How is ISO determined?

[Positron]

As sensor technology as well as algorithms for interpolating data (in firmware) improve, the ability to squeeze more usable pixels out of the camera at high ISO improves. If Canon wanted, they could release a camera today with ISO ten billion just by taking the image at base sensitivity (assume ISO 100) and ratcheting the exposure up +27 EV (give or take). It would look similar to what would happen if you tried the same thing in Photoshop -- not pretty. They always play a balancing game between pushing the highest numbers possible for marketing purposes and trying to only give the user the option of what's vaguely usable so customers don't get disgruntled when they buy a camera that can go up to ISO 13,421,772,800 and the images look terrible... it's bad for business.

[Jon Ruyle]

If Canon wanted, they could release a camera today with ISO ten billion just by taking the image at base sensitivity (assume ISO 100) and ratcheting the exposure up +27 EV (give or take).

This would be somewhat problematic since Canon only has a 14 bit raw

[ChadS]

This would be somewhat problematic since Canon only has a 14 bit raw

The quantization of the voltage is independent of the amount of gain added.

[Jon Ruyle]

The quantization of the voltage is independent of the amount of gain added.

Ideally, sure. But Canon currently achieves ISO above 1600 by pushing the raw.