Who writes in the Rollback segment and When?

[Tifa]

Hi everybody,
I have some questions about the rollback segment, i searched in the forum before i opened this thread and i didn't find any answer (may be I didin't search enough...). That is my questions:

I don't find in my documents any information about the process who writes in the rollback segment. Normally it's the DBW since it is THE process that writes in data files by definition... but I'm not sure !

I want to know too when exactly rollback segment is written, i know it includes the "before" image of the modified data, but I don't know when this image is written. In all document I read, they speak only about writing of the dirty blocks in data files, but not of the "before" image !!!

My third and final question is the following:
When a little bit of information is modified in a data block; Do we keep in the rolloback segment the whole of the data block, or only a part of the block, the one which was modified ?

Thank you very much for your help.