heap notes

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Some notes on using the heap instructions.

The instructions CRTHS, DESHS, ALCHSS and FREHSS perform very well and are 
completely bug free.

Use MATHSAT after your module or pgm has freed all its allocations. This is a 
good way to chk that you really are freeing all that has been allocated.

Allocating from your own CRTHS heap instead of the default heap is a good idea. 
This way the harm done by coding errors which overwrite an allocation can be 
limited to the code in question. This also will enable MATHSAT as described 
above to be useful.

The heap space mark functionality should be ignored. There is a bit of 
documentation on marks, so there must be a good reason for them.  Maybe marks 
are used to implement a function call stack. But I dont think there is much use 
for it.

There are 2 heap instructions not provided by MI.
TSTHSS - test that an allocation has been freed.
MATHSSAT - materialize the size of an allocation.
Both can be implemented using MATHSAT, but that would run too slow for 
production work.  The machine has this info available since it uses it when you 
FREHSS,  so these instructions could be easily provided.

It is best if you over allocate your allocations by 16 bytes or so. Use this 
space in front of the allocation to store the size of the allocation and maybe 
the name of the module that did the allocation. This helps to identify the 
source of allocations which have not been freed.  It also helps when trying to 
trap bugs which overwrite their allocations.

Steve Richter



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