auto increment at limits?

[wxdqz]

Hello all,

I'm just curious how auto increment columns work. What happens when an auto increment column reaches it's upper limit? Does it loop back around and use unused values? For example if I have an unsigned tinyint and I get to the max integer, does mysql automatically start writing over the integers starting at zero or does it look for free integers. Also what happens if all values are taken (no free integers)?

Thanks in advance,
-- tale