The primitive operation Grab is an operation in the reference layer. It maps the object specified by a persistent pointer in the main memory and returns the reference pointer to the object. It returns null in failure.
While an object is grabbed, the object is kept in the main memory and the reference pointer to it is guaranteed to be valid.
There must be two types of grabbing operations. One is the exclusive grabbing which allows modification of the object. No two persistent pointers can be converted to the reference pointer by the operation simultaneously.
The other is the read-only grabbing which forbids modification of the object and allows only reference of the object. More than one persistent pointer can be converted to the read-only reference pointer by the operation simultaneously.