Important: This document is part of the Legacy section of the ADC Reference Library. This information should not be used for new development.
Current information on this Reference Library topic can be found here:
The Memory Manager has been rewritten for Apple's PowerPC Macintosh computers. The new Memory Manager uses native PowerPC code and better algorithms for faster performance. It is also not as forgiving to ruthless programmers who break the rules defined in Inside Macintosh and Macintosh Technical Notes.
First a conceptual clarification. The Memory Manager as documented in Inside Macintosh is a storage allocator. It organizes memory into "heaps" or "heap zones." Its architecture was not designed to support address space mapping, protection, or interrupt time allocation.
The new Memory Manager uses the existing API that is documented in Inside Macintosh. If you code to the Macintosh API, you will inherit benefits of the new high-performance Memory Manager.
Things Not to Do
Here is a list of no-nos that may have appeared to work in the old Memory Manager, but may not with the new Memory Manager.
Don't Dispose Blocks Twice
Never dispose of a memory block twice, and don't attempt to operate on a
disposed memory block. Although many traps return
There are many subtle ways that a memory block can be disposed of twice.
Consider the following example. A programmer calls
Stay Out of Free Blocks
Do not modify data that exists in a free block, and don't attempt to read data or execute code out of a disposed memory block.
Don't Use Fake Handles or Fake Pointers
Call Memory Manager routines only on memory blocks that were created by the
Memory Manager. Any call that operates on a pointer block must operate on a
pointer block created by the Memory Manager. Do not call
Don't Make Calls at Interrupt Time
Since all calls either move memory or set low memory global variables (except
Don't Touch the Data Structures
Do not access or modify Memory Manager data structures directly. This includes
relocatable and nonrelocatable block headers, the heap header, and unused
master pointers. If there is an API available, use it. For example, there is an
API for locking and unlocking a handle. If you don't use it, you will break in
the future. Similarly, don't touch low memory global variables that are
maintained by the Memory Manager. Use
Keep Your Caches Coherent
Be careful with processors that have dual instruction and data caches, such as
the Motorola 68040. Since many clients manipulate code as data, the old Memory
Manager flushes caches frequently for compatibility. The new Memory Manager
flushes caches only when it moves blocks around. When in doubt, call
New Rules to Live By
The new Memory Manager uses larger size block headers and a larger zone header. Clients must leave some slop space for future versions of the Memory Manager. When calculating how large to make a heap, use the following sizes:
Overhead for heap header and trailer 256 bytes
Overhead for each Memory Manager block 32 bytes
Don't Assume Any Stack/Heap Relationships
Don't assume that memory blocks cannot be above your stack. In the future, your
stack may be some other place than it is now. However, do call
Create Dynamic Heaps Inside Other Memory Manager Memory Blocks
To create subheaps, allocate a Memory Manager block and call
Things It's Still OK to Do
It is recommended that you call
Use Existing Tools to Your Advantage
Most popular debuggers for the Macintosh have a heap scramble feature. Use it to debug your code. This feature will ensure that subtle bugs are caught before shipping products to your customers. Similarly, use Zap Handles, Even Better Bus Error, and Double Trouble to ensure compatibility in the future. Although these utilities might not work with the new Memory Manager, you can test with them today using the old one.
With the new high-performance Memory Manager, your applications will run faster. However, you need to follow the rules to be compatible in the future.
Inside Macintosh: Memory
Macintosh Technical Note: "OV 04 - Compatibility Why and How"
Macintosh Technical Note: "OV 11 - The Joy of Being 32-Bit Clean"
Macintosh Technical Note: "HW 06 - Cache as Cache Can"