ZBasic Language Reference
96
ZBasic Microcontrollers
3.29 Controlling the Heap Size and Main() Task Stack Size
The diagram below illustrates how RAM is allocated between defined variables, task stacks and the heap.
The static allocation area, located at the beginning of RAM, includes all module-level variables defined by
your application and all statically allocated system variables. This includes the task stacks that are
statically allocated by your application.
For native mode devices, the task stack for Main() occupies all of the RAM between the end of the static
allocation area and the heap limit. The task stack grows downward from the heap limit while the heap
grows downward from the end of RAM. For native mode ZX devices, the heap limit is set at 512 bytes
from the end of RAM by default. For generic ZBasic devices, the default heap size depends on a number
of factors including RAM size and Program Memory page size.
For VM mode devices, the space between the end of the static allocation area and the end of RAM is
shared by default between the Main() task stack and the heap. The task stack grows upward from the
end of the static allocation area while the heap grows downward from the end of RAM. There is no
specific heap limit in the default case but you may specify one as described below.
If a heap limit exists, the heap will not grow beyond that point. When the heap is exhausted, further
allocation attempts will fail, possibly resulting in malfunctioning of your application. The heap is used for
storing most RAM-based strings (but not BoundedString or fixed-length string types) including strings that
are returned by functions you define in your application and System Library functions that return strings.
Additionally, for devices that use internal Flash memory for Program Memory (including all generic
devices), when a Program Memory write operation is performed a temporary buffer is allocated from the
heap. The size of this buffer is equal to the Flash page size for the microcontroller, typically 256 bytes.
This additional temporary use of heap space must be considered if your application performs write
operations on Program Memory. Lastly, the System Library function System.Alloc() may be used by
your application to allocate memory from the heap. All of these uses must be considered when
determining how much heap space is needed by your application. The function
System.HeapHeadRoom() may be helpful for determining the minimum heap size for your application.
You can specify the heap limit setting in several different ways, each of which may be useful in different
circumstances. Perhaps the simplest method of setting the heap limit is to specify the desired size for the
Main() task stack. This can be done using the directive Option MainTaskStackSize in your Main
module or by using the compiler option –-main-task-stack-size. In both cases, the heap limit is set
by adding the specified size to the address of the end of the static allocation area.
The second method of setting the heap limit is to specify the desired size of the heap using the directive
Option HeapSize in your Main module or by using the compiler option –-heap-size. The heap limit
is set by subtracting the specified value from the address one past the end of RAM. For devices that
have external RAM, the end of RAM is determined at startup. If you have external RAM, you may specify
that all of the external RAM should be used for the heap by specifying the heap size using the special
values 65535 or &HFFFF.
The final method for setting the heap limit is to specify an absolute address using the directive Option
HeapLimit in your Main module or by using the compiler option –-heap-limit. This method is
recommended for use only by advanced programmers who understand completely how memory is
allocated in a ZBasic application.
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