The Graphic Accelerator Guide

What Does A Video Card Do, What Affects The Performance?

We have to realize that the data as soon as it leaves the CPU has to go through 4 steps until it finally reaches the monitor:

• From the bus into the Video Chipset, where it's processed (digital data)
• From the Video Chipset into the Video Memory, to store a mirror of the screen picture here (digital data)
• From the Video Memory into the Digital Analog Converter (= RAM DAC), to read out the screen mirror and convert it for the monitor (digital data)
• From the Digital Analog Converter to the Monitor (analog data)

As you can see, except the step from the RAM DAC to the monitor, each step is some kind of a bottleneck and crucial for the overall performance of the graphical subsystem. The slowest step is the one which determines the overall speed. Lets now discuss, what these single steps mean and what actually happens:

The Transfer Of Data Between CPU And The Video Chipset

This bottleneck is mainly depending on the bus type and speed, the motherboard and its chipset. The fastest bus system at present is the PCI bus, so you will have slower performance with VL bus, ISA, EISA and NuBus (only for Macs). The PCI bus however doesn't always run at highspeed of 33 MHz, so with a Pentium 75, P90, P120, P150 you'll have a PCI bus speed of only 25 (P75) or 30 MHz, which already here decreases the performance of the graphical subsystem. Later chipsets also offer faster PCI performance, so the Intel 430HX chipset offers a faster PCI performance than the Intel Triton 430FX chipset . Last but not least it's down to the motherboard how good the PCI performance is. Even much faster than PCi is the new AGP . As the name already says, it's not a bus, it's a port. This means you can only run one device on it, the graphic device. It can runs at 66 MHz and can tranfer data at the rising and falling edge of a clock circle (x2 mode). This makes it at least double as fast as PCI, but this does not necessarily result in double performance of AGP graphic cards, because the data transfer bandwidth is not the limiting factor of current graphic cards.