question about video card memory configuration

[toyota]

what is the technical reason a 512 bit card will only have memory amounts in 1gb, 2gb, 4gb and so on? basically someone is asking me why a 512 bit card cant have 3gb of memory and I am not sure how to answer that.

 

[Douglar]

It has to do with the number of data pins on the ram chips.

If each ram chip has 64 data lines, it takes multiples of 8 chips to do a 512 bit bus.
If each ram chip has 32 data lines, it takes multiples of 16 chips to do a 512 bit bus.

Since all the chips have to be the same and you can't do 1/2 a chip, math determines the possible memory configurations from here on out.

 

[toyota]

basically each memory controller needs the same amount ram?

 

[Douglar]

basically each memory controller needs the same amount ram?

Yes, that and the number of data lines used on the chips determines how much ram can be used on each controller.

If you have a 64bit bus on each GPU controller and you use chips with 32 data lines on each chip's controller:
512 Mbit chips on each gives you 1 GByte total
1 Gbit chips on each gives you 2 GByte total
2 Gbit chips on each gives you 4 GByte total
They don't make 1.5Gbit chips or 3Gbit chips.

 

[Arkadrel]

Think of the bus, as a high way.... and the memory as trucks that run on it.
..
..
.

jk

Anyways its best it matches up, that way theres no "waste/delay(performance hit)/errors that occure"

 

[TemjinGold]

I think an easy analogy (if that person isn't technically inclined enough to understand the above) is to say think of your ram sockets. Say you had 4 ram sockets on your motherboard and that you had to meet the following constraints when buying ram:

1) You can only use the same size ram modules.
2) You must fill all the sockets.

Your friend should be able to see why certain sizes can't work now.