mvcc and lock

Hi all<br />I have read some code on transaction part.<br />When the new transaction starts, it record the snapshot of
databasecontaining the current transaction id,etc. So depending on the snapshot<br />, the transaction decide which
tupleis visible.<br />But transaction could also be implemented by lock. so I am not sure how <br />the transaction is
implemented,by MVCC or Lock, or by both?<br />In my option, when tuple is processed in readonly mode(select), MVCC is
enough.but when tuple is changed, lock is used. I am not sure whether the explanation is correct.<br />Thanks for any
comments.<br/>Best regards.<br />josh