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Wednesday, March 08, 2006

7800 GT overclocking myth exposed!

I was reading a review of Gainward BLISS 7800 GT 'Goes Like Hell' 512 MB (wow!) with rating clocks of 450/1300 (1.4 ns memory) at PureOverclock when their explanation of the nature of overclocking 7800 GT struck me. Most people know that not every parts in the G70 core is clocking at the same rate at any time. PureOverclock break it down further by explaining what root clock, vertex clock, shader and ROP clock mean.

I don't actually understand it fully but here's what they said about it:

"You’ll probably need some more details to understand this fully:

• The vertex clock is always 40 MHz higher than the root clock.
• The shader and ROP clocks increase every time you hit a 27 MHz oscillation of the vertex clock (a vertex clock of 486 MHz is 18x27 and so the shader and ROP clocks will increase).
• When the vertex clock hits a 27 MHz oscillation the shader and ROP clocks will be 27MHz less than the root clock when the oscillation takes place. The shader and ROP clocks will not increase until the next 27 MHz oscillation of the vertex clock occurs. Another way to calculate shader and ROP clocks would be that both of them will be 13.5 MHz higher than the root clock.

For example, if you select a root clock of 445 MHz you will have a vertex clock of 485 MHz and the shader and ROP clocks will be 432 MHz.

Why will the shader and ROP clocks be 432 MHz? Because the last 27 MHz oscillation of the vertex clock took place at 459 MHz (27x17). If we now take what we know, which is that the shader and ROP clocks will always be 13.5 MHz higher than the vertex clock was at its last oscillation we can easily predict when the shader and ROP clocks will make a jump."

Head over to the article for the full story. It's very good for GT overclockers out there like me.

Anyway, the point is that somehow Gainward's 7800 GT that's being reviewed does not exhibit this nature. It seems that all those clocks are clocking at the same clock as root clock. Whether this is an advantage or not, I don't know but certainly this card is full of surprises.

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