At 11:57 AM -0600 4/1/01, Victor B. Putz wrote:
>I'm attempting to port the SmallEiffel Eiffel compiler to the PalmOS
>platform. Most everything is going well, but the Garbage Collector
>needs a bit of help--I have to write a routine which will look at the
>application stack and processor registers to see if any pointers to
>GC'able objects are located therein.
>
>For other architectures, this procedure consists of:
>
>1) Push the registers onto the stack
>2) Start at the top or bottom of the stack with a pointer to a pointer
>3) Increment (or decrement) the pointer-to-pointer, and send the
>dereferenced contents to a marking function
>4) Marking function determines if the dereferenced pointer points to a
>GC-able object and marks it if that's the case.
>
>How do I do this in the PalmOS platform? I tried getting the stack
>parameters with SysGetStackInfo, but I'm not sure how to check the
>registers... and since pointers are 32-bit, is stepping through the
>stack with a "pointer to a pointer" really a good idea? Are stack
>pushes aligned to 16 or 32 bits?
>
>Any help would be, well... helpful. It's just not right without the
>garbage collector.
Victor,
I don't think that implementing a garbage collections scheme is the
right thing on a Palm device. The dynamic heap from which memory
allocations are performed is just too constrained. If you have
filled up the heap, even if it's largely recoverable garbage, there's
no way for another process (or even the OS) to force a garbage
collection if it needs some of that space.
As for the specific problems you're facing, I think that the
SmallEiffel examples provided for the other operating systems should
be sufficient. They have (fairly portable) facilities for getting
the current stack bounds (by taking the addresses of variables on the
stack), as well as for putting the registers onto the stack (using
setjmp). Pointer addresses can be stored on 16-bit boundaries, so
you need to walk the heap taking that into account.
Fortunately, Eiffel's garbage collection is portable to the extent
that you don't need to know the Palm OS Memory Manager's data
structure format. However, know that some day (I have no idea when)
I may be modifying Palm OS Emulator to check for accesses to various
parts of the stack. Some applications have bugs that cause them to
overwrite parts of the stack, leading to crashes. It would be nice
if the Emulator detected those bugs and reported them as stack
overwrites, as opposed to a generic crash. When that facility is
added, your "walk the stack" algorithm maybe flagged as making
improper memory accesses.
--
-- Keith Rollin
-- Palm OS Emulator engineer
P.S. Check out the new e-mail address. [EMAIL PROTECTED] will work
for a while, but anyone mailing me needs to update to the new one
eventually.
--
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