[ajanuary]

Before ARC they used manual reference counting. At runtime each object has a count of how many things have declared they have a reference to it. You can increment that count by doing [obj retain] whenever you know you're keeping a reference to it that will outlive the stack frame. When you're done with the object you can do [obj release] to decrement the counter. When the counter hits zero, the memory for the object can be freed (I don't actually know if it's done immediately or added to a list of objects to free later).

What the automatic part of ARC does is, at compile time, analyse the code and automatically add the retain and release messages. If it determines the reference will outlive the stack frame, it adds a retain message. if it determines the reference can no longer be dereferenced (maybe because another object reference is assigned to that variable) it adds a release message.

The runtime characteristics are the same as ARC, it just means the programmer no longer needs to work out where to add the retain and release messages (though they still need to think about reference cycles).