Page 2

Inside the graphics core

Welcome! I see you were able to let go of that first page :)

Let's chat a little about the G71. It's a generic little story I tell you in all our reviews. What's G71 you ask ? This is the codename of the chip from it's development stages. You all know GeForce 7800 had the codename G70.

So in essence there are three factors changed on the G71 versus G70. A smaller fabrication process (at 90nm), a higher core clockspeed and of course faster memory bandwidth. That G71 chip is different though, as the transistor count went down. The 7800 GTX for example has 304 million transistors where the new 7900 GTX has 278 Million so there are differences for sure.

The G71 has been manufactured at NVIDIA's latest 90nm fabrication process. Smaller fabrication processes of course offer several advantages. You "could" push more transistors on the GPU as you have more space to work with, but another nice advantage is a smaller chip design. That means you need to cram in less voltage into the silicon and that saves on heat. Less heat means means higher clockspeeds and that's why today's product is clocked roughly 100-150 MHz faster then it's predecessor the 7800 GTX 512MB. So to sum this up, less transistors can produce an equal amount of performance as the clock frequency of the processor becomes faster.

Since we are on the topic of the graphics core, inside it there are precisely eight vertex units active. Also the number of pixel pipelines are identical to the 7800 GTX, there are twenty-four of them. Let me enlighten you with briefly what happens in the pixel pipeline for you to understand its importance. Each pixel that is rendered on your screen goes through a pipe where it'll receive its complex color/effect etc. Each time that pixel is altered it'll pass through the pixel pipeline, one pass is one clock cycle.

Does that ring a bell ? One clock cycle ? Yes it does, it relates to MHz. One MHz represents one million cycles per second. The speed of microprocessors, called the clock speed, is measured in megahertz. For example, this G71 graphics processor runs at 650 MHz and thus executes 650 million cycles per second. Each computer instruction requires a fixed number of cycles, so the clock speed determines how many instructions per second that processor can execute.

The 7800 GTX throughout the Series 7800+ line has had eight ROPs. ROP is short for Raster OPeration and is a portion of a pipeline responsible for AA, Blending and Z-Buffer compression. Simply stated a ROP is basically the output engine of a pixel shader pipeline. The pipeline is scalable, each pipe is available at any time in sets of 4, which we call quads.

Of course next to the GPU the most important thing is its memory to work in. We call this the framebuffer, and the faster it is the higher your memory bandwidth to work in will be. Suffice to say that the 7900 GTX 512MB is armed with the latest and fastest memory available. The XXX version comes with a clock of 1800 MHz and that's a lot of bandwidth for sure.

Please focus on the chart below where I cite the more important specs.

Product Name

# pixel processors

# vertex processors

Bus width

Memory Type/Amount

GPU Speed

RAM Speed

GeForce 6800 Ultra **

16

6

256-bit

GDDR3/256MB

400MHz

1100MHz

GeForce 6800 GT

16

6

256-bit

GDDR3/256MB

350MHz

1000MHz

GeForce 6800

12

5

256-bit

GDDR/128MB

325MHz

700MHz

GeForce 6800 LE

8

4

256-bit

GDDR/128MB

320MHz

700MHz

** Not manufactured anymore - Spec are reference specification. Speeds can differ per model and manufacturer

In many ways the product is feature wise 100% similar to the entire 7800 series. This means you'll be able to select resolutions up to a fantastic 2560x1600 (and actually play games at that resolution) thanks to the dual link DVI connectors.