When you execute one of these DBCC commands, the Database Engine creates a database snapshot and brings it to a transactionally consistent state. The DBCC command then runs the checks against this snapshot. After the DBCC command is completed, this snapshot is dropped.

Sometimes an internal database snapshot is not required or cannot be created. When this occurs, the DBCC command executes against the actual database. If the database is online, the DBCC command uses table-locking to ensure the consistency of the objects that it is checking. This behavior is the same as if the WITH TABLOCK option were specified. An internal database snapshot is not created when a DBCC command is executed:

• Against master, and the instance of SQL Server is running in single-user mode.
• Against a database other than master, but the database has been put in single-user mode by using the ALTER DATABASE statement.
• Against a read-only database.
• Against a database that has been set in emergency mode by using the ALTER DATABASE statement.
• Against tempdb. In this case, a database snapshot cannot be created because of internal restrictions