Why does merkle root change (in the data part of getwork)?

[Martin P. Hellwig]

Hi all,

I am making a parser in python for the 'data' part of the getwork json-rpc call.
All well so far but I am getting a bit confused why merkle root changes.

According to https://en.bitcoin.it/wiki/Block_hashing_algorithm it should change every time a transaction is accepted. But as far as I can tell no transaction change has been incoming (checking with tcpdump) and even when I do two repeated calls so that the timestamp is the same it still changes.

What else contributes to merkle root being different on subsequent getwork calls?

[Pieter Wuille]

The first transaction is always the coinbase, to claim generated coins and fees. Instead of an normal input script, there is arbitrary data, used as an additional nonce. This either this nonce or the block header's timestamp is changed for every invocation of getwork.

[Martin P. Hellwig]

Thanks!