 lanes clocked at the highest rate each line can handle. Think of it like a relay race of four competing teams and four legs in the race. In the parallel AGP model at the end of each leg, all of the runners would have to wait for the slowest one to arrive before taking off on the second leg. At each hand-off of the baton, all teams have to wait for the slowest runner to make it there. With PCI Express, the signals are sent each at their own clock rate. When a runner hands off the baton, the next runner on his team can immediately go. The individual runners are not any faster, but by removing the coordination time at each handoff point, the overall process is sped up. |
6. Say Goodbye to the North and South Bridges
Up until now, most PC motherboards had two major chips on them other than the microprocessor. These are the Northbridge and the Southbridge chip sets. PCI Express deals with the high bandwidth/low bandwidth problem in a much more elegant way. Notice above I said up to 32 lanes. The number of lanes for an I/O device can vary from 1 all the way up to 32, doubling every time. That means there are I/O configurations for 1X, 2X, 4X, 8X, 16X and 32X. Slower devices can use 1X or 2X. For ripping performance, like a video card, 16X or 32X does a better job. Even the size of the connector varies from very short for the 1X interface up to a much longer 32X connector. What is really mind blowing is that different speed cards can, within limits, be plugged into different size PCI Express slots! |
The Northbridge manages data transactions between the CPU, the RAM, and the video card. The Southbridge funnels data to and from all the rest of the devices connected to the I/O bus. Northbridge does high-speed data movement while the Southbridge deals with the slower moving I/O.7. Make the Hardware Simple!
To abandon the legacy of PCI and AGP must require some huge advantages, and PCI Express has them. It’s too bad that they named it PCI Express because that sounds like an evolutionary change, but PCI Express throws out the whole concept of the I/O bus and replaces it with something very revolutionary. As I mentioned, the switch from a parallel bus to lanes, which are really a bundle of serial buses, made the timing and routing of signals much easier for the hardware designer. Another advantage is that the I/O bus is scalable from one lane (1X) for simple and slower devices like a USB controller all the way to 32X for the power-hungry speed demons, such as a high performance video card. The simple devices only need a few wires run to them, saving circuit board space and many pins on the IC packages. |
8. Keep the Software
Even though PCI Express is a revolutionary improvement in PC hardware, it is transparent to the software that runs on it. Yes, you need to replace both the Motherboard and all the plug-in Adapter Cards to convert to PCI Express, but the same operating system (Windows XP, Linux, etc.) can run on the new configuration. |
9. Save the Juice
|
Our laptops are a marvel of energy-saving ingenuity, since they have to run on a battery and battery life is one thing we value in a small, portable computer. Even our desktop machines have to pay attention to power savings, so we don’t need too many fans to keep them cool. For the simple I/O devices in your computer, they only need one bus driver for PCI Express 1X instead of 32 for the old PCI bus. That saves a lot of power. Even the faster devices can turn off lanes when they are not busy and conserve energy that way.Final Words
If you are thinking about a new gaming machine or an upgrade to your old faithful box, check out the offerings of PCI Express motherboards and video cards before spending money on older AGP models. You have to choose between sticking with the older technology, or making the leap to the new way of doing things. Yes, many of the new PCI Express motherboards have PCI slots to accommodate older adapter cards, but not for video. You pretty much have to match both the motherboard and video card to have PCI Express performance. That means there are bargains in high-end AGP motherboards and video cards as existing stocks are flushed out in favor of PCI Express. On the other hand, the PCI Express-based machine will carry you farther into the future. |
Did you find this article helpful?
Do you have a question or comment?
|
|||||||||||
