Why sd card/pendrives are 4GB, 8GB, 16GB, 32GB, 64GB etc and not 6 GB, 11GB, 45GB etc?

[TechnoReact-Site]

Why sd card/pendrives are 4GB, 8GB, 16GB, 32GB, 64GB etc and not of some other sizes? Whats the tech behind this?

 

[Mad Cabbie]

I can't remember now, but I think it is to do with the way in which the drives store data. If you have a data address at 32bit, this means that you have 2^32 addresses present in the memory. If your data is 32 bits then it is stored as 4 bytes in sequential addresses, at 2^32 which gives 4gb. To make a stick 8gig, you add one to the bus, giving you 2^33, which gives 8gig. This is pretty easy to realise on the chip, but can increase the issue of complexity.

For something like a 6gig or 12gig, you have to use two drives, ie: 6 gig would require a 2 gig and a 4 gig combined, 12 gig would require an 8 + 4 gig. This increases the complexity, and cost for manufacturers. The cheaper option is to add 1 bit to the address bus giving twice the amount of addresses available, adding two bit to the address bus gives 16gig and so on..... So for minimal complexity and outlay, adding a bit results in the SD card doubling.