when an update statement is executed in oracle database

shortly, 
1. write the modified data to the archive redo log file and 2. update control file to indicate the most recent checkpoint 


in detail 
1. The data set for the update is taken from the DB CACHE.

2. The ‘Pre Change’ data is written to the UNDO CACHE.

3. The ‘Post Update’ data is rewritten to the REDO CACHE as a delta (a piece of data that contains enough information to reconstruct the original data). The ‘Post Update’ data from the UNDO CACHE is also written to the REDO CACHE (alternatively known as the REDO LOG BUFFER).

4. After a COMMIT is issued the LOG WRITER Process (LGWR) writes the REDO CACHE to the REDO LOG FILES.

5. If ARCHIVELOG is enabled then the ARCHIVER Process (ARCH) writes a copy of the REDO LOG FILES to an alternative destination(s) (Up to 10) before the REDO LOGS are reused.

6. The CHECKPOINT Process (CKPT) is called regularly and this ensures that changed data (dirty blocks) are written from the DB CACHE and back to the datafiles. In addition this process updates the CONTROL FILES.