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Apple IIGS
#78: Bank Alignment and Memory Management 1 of 2
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Technical Notes
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Apple IIGS
#78: Bank Alignment and Memory Management
Revised by: Matt Deatherage May 1992
Written by: Matt Deatherage March 1990
This Technical Note discusses the way the Memory Manager deals with requests for memory that
is already in use, and why this can be really annoying.
Changes since March 1990: Included new information about some smarter algorithms in System
Software 6.0 and later which can avoid problems some of the time.
The Memory Manager is a sophisticated software module that provides the framework for the
allocation, moving, management, and disposal of blocks of memory; however, it’s not magic.
When you ask the Memory Manager for a block of memory and it’s not immediately allocatable,
the Memory Manager starts through the procedure for purging, compacting, and calling out-of-
memory (OOM) queue routines until, at the end of its rope, it finally gives up and returns error
$0201. The exact procedure is repeated below, taken from Volume 3 of the Apple IIGS Toolbox
Reference. Note that each successive step is only taken if, after the previous step, the requested
memory still isn’t available.
1. Calls each OOM queue routine until either all routines have been called or until one
OOM queue routine reports that it has freed enough memory to satisfy the request.
2. Compacts memory.
3. Purges all level 3 handles. If this frees enough memory, compaction occurs.
4. Purges all level 2 handles. If this frees enough memory, compaction occurs.
5. Purges all level 1 handles. If this frees enough memory, compaction occurs.
6. Calls each OOM queue routine again until all have been called or until one OOM
queue routine reports that it has freed enough memory to satisfy the request.
7. Gives up and returns error $0201.
This strategy works pretty well—as long as the request is for a block of memory wherever it fits. If
someone has asked the Memory Manager for memory at a specific address, things get stickier.
Suppose that you’ve asked the Memory Manager for a handle starting at the beginning of bank 2,
and that something else (i.e., the ProDOS FST) is already using that memory. The Memory
Manager notices that the handle isn’t immediately available, so it starts going through the listed
procedures. Since the handle for the ProDOS FST is neither purgeable nor movable and GS/OS
isn’t likely to give it up in an OOM queue routine, the request fails and the Memory Manager
returns error $0201.
However, the Memory Manager went through all the steps listed to get to the seventh step, the
error. The Memory Manager has no way to know that one of the OOM queue routines isn’t going
to give up that particular handle and allow the request to be fulfilled. The OOM queue routines