🚀 go-pugleaf

jlowrey@skat.usc.edu (John 'Fritz' Lowrey) writes:

>	1:  Once the overlay module is no longer needed, is the memory
>		allocated to it freed to the data segment?

Not with most overlay managers.  The only one that I know of that does
this is the one that comes with the Zortech C/C++ compiler called the
'V' (for virtual code) memory model.  Actually the space for the overlay
doesn't come from the data segment it comes from the heap and will be
returned to the heap whenever malloc detects that a failure is about to
occur.  Also note that if malloc doesn't need the space the overlays
may ALL be pulled into memory and once loaded there is virtually no
overhead for calling functions residing in the overlays.

Also important to note is that there are very few (zero?) overlay managers
that support overlaying of data.  All that I am aware of just overlay
code.

>	2:  In a related question, I assume procedures from one overlay
>		module may call those in anther (which seems fairly 
>		inefficient, but logical), how is the memory for these
>		overlay functions dealt with?

Again it depends on the manager.  With most managers you have to manually
specify which modules may be resident simultaneously and where they reside.
Calling a function that will reside on top of the caller can be bad news
(reboot time).  This is very difficult to test for because the call may
only occur during some obscure condition or your code grows a bit more.
The Zortech overlay scheme allows functions to call another other function
safely (admittedly it slows down a bunch) even if it overlays itself.


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