SQL Server - How to lock a table until a stored procedure finishes

I want to do this:

create procedure A as
  lock table a
  -- do some stuff unrelated to a to prepare to update a
  -- update a
  unlock table a
  return table b

Is something like that possible?

Ultimately I want my SQL server reporting services report to call procedure A, and then only show table a after the procedure has finished. (I'm not able to change procedure A to return table a).

Needed this answer myself and from the link provided by David Moye, decided on this and thought it might be of use to others with the same question:

CREATE PROCEDURE ...
AS
BEGIN
  BEGIN TRANSACTION

  -- lock table "a" till end of transaction
  SELECT ...
  FROM a
  WITH (TABLOCK, HOLDLOCK)
  WHERE ...

  -- do some other stuff (including inserting/updating table "a")



  -- release lock
  COMMIT TRANSACTION
END

BEGIN TRANSACTION

select top 1 *
from table1
with (tablock, holdlock)

-- You do lots of things here

COMMIT

This will hold the 'table lock' until the end of your current "transaction".

Use the TABLOCKX lock hint for your transaction. See this article for more information on locking.