Why the HEAP region ?

Joel Sherrill joel at OARcorp.com
Thu Jun 12 13:45:40 UTC 1997

On Thu, 12 Jun 1997, Chris Johns wrote:

> I was wondering why a separate region is created in the libc support
> code for the heap ?
> The bsp start code creates a region called the work space region. Why is
> this region not used ?

The Work Space Area is not a region (it is implemented directly using
the Heap Handler in the supercore) and is exclusively for use by RTEMS for
object control blocks, task stacks, etc.  The amount of memory required
for this can be calculated based on your configuration table and a couple
of other parameters.

The C library/program heap is a region because that is what is user
accessible and supports concurrent access.  The division between
application and executive space is intentional.  Long ago there was talk
about redoing the newlib heap management to use a mutex to provide shared
access to a heap but right now each task must have its own heap to get any
time of reentrancy in newlib.  That is the original driving factor behind
the RTEMS malloc code.  Long term, we will probably use the proposed
newlib improvements to malloc when they show up.

> I would like to give all the memory after the bss section to the work
> space, and have the heap get memory from there. This way I do not have
> to adjust the work space when a new task is added and the stack required
> cannot be allocated.

If this is all you are concerned about, there are extension hooks to
manage the allocation of your own task stacks.  I think these hooks
unfortunately do not pass in a requested size but I think Tony Bennett has
patches for this he has not collected up yet.

> This is an even greater issue when code is downloaded to a running
> target (I know number of tasks etc are also fixed, but that is another
> issue).

This is a more fundamental issue and most certainly tougher to address.  I
suppose that something could be done about it but it would require some
fundamental rework of the object manager especially ids and the local

Joel Sherrill                    Sr. Computer Scientist
joel at OARcorp.com                 On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
   Support Available             (205) 722-9985

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