There's a post somewhere in my 3000 posts where I describe the code when new transactions are added to the next block that is being hashed by Bitcoin. Without a quick search finding my post, essentially a new merkle tree and candidate block is created from the current "best" transactions in the memory pool every minute by Bitcoin, and that is what is hashed when mining.

(edit - found the post:)

Quote from: deepceleron on July 30, 2013, 08:28:54 AM

Bitcoin maintains a memory pool of all valid transactions that are waiting for a block. This contains all transactions that are valid and at least meet the minimum anti-spam fee rules. It also includes "orphan" transactions, which are not yet funded by a payment in the blockchain. These are relayed to other Bitcoin nodes when first seen.

When mining, Bitcoin assembles a sub-set of the memory pool transactions into a block (there may be some or many memory pool transactions that won't fit in the structure of the current block with the miner's rules), and keeps mining the same block data until there are new transactions and it's been over 60 seconds, if 4 billion hashes are done, or if a new network block is announced.

Main.cpp

Code:

// Check for stop or if block needs to be rebuilt
   boost::this_thread::interruption_point();
   if (vNodes.empty())
   break;
   if (nBlockNonce >= 0xffff0000)
   break;
   if (nTransactionsUpdated != nTransactionsUpdatedLast && GetTime() - nStart > 60)
   break;
   if (pindexPrev != pindexBest)
   break;

Only when these conditions happen is a new Merkle tree built and a new block assembled with another set of transactions (if the block is full, transactions can be bumped out of the temporary block by newer ones with higher priority or more fees). A flood of transactions won't create a CPU denial-of-service attack (for OP here).

The transactions (and other block data) that would be included in a block by your Bitcoin is available for your viewing with the Bitcoin RPC command getblocktemplate.

When mining for a pool, their Bitcoin assembles blocks the same way (although they may have tweaked the rules). The only difference is pools have an additional layer of software that assigns work to specific miners and keeps track of what miners are working on, and will reject unassigned work submissions.

Quote from: deepceleron on November 25, 2013, 10:13:51 PM

Quote from: deepceleron on December 05, 2012, 06:54:51 AM

Quote from: deepceleron on November 24, 2012, 08:39:36 AM

Deepceleron's razor #17: If something is worth saying to a noob, deepceleron has probably already said it.