An RPC method used by a miner to get hashing work to try to solve.
(Just ignore the midstate until you understand the internals of SHA256.)
calculate: hash = SHA256(SHA256(data))
If not, increment the [Nonce] that is a number stored in portion of the data that starts 640 bits in (bytes 76 to 79), and try again.
If the incremented portion overflows, get new work.