Note that mining for real ether only makes sense if you are in sync with the network (since you mine on top of the consensus block). Therefore the eth blockchain downloader/synchroniser will delay mining until syncing is complete, and after that mining automatically starts unless you cancel your intention with miner.stop().
Once synced and mining commences, geth will keep the blockchain in sync while it hands off work packages to ethminer for ethminer to perform the mining computations.
Best Answer
For mining Ethereum it is necessary to build a so called DAG. This post explains it.
Ethereum has AFAIK the highest demand to resources to mine amongst all currencies.
According to this reddit post mining with your card might not work due to insufficient memory. Seems like you ran into that problem.