AppEngine_Heap provides a sliding heap manager.
· Introduction
· Anchors
· Warnings
· Notes
· Example
Introduction
AppEngine provides a sliding heap manager. This is unlike RISC OS's
own OS_Heap heap manager in that it ensures any unused
memory inside the heap is returned to the free memory pool.
This is achieved by allowing blocks claimed from the heap to slide (move
about within the heap) and by referencing each of the blocks indirectly via a
fixed address known as an anchor. Doing this allows the heap manager to
squeeze out any free space from the heap without this affecting the client
application.
Any heap operation can cause claimed blocks position in memory to move.
One significant difference between this heap manager and others is that it
never allows gaps of free space to appear among the blocks. As soon as any
free space is created, then the heap manager will rearrange the heap such
that the block of free space is removed. This means that the heap is always
kept at its smallest possible size.
Anchors
Anchors are located at the base of the heap immediately before the blocks.
Initially the heap manager allocates enough space for eight anchors and hence
eight heap-managed blocks. If you claim a ninth block, then the heap manager
will make space for eight more anchors, and so on.
The space allocated for anchors cannot be freed unless the heap is destroyed.
For this reason it is strongly recommended that you should maintain a
blocks_allocated counter in your own heap interface code, and when it
becomes zero, you should delete the heap. When a claim for a block is made
and blocks_allocated is zero, you should (re-)create the heap.
Warnings
i. When the heap manager is active and is residing at the top of your
application's slot, you should never use Wimp_SlotSize
(or END= in BASIC since that calls Wimp_SlotSize). Neither should you attempt to
store anything above the heap.
ii. Executable code may not be contained in dynamic areas, so you
should avoid keeping code in a heap-managed block.
iii. The internal structure of the heap will not be documented here. To
ensure future compatibility you should only use the defined software
interfaces.
Notes
i. The heap manager is very frugal, it doesnt align blocks or block
sizes to 16-byte boundaries as some others do.
ii. It is written in 100% hand-crafted assembler and I've optimised
certain parts of it for the StrongARM, despite not owning one. (In fact
it was written on my 2Mb ARM 2 A3000. :-)
iii. It is coded to work if using memory above 2Gb, which I understand
will happen if you have 128Mb or more memory (this remains untested).
Example
REM Create a heap residing above BASIC's workspace
SYS "AppEngine_Heap",0,HIMEM,"" TO ,heap%
REM Claim a 2000 byte block
SYS "AppEngine_Heap",2,heap%,2000 TO ,anchor%
REM Find the base address of the block
address%=!anchor%
REM Add 48 bytes to the end of the block
SYS "AppEngine_Heap",4,heap%,anchor%,48 TO ,success%
REM Delete the block
SYS "AppEngine_Heap",3,heap%,anchor% TO ,anchor%
REM Delete the heap
SYS "AppEngine_Heap",1,heap% TO ,heap%
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