Explain to me how memory width (128/192/256 bit, etc) is related to memory amount.

[MaxAwesome]

Hello guys!

I'm asking this out of curiosity, because I've been reading GTX 660 TI reviews, and in some forums people are saying that it's ridiculous to have 192bit memory bus paired with 2GB of VRAM, because it can't use it all.

Why is this the case? How does the bus width affect amount of memory on a GPU?

[mlee49]

Memory bus can be thought of literally as lanes of traffic. More lanes dedicated for traffic, the greater the flow. The graphics processor is connected to the RAM on the card via a memory bus.

As far as affects on gpu memory bus, I've seen mixed results. A wider bus doesn't simply mean the card is faster or will perform better. From my experience, I purchased an eVGA GTX 275 FTW with a wider memory bus and my gpu memory overclocked better than stock.

[MaxAwesome]

Originally Posted by mlee49

Memory bus can be thought of literally as lanes of traffic. More lanes dedicated for traffic, the greater the flow. The graphics processor is connected to the RAM on the card via a memory bus.

As far as affects on gpu memory bus, I've seen mixed results. A wider bus doesn't simply mean the card is faster or will perform better. From my experience, I purchased an eVGA GTX 275 FTW with a wider memory bus and my gpu memory overclocked better than stock.

This much I know.

You actually didn't mention anything about memory size and how it's related to bus width

I'll give the example above again: how does 192bit bus interfere with the 2GB of VRAM available on the GTX 660 Ti? Can all the VRAM be adressed or not? Is that the problem?

[Zubasa]

The way it works is quite simply the bus width pretty much controls the number of memory chips that can be used on the card.
A GPU with 256-bit bus has 8 memory chips minimum, since each memory chip have a 32-bit wide bus.
Some cards with double the memory amount will have 16 memory chips.
The 8 extra chips usually on the back of the card shares the bus with the 8 chips soldered to the opposite (front) side of the card.

The 660TI is a 192-bit bus which normally will have 6 memory chips resulting in 1.5GB of memory.
But in this case nVidia added 2 memory chips to the back of the 660TI 2GB to achieve this.
The 2 extra chips shares the bus with 2 memory chips on the front side of the card.


The first card that I know to employ this technique was the 550Ti.
All the vram can be addressed yes, but it is quite an unorthodox way to have such a set up.

[Frag Maniac]

Yep, as said it's bandwidth, or 256bit being like 4 lanes of traffic (data), 192 like 3 lanes, 128, like 2, etc.

That said, many have speculated the Ti 660 with 192bit interface will be crippled, but others have pointed out that it appears Nvidia has found a way to make better use of less bandwidth.

So it's not just specs, a lot of it has to do with improvements in architecture and how well they can make use of a given GPU's specs.

[TRWOV]

The only drawback I see from the 192bit bus is that the last 512MB of those 2GB are going to deliver less bandwith as it uses a shared 64bit link, so if your GPU memory usage goes >1.5GB you could get an fps hit... at least that's how I understand it, please correct me if I'm wrong.

[hat]

Originally Posted by Zubasa

The way it works is quite simply the bus width pretty much controls the number of memory chips that can be used on the card.
A GPU with 256-bit bus has 8 memory chips minimum, since each memory chip have a 32-bit wide bus.
Some cards with double the memory amount will have 16 memory chips.
The 8 extra chips usually on the back of the card shares the bus with the 8 chips soldered to the opposite (front) side of the card.

The 660TI is a 192-bit bus which normally will have 6 memory chips resulting in 1.5GB of memory.
But in this case nVidia added 2 memory chips to the back of the 660TI 2GB to achieve this.
The 2 extra chips shares the bus with 2 memory chips on the front side of the card.
http://www.techpowerup.com/reviews/P...ages/front.jpghttp://www.techpowerup.com/reviews/P...mages/back.jpg

The first card that I know to employ this technique was the 550Ti.
All the vram can be addressed yes, but it is quite an unorthodox way to have such a set up.

They're still doing that goofy shit? I haven't seen that in a long time.

[Hilux SSRG]

Taking from all these comments, it seems to me that 2gb on a 660Ti is more marketing than usefulness. Now I wonder about those 3gb 660Ti?

[mlee49]

Originally Posted by MaxAwesome

This much I know.

You actually didn't mention anything about memory size and how it's related to bus width

I'll give the example above again: how does 192bit bus interfere with the 2GB of VRAM available on the GTX 660 Ti? Can all the VRAM be adressed or not? Is that the problem?

I would only tell you what I know.

So, back to traffic: Larger bus handles more memory, but higher memory(ie 2GB) thats alot of bits(cars) on the bus(road/lanes).

So just widening a highway and adding a lane does not mean traffic jams won't happen. Opening lanes for more cars is fine, but adding more cars(ie vram) will just fill up the lanes just as fast.

In theory, all the VRAM can be addressed. In reality, there is a buffer that will limit the amount of VRAM addressing before performing some other action(like offloading it to the cpu).

If Nvidia was a city planner and hoped to add this ultra fast highway with 4 lanes of traffic, but said "This highway will handle the amount of traffic 2 other highways" then they might have just created a bigger traffic jam than before.

Nvidia is smart though and certainly will use smart techniques to ensure this larger vram is warranted. Think Physx

[qubit]

Originally Posted by Zubasa

The way it works is quite simply the bus width pretty much controls the number of memory chips that can be used on the card.
A GPU with 256-bit bus has 8 memory chips minimum, since each memory chip have a 32-bit wide bus.
Some cards with double the memory amount will have 16 memory chips.
The 8 extra chips usually on the back of the card shares the bus with the 8 chips soldered to the opposite (front) side of the card.

The 660TI is a 192-bit bus which normally will have 6 memory chips resulting in 1.5GB of memory.
But in this case nVidia added 2 memory chips to the back of the 660TI 2GB to achieve this.
The 2 extra chips shares the bus with 2 memory chips on the front side of the card.
http://www.techpowerup.com/reviews/P...ages/front.jpghttp://www.techpowerup.com/reviews/P...mages/back.jpg

The first card that I know to employ this technique was the 550Ti.
All the vram can be addressed yes, but it is quite an unorthodox way to have such a set up.

That's some weird sht and not something I'd like to see on my card. It should always be kept to a power of 2 for optimal design.

Good explanation.