TMU and textures per pass

[MistaPi]

From what I understand the TMU adds texture to the polygon, so a gfx card with 1 TMU can add 1 texture per pass and with multi-texturing the card has to run multiple passes?
How is it then possible that 8500 can add 6 textures per pass and 9500/9700 can add 8 per pass?

Many of you will probably smack your hand against your forehead when you are reading this newb question, but I would really like to understand this.

 

[arjan de lumens]

Most modern GPUs have a 'loopback' feature, so that if the pixel pipeline does not contain enough TMUs to apply all textures needed for a pass, affected pixels are looped through the TMU over and over again until all needed textures are applied.

 

[Dave Baumann]

http://www.beyond3d.com/reviews/ati/radeon9700pro/index.php?page=page17.inc#multipass

 

[Pete]

A pass does not always equal a clock cycle. So a 300MHz GPU runs 300 million cycles a second, but that doesn't mean it's completing 300 million passes a second.

 

[DemoCoder]

Some cards can "loop back" to do the next texture in the next clock cycle. However, older cards would use a "pipeline combine" mode where a TMU was "borrowed" from a neighboring pipeline, halving your pixel fillrate.

 

[MistaPi]

Ahh.. of course, loop-back. Thanx alot Dave.

I really love this forum and of course Beyond3D in general.