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Self installation

In order to make persistence transparent, it is necessary to over-ride the built in heap mechanism of Pascal. Syntactically the interface to the Pascal heap is provided by two `procedures': new, and dispose. These differ from other procedures in not being strictly type checked. One can pass to new a pointer to any type, and it will create a new instance of the type. This violation of the type rules is an indication that these procedures are not what they seem to be. In fact they are not procedures at all, but a piece of `procedure like' disguising what the compiler really generates.

A moments thought will reveal that for new to work it really requires two parameters: a var parameter for the pointer to be returned and a second hidden parameter specifying either the size or the type of the object being created. The Borland Pascal compiler makes the underlying procedure directly available as:

procedure GetMem(var P: Pointer; Size: Word);

In this form it is useful for creating arrays on the heap whose size is not known until run time. Dispose is similarly translated by the compiler into a call on FreeMem.

During its startup sequence the persistence manager replaces the first instruction of GetMem with a jump to its own store allocator. The first instruction of FreeMem is replaced with a return instruction, since in a system with a garbage collector, freeing memory is a null action.

The extended syntax that is used with constructors new(p,init(a,b,...)) is translated by the compiler directly into a call on the user declared constructor init. At the start of the constructor method the compiler plants a call to another store allocation routine , call it PrivateAlloc, which, instead of being passed the objects size, is passed its VMT offset. Since the first word in a VMT always contains the size of the object, this provides sufficient information to create the object.

The persistence manager patches PrivateAlloc to branch to an allocation routine that searches the list of persistent classes for the one with the appropriate VMT and then allocates an object from the private heap of that class. For this to function, the class list must have been updated to have entries for all persistent classes. Thus as part of its initialisation sequence, the persistence manager searches the data segment for VMTs and registers all the persistent classes whose VMTs it finds.

next up previous
Next: Loading Up: Data Structures Used Previous: Pointers to virtual

W Paul Cockshott
Fri Sep 6 10:29:18 BST 1996