No, for available-for-sale securities, it works in different way...

let us take the same example,

AFS - Original cost $100

FV at Dec.31, 2008 $80

Unrealized loss goes to Other Comprehensive Income, not IS

Thus, it has not been charged to earnings yet.

AFS sold in 2009 for $90

The entry relating to unrealized loss transferred to OCI is reversed.

We compare the original cost and selling value

Realized loss = 100-90 = $10

This $10 is now charged to earnings, that is, IS

For held-to-maturity, I had provided the wrong answer in error. My apologies. As I said in another post after that, the accounting is different for held-to-maturity securities.

The unamortized cost affects the interest expense/revenue for that security.

Don't think too much right now. You would cover it in "Bonds" section of the study material.