Mandatory processes

Hi,

When i started my oracle database,i found the following oracle background processes are up & running.

oracle 253970 1 0 09:26:19 - 0:00 ora_reco_dm
oracle 258126 1 0 09:26:18 - 0:00 ora_pmon_dm
oracle 376978 1 0 09:26:19 - 0:00 ora_cjq0_dm
oracle 549008 1 0 09:26:19 - 0:00 ora_smon_dm
oracle 557160 1 0 09:26:18 - 0:00 ora_ckpt_dm
oracle 577688 1 0 09:26:18 - 0:00 ora_lgwr_dm
oracle 626720 1 0 09:26:19 - 0:00 ora_qmn0_dm
oracle 1024142 1 0 09:26:19 - 0:00 ora_qmn1_dm
oracle 1028234 1 0 09:26:25 - 0:00 ora_i201_dm
oracle 1069170 1 0 09:26:18 - 0:00 ora_dbw0_dm

From the above observation,I just wants to clear what are all the mandatory processes in 9i?

Clear my following doubts,
1) Is RECO mandatory process from 9i onwards or not?
2) When qmn0 & cjq0 processes becomes mandatory & starts with the database?
3) Is process ora_i201_dm mandatory or it depends on the
platform(i'm using AIX 5L)?

Thanks